# Ventilate heat guides (English, full text) Source: https://getventilate.app/guides Index: https://getventilate.app/llms.txt --- # Your home --- ## How to ventilate your home during a heatwave Most homes without air conditioning have more room to manoeuvre than people think. The difference between a flat that stays bearable and one that becomes an oven is rarely the building itself. It is **timing**: which hours you open, which hours you shut, and what you do about the sun hitting the glass. ## The one rule underneath everything Open when the air outside is **cooler** than the air inside. Close when it is warmer. That sounds obvious, but it is the opposite of what most people do. The instinct on a hot day is to throw the windows open for fresh air. At 3 pm in a heatwave, that instinct actively heats your home. Every hour a window stands open against 35 °C air, your rooms move closer to 35 °C. The corollary matters just as much: when the outdoor temperature finally drops below your indoor temperature, usually well after sunset, that is your window of opportunity. It may only last a few hours, and it is the only free cooling you get. ## Three things to get right **Shading.** Sun coming through glass is a much bigger heat source than warm air leaking in. Blocking it outside the window is far more effective than blocking it inside. See [protecting your windows from the sun](/guides/ventilating-your-home/protect-your-windows-from-the-sun). **Timing.** The pattern over a heatwave is not a single decision but a daily rhythm of keeping heat out, flushing heat at night, and holding the cool you gained. See [the three phases of heat](/guides/ventilating-your-home/phases-of-heat). **Airflow.** Where you put a fan depends entirely on whether your home has windows on one side or on opposite sides. The right placement moves a lot of air. The wrong one just makes noise. See [where to place a fan](/guides/ventilating-your-home/where-to-place-a-fan). Layout, courtyards, top floors, and night flush are unpacked in [how your apartment layout sets the ceiling](/guides/apartment-layouts). **Air worth letting in.** Cooler outdoor air can still be dirty, full of pollen, glaring through a sunny pane, or too sticky. Temperature still leads. See [when cooler air is not worth letting in](/guides/air-worth-letting-in). ## Your layout sets the ceiling A flat with windows on two opposite façades can be flushed of hot air in minutes once outdoor air is cooler, because air has somewhere to go. A single-sided flat, with all its windows on one façade, has no natural through-draught and depends much more on fan placement and patience. Neither is a lost cause. But knowing which one you have changes what you should do, and it is the first thing to work out before you start moving fans around. More in [apartment layouts](/guides/apartment-layouts). ## Safety first None of this replaces medical judgement. If someone is showing signs of heat exhaustion, confusion, or heatstroke, ventilation advice is not the priority. Follow your local health agency's guidance and get help. --- ## The three phases of heat in your home During a heatwave your home moves through a repeating daily cycle. Treating it as one continuous emergency leads to the classic mistakes: windows open at noon, shutters open in the afternoon, everything shut at 2 am when the outdoor air had finally turned useful. Split the day into three phases instead, and each one has a clear answer. ## Phase 1: keep heat away This is the daytime phase, roughly from mid-morning until the outdoor temperature drops back below your indoor temperature. Usually that is late afternoon or early evening, not sunset. The goal is defensive. **Nothing you do now makes the flat cooler.** It only makes it warmer more slowly. - Close windows on any façade where outdoor air is warmer than indoor air - Close shutters, blinds, or curtains on sun-facing windows, ideally on the outside of the glass - Turn off anything that produces heat and is not needed: ovens, desktop computers, incandescent lights, tumble dryers - Accept that the flat will warm up a little regardless. The aim is to arrive at evening at 27 °C instead of 32 °C ## Phase 2: cool down The moment the outdoor air is genuinely cooler than the indoor air, the strategy inverts completely. This is your only free cooling, and it is time-limited. - Open as many windows as you can, as wide as security and safety allow - Prioritise a through-draught if your home has windows on opposite sides - Use a fan to push the exchange along, placed according to [your layout](/guides/ventilating-your-home/where-to-place-a-fan) - Keep going as long as the outdoor air stays cooler, which often means overnight Bear in mind that indoor temperature lags. Your walls and floors are still radiating stored heat, so the room will not drop as fast as the outdoor reading does. ## Phase 3: maintain reasonable warmth By early morning the outdoor air starts climbing again. There is a crossover point where continuing to ventilate stops helping and starts hurting. Close up before that point, not after it. Every degree you saved overnight is capital you are now protecting for the day ahead. Shutters go back down on the east-facing windows first, because that is where the morning sun lands. ## And look after your body The building is only half of it. Fans, hydration, and light clothing do work that ventilation cannot, especially during Phase 1 when the flat is warming and there is nothing more to do about it. See [how to cool your body down](/guides/staying-cool-indoors/how-to-cool-down). ## Where an app helps The hard part of this is not understanding the phases. It is knowing exactly **when** the crossover happens today, for your flat, given your indoor temperature and the local forecast. That moment shifts by hours from one day to the next, and it is different for a top-floor studio and a ground-floor flat on the same street. Ventilate works that out and tells you which phase you are in, window by window. What matters is your safety: always follow your local health agency for further recommendations. --- ## How to protect your windows from the sun If you only change one thing during a heatwave, change this. In a typical flat, **direct sunlight through glass brings in more heat than warm air ever does**. A single unshaded south or west-facing window can add more heat to a room than everything else combined. ## Outside beats inside This is the part most people get wrong. Interior curtains and blinds do help, but far less than exterior ones. Once sunlight has passed through the glass, its energy is already inside the room. An interior curtain absorbs it and then re-radiates most of it into the space behind. An exterior shutter, blind, or screen stops the energy before it crosses the glass at all, and what it absorbs is carried away by outdoor air. If you have both, use the outside one. If you have only interior blinds, light-coloured and reflective beats dark and absorbent. ## The window protection checklist - Close shutters or blinds on sun-facing windows during the day, before the sun reaches them rather than after - Use reflective shades or light exterior screens wherever you can fit them - On older buildings, whitewash on exterior walls and glass still reflects a lot of sun, and washes off in autumn - Light-coloured curtains beat dark ones when interior blinds are your only option - Do not forget skylights and roof windows, which take the sun almost straight on and are often the hottest surface in the flat ## Follow the sun around the building Windows do not all need shading at the same time, and shading a window that is in shadow costs you daylight for nothing. - **East** windows take the morning sun. Shade them early, open them up in the afternoon - **South** windows take the middle of the day, at a high angle. Overhangs, awnings, and balconies above help a lot here - **West** windows are the difficult ones. Late afternoon sun arrives low and horizontal, straight into the room, at the hour when the flat is already at its warmest - **North** windows in the northern hemisphere get very little direct sun and rarely need shading This is why per-window advice beats a single rule for the whole home, and why the orientation of each window is worth recording once. ## What about the glass itself Solar control film is a real improvement on plain single glazing and is a reasonable option for windows you cannot shade from outside. It is a permanent trade-off though: it cuts winter solar gain and some daylight along with the summer heat. Shade you can open and close is almost always the better answer. ## Then think about air Shading is the defensive half. Once the outdoor air cools below your indoor temperature, the job switches to moving air through the flat as fast as you can. That is a different problem with a different answer: see [the three phases of heat](/guides/ventilating-your-home/phases-of-heat) and [where to place a fan](/guides/ventilating-your-home/where-to-place-a-fan). --- ## Where to place a fan in a single-sided or cross-ventilated home A fan does not cool the air. It **moves** it. The motor even adds a small amount of heat to the room. Everything useful a fan does comes from either evaporating sweat off your skin or shifting air between inside and outside, and the second one depends completely on where you put it. ## First, work out your layout Look at where your windows are, not how many you have. **Single-sided**: every window is on the same façade. Air has no natural path through the flat, so an open window mostly exchanges air with itself. Common in small flats, studios, and anything on the corridor side of a block. **Cross-ventilation**: you have openable windows on two different façades, ideally opposite ones. Air can enter one side and leave the other. This is dramatically more effective, and it is worth opening an interior door to unlock it. ## Single-sided homes With one façade, you have to create the pressure difference yourself. - After sunset, once the outdoor air is cooler, place a floor fan **on the windowsill facing into the room**, angled slightly upward - Angling upward matters: it throws the incoming cool air across the ceiling rather than into a wall, and it mixes the room instead of stirring one corner - If you have two windows on the same façade and they are reasonably far apart, use one as intake and put the fan in the other facing outward - Open interior doors so the moving air has somewhere to spread ## Cross-ventilation homes Here you are steering a flow that already wants to happen. - Open the **intake** wide, choosing the shady or greener side, a courtyard rather than a sun-baked street where you have the choice - Open the **exhaust**, typically the street side, just a crack - Place the fan **1 to 2 metres back from the exhaust window, facing outside** The crack is the counter-intuitive part. Narrowing the outlet speeds the air leaving through it, which pulls harder on the intake side and increases the total exchange. Opening everything equally gives you a gentle drift instead of a flush. ## Fans and people For cooling a person rather than a room, the rules are different. Use oscillating mode rather than a jet aimed at your face, which dries eyes and airways over a long evening. And in an empty room, switch the fan off: it is heating the air very slightly and cooling nobody. More on this in [how to cool your body down](/guides/staying-cool-indoors/how-to-cool-down). ## Timing still rules None of this helps if the air you are pulling in is hotter than the air you already have. Fan placement is a Phase 2 activity: it belongs to the hours when outdoor air is genuinely cooler than indoor air. During the day, the fan cools **you**, and the windows stay shut. See [the three phases of heat](/guides/ventilating-your-home/phases-of-heat). --- ## Why your home stays hot after a heatwave The heatwave breaks. Outdoor temperatures fall back to something reasonable. And your flat is still uncomfortable, sometimes for days afterwards, which feels like the building is broken. It is not. Your home is doing exactly what it is built to do, just at the wrong moment. ## Buildings store heat Concrete, brick, plaster, tile, and even furniture absorb heat during hot days and release it slowly once the surrounding air is cooler. Builders call it thermal mass, and it is normally an asset: it is what stops a stone house swinging wildly between day and night temperatures. During a long heatwave, though, that mass charges up. Once your walls are holding several days of stored heat, they keep radiating it into the rooms regardless of what the forecast says. **The air outside cools in hours. The building cools over days.** Well insulated homes are the most striking case. Insulation slows heat transfer in both directions, so a flat that resisted the heat impressively for the first two days will also be the slowest to let it go afterwards. ## What to do about it The answer is the same as during the heatwave, just carried on for longer than feels necessary. - **Keep night ventilation going.** Every cool night is a chance to pull stored heat out of the structure. Stopping after one comfortable evening wastes it - **Use fans to mix the air.** Heat sits near the ceiling and in still corners. Moving air brings it into contact with the cooler air you are letting in, so it can actually leave - **Keep shading during the day** while the flat is still above outdoor temperature. Fresh sun on the glass tops the battery back up - **Be patient.** Expect a gradual slope over two to four days, not an overnight fix ## The temperature you feel is not the temperature you read Warm walls radiate directly onto your skin, which is why a room at 26 °C can feel unpleasant when the surfaces around you are at 29 °C. This is also why the flat feels better as soon as you get some air moving, before the thermometer has moved at all. If you are tracking indoor temperature, do not be discouraged by a slow curve on the way down. Surface temperatures fall first, and comfort follows before the numbers fully catch up. ## Do not close up too early The most common mistake in the days after a heatwave is reverting to normal habits: windows shut at night, shutters open in the day, because the crisis is over. That combination reheats a flat quickly and undoes the recovery you had been building. Keep the heatwave rhythm until indoor and outdoor temperatures actually converge. Ventilate keeps ranking what matters now and can guide window timing until your indoor comfort catches up with the weather outside. --- ## How your apartment layout sets the cooling ceiling The outdoor forecast is shared. Your indoor temperature is not. A ground-floor courtyard and a top-floor studio on the same block can be several degrees apart by late afternoon, even when they opened and closed at the same clock times. Layout is the reason. It decides whether air can actually leave, which façade is the cooler intake, how much sun the roof dumps into the rooms, and whether a balcony door is a gift or a radiator. ## Start with the façades Look at where the windows are, not how many you have. **Single-sided** homes have every opening on one wall. **Cross-ventilated** homes have openings on two different sides. That split is the first thing the Ventilate engine asks, because fan placement and flush speed both hang on it. See [single-sided or cross-ventilation](/guides/apartment-layouts/single-sided-or-cross-ventilation). Then look at what is *outside* each window. A courtyard with plants is usually a better intake than a sun-baked street. See [courtyard or street](/guides/apartment-layouts/courtyard-or-street-windows). ## Height, night, and the size of the opening **Top floors** sit under the roof. They heat faster by day and give the heat back all night. See [why the top floor stays hot](/guides/apartment-layouts/top-floor-heat). **Night flush** is the only free cooling most European flats get. It only works if outdoor air is actually cooler, and in a city [heat island](/guides/heat-and-cities/urban-heat-islands) that stretch is shorter. See [how to flush heat at night](/guides/apartment-layouts/night-flush-ventilation). **Balcony doors** move a lot of air when they should be open, and a lot of sun when they should be shut. Small high windows do the opposite. See [balcony doors and small windows](/guides/apartment-layouts/balcony-doors-and-small-windows). ## What the app does with this Onboarding maps layout, window type, and what sits outside each pane. Advice is per window, not one rule for the whole home. Fan placement still lives with the timing guide: [where to place a fan](/guides/ventilating-your-home/where-to-place-a-fan). Once layout is working, the next question is whether that cooler outdoor air is [worth letting in](/guides/air-worth-letting-in). --- ## Single-sided flats versus cross-ventilation Look at where your windows are, not how many you have. **Single-sided**: every openable window is on the same façade. Air has no natural path through the flat. An open window mostly exchanges air with itself. Common in studios, corridor-side flats, and anything that only faces the street or only the courtyard. **Cross-ventilation**: you have openable windows on two different façades, ideally opposite ones. Air can enter one side and leave the other. Opening an interior door is often what unlocks it. Neither is a lost cause. They are different jobs. ## Single-sided: you create the pressure With one façade you have to manufacture the draught. - After sunset, once outdoor air is cooler, put a floor fan **on the sill facing in**, angled slightly upward - If two windows on the same wall are far enough apart, use one as intake and put the fan in the other facing out - Open interior doors so the moving air has somewhere to go - Expect the flush to take longer. Start as soon as outdoor air is cooler, not at midnight ## Cross-ventilation: you steer a flow Here the air already wants to move. - Open the **intake** wide: shady side, greener side, courtyard rather than a baked street - Open the **exhaust** just a crack, usually the street side - Place the fan **1 to 2 metres back from the exhaust, facing out** Narrowing the outlet speeds the air leaving through it, which pulls harder on the intake. Opening everything equally gives you a gentle drift instead of a flush. More placement detail: [where to put a fan](/guides/ventilating-your-home/where-to-place-a-fan). What sits outside each pane matters too: [courtyard or street](/guides/apartment-layouts/courtyard-or-street-windows). ## Timing still rules None of this helps if the air you pull in is hotter than the air you already have. Layout is a Phase 2 tool. During the day the windows stay shut and the fan cools **you**. See [the three phases of heat](/guides/ventilating-your-home/phases-of-heat). --- ## Courtyard windows versus the street Two windows in the same flat can face two different climates. A **courtyard** with plants, shade, or a light well is often several degrees cooler in the evening than the street façade of the same building. A **street** window sits above asphalt that has been storing sun all day. That is the local face of an [urban heat island](/guides/heat-and-cities/urban-heat-islands). ## Prefer the sheltered side as intake When you have a choice, pull air from: - Trees, a park, a planted courtyard - The façade that stayed in shade through the afternoon - Water, if you have it and the air is moving Push air out through: - The street, the traffic, the sun-baked wall - The side that was in sun all day That is the same ranking Ventilate uses when you say what sits outside each window. City and street lean toward exhaust. Garden and trees lean toward intake. A tight courtyard can also go stagnant: open it when it is clearly cooler, not just because it is inside the block. ## If you only have the street Plenty of flats are single-sided onto a road. You cannot invent a courtyard. - Wait until outdoor air is actually cooler. On a heat-island street that may be later than the forecast for the countryside - Use a fan on the sill to pull night air in, then mix the room - Keep daytime shutters down. The street façade is usually the solar load - If air quality is poor, keep openings short: [dirty air versus indoor heat](/guides/air-worth-letting-in/dirty-air-versus-indoor-heat) ## Noise is a clue, not the rule A loud street is often also a hot, dirty one. Closing for noise at 2 am can trap the day's heat. If you must keep a street window mostly shut, crack a smaller opening and run the fan, rather than sealing the only path you have. Night flush still has to happen somehow: [how to flush heat overnight](/guides/apartment-layouts/night-flush-ventilation). --- ## Why top-floor flats stay hotter Heat rises. Roofs bake. Those two facts stack. A top-floor flat sits under a surface that has taken the full sun load, often with little insulation above it in older buildings. The rooms below you are also storing heat and sending some of it up. By late afternoon the top floor is often the hottest home on the staircase. By 11 pm it may still be giving that heat back, while a shaded ground floor has already started to recover. This is the building-scale version of a [heat island](/guides/heat-and-cities/urban-heat-islands): dark surfaces store the day and release it after dark. ## Daytime is stricter here - Shutters and exterior screens on every sun-facing pane, earlier than the floors below - Roof windows and skylights are solar heaters. Cover them if you can - Balcony doors on the top floor are large glass. Treat them as [windows you shade first](/guides/apartment-layouts/balcony-doors-and-small-windows) - High UV on a clear day is a close-the-sunny-side signal: [high UV](/guides/air-worth-letting-in/high-uv-and-sunny-windows) You will not make a top floor cool at 3 pm. You can arrive at evening a few degrees less hot, which is the whole game. ## Night has to work harder The useful outdoor window is the same for the whole street. A top floor needs more of it, because there is more stored heat to dump. - Start the flush as soon as outdoor air is cooler, not after you go to bed - Keep it going overnight if the air stays cooler - A fan matters more here than in a ground-floor courtyard. See [single-sided or cross-ventilation](/guides/apartment-layouts/single-sided-or-cross-ventilation) - Close before the morning crossover, then get the east shutters down first Well insulated modern roofs hold heat longer once it is in. Thick old walls delay the peak but also delay the cool-down. Either way, the night flush is not optional on the top floor. See [night flush](/guides/apartment-layouts/night-flush-ventilation). --- ## How to flush heat out of a flat at night A heatwave day is defensive. The night is when you actually dump heat. Once outdoor air is cooler than indoor air, open as wide as security allows and keep a path through the home. Walls, floors, and furniture give back what they stored. You are not trying to match the outdoor thermometer by midnight. You are trying to arrive at sunrise a few degrees down, then close up and protect that gain. That is Phase 2 and Phase 3 of [the daily heat cycle](/guides/ventilating-your-home/phases-of-heat). ## The window is shorter in a city [Urban heat islands](/guides/heat-and-cities/urban-heat-islands) hold the day's sun in asphalt and walls. Rural air may drop at 9 pm. Street air often stays warm until midnight or later, and it climbs again earlier in the morning. So the flush: - **Starts later** than a countryside forecast suggests - **Ends earlier** than you would like - **Matters more**, because you recover less overnight City-by-city, that lag is not the same. Paris nights stay warm in dense arrondissements. Berlin parks help some streets. See [heatwave playbooks for European cities](/guides/city-heatwaves). ## How to actually flush - Confirm outdoor air is cooler than indoor air. Guessing from the clock is how people open at 7 pm onto 32 °C streets - Open the cooler, shadier, greener side wide. Crack the hot street side for exhaust if you have two façades - Use a fan to speed the exchange: [layout and fans](/guides/apartment-layouts/single-sided-or-cross-ventilation) - Keep interior doors open so bedrooms share the flush - If dew point is high, prefer a burst rather than all night wide open: [dew point](/guides/air-worth-letting-in/dew-point-best-moment-to-open) - Close before the morning crossover. East shutters down first After the heatwave the same night routine still helps for a few days, because the building is still giving heat back. See [why the home stays hot afterwards](/guides/ventilating-your-home/after-the-heatwave). --- ## Balcony doors versus small high windows Not every opening is the same hole in the wall. A **balcony door** or sliding door is a tall sheet of glass. When it should be open, it is the best intake or exhaust you have. When it should be shut, it is a radiator if the sun hits it. A **small high window** is the opposite: weak airflow, less sun load, often too high for a floor fan on the sill. ## Doors: shade by day, flush by night - Exterior shutters, a screen, or a reflective blind on the door during Phase 1. Interior curtains are a poor second - At night, the door is usually the opening to use, especially on the cooler façade - A floor fan works well across a door track or just inside the opening. Standing fans sit back from the track - Street-facing balcony doors pick up heat-island air and noise. Prefer them as exhaust if you also have a courtyard window Sun plus a west balcony is a classic top-floor trap. Close and shade that door while UV is high: [sunny-side windows](/guides/air-worth-letting-in/high-uv-and-sunny-windows). ## Small and high windows - They rarely make a through-draught on their own - A floor fan on the sill is often impossible. Use a standing fan in the room, or treat them as the exhaust crack in a cross-vent setup - They are still worth opening at night if they are your only openings. Weak flush beats no flush - By day, even a small pane on the sunny side should be shaded. Glass is glass ## Mix them on purpose If you have a balcony door on the courtyard and a small window on the street, you already have a decent cross-vent pair: door wide for intake, small window cracked for exhaust. That is the same idea as [courtyard versus street](/guides/apartment-layouts/courtyard-or-street-windows). If you only have one small window, you are in single-sided territory. Patience and a fan: [single-sided layouts](/guides/apartment-layouts/single-sided-or-cross-ventilation). --- ## When cooler air is not worth letting in Free ventilation advice answers one question: is the air outside cooler than the air inside? That is the right first question. It is not always the last one. Cooler outdoor air can still be **dirty**, **full of pollen**, **glaring through a sunny pane**, or **too sticky to cool you**. Opening wide then trades a little indoor heat for a worse room. ## Temperature still leads None of these extra signals should trap you in a dangerously hot room. If indoor heat is already severe and outdoor air is still cooler, a **short opening** can still dump heat. The overlay never opens a window that temperature and sun had already closed. It only caps a wide opening toward something smaller, or a close. That trade-off is the whole cluster. ## Four reasons cooler air can be a bad deal **Dirty air.** Poor outdoor air (European AQI) is a reason to keep openings small. Indoor heat during a heatwave is also a health risk. Those two facts pull in opposite directions. See [dirty outdoor air versus indoor heat](/guides/air-worth-letting-in/dirty-air-versus-indoor-heat). **Pollen.** Pollen only changes the advice if you said you are sensitive. High counts mean a smaller opening. Very high counts mean closed, unless the room is already dangerously hot. See [pollen and whether to open](/guides/air-worth-letting-in/pollen-and-open-windows). **High UV on the sunny side.** A UV index of 6 or more is also when sun on glass dumps heat into the room. Close the sunny side. Leave the shade side free to follow the temperature. See [high UV and sunny windows](/guides/air-worth-letting-in/high-uv-and-sunny-windows). **Sticky air.** Relative humidity is a percentage. [Dew point](/guides/air-worth-letting-in/dew-point-best-moment-to-open) is the temperature that makes air feel heavy. When it is high, a short opening is usually better than throwing every window wide. ## Timing and layout still come first Work out [when to open at all](/guides/ventilating-your-home) before you worry about air quality. A single-sided flat and a cross-ventilated one do not flush the same way: see [how your layout sets the ceiling](/guides/apartment-layouts). In a city, the outdoor air you are judging is also shaped by the [urban heat island](/guides/heat-and-cities/urban-heat-islands). Nights start later. The useful window is shorter. That is why these extra signals matter more in a dense street than in the countryside. Ventilate Pro feeds pollution, pollen, UV, and dew point into the same window advice. Core timing, history, and the toolkit stay free. This is not medical advice. If you have asthma, a diagnosed allergy, or signs of heat illness, follow your clinician and your local health agency. --- ## Dew point, not humidity, tells you when to open The number people mean when they say the air feels sticky is **dew point**, not relative humidity. Relative humidity is a percentage of how full the air is compared with how much moisture it *could* hold at that temperature. It swings when the thermometer swings. Dew point is the temperature where water in the air would start to condense. It stays put as the day warms, so it is a better read of how muggy the air actually is. High dew point means muggy air that does not cool you well, even if the outdoor thermometer is a little lower than the room. ## What the numbers feel like These are rules of thumb, not medical thresholds: - Below about 10 °C: dry - 10–16 °C: comfortable - 16–20 °C: slightly heavy - Around 20 °C and above: muggy - Past about 22 °C: oppressive When outdoor dew point is high, or close to your indoor temperature, a **short opening** is usually better than throwing every window wide. You still dump some heat. You do not fill the flat with air that will not evaporate sweat. ## The dew point checklist - Trust dew point more than the humidity percentage - Open wide when dew point is comfortable and outdoor air is cooler - Prefer a burst of air when it feels heavy or sticky Ventilate Pro uses outdoor dew point in the window advice. Temperature still leads. Dew point only narrows a wide-open call into a shorter one. See [when cooler air is not worth letting in](/guides/air-worth-letting-in). This is not medical advice. If heat is making you unwell, follow your local health agency. --- ## Dirty outdoor air versus indoor heat Poor outdoor air is a real reason to keep windows mostly closed. Indoor heat during a heatwave is also a health risk. Those two facts pull in opposite directions. The instinct, when the air-quality map turns orange or red, is to shut everything. In a mild room that is the right call. In a flat that is already 28 or 30 °C, shutting the last cooler outdoor air out can make the heat worse. ## Temperature still leads The app does **not** close everything the moment air quality looks bad. - **Poor** outdoor air (European AQI around 60 or more): prefer a **short opening** rather than wide open, unless the room is already very hot - **Very poor** air (around 80 or more): close, **unless** indoor heat is severe (about 28 °C or more) and outdoor air is still cooler. Then a short opening still dumps heat - Cooler outdoor air can still help a hot room. The overlay never opens a window that temperature and sun had already closed PM2.5 is the fine-particle number behind a lot of AQI colour. It is useful context, not a medical diagnosis. In a dense street the same AQI often sits on top of a [heat island](/guides/heat-and-cities/urban-heat-islands): nights stay warmer, so you have fewer hours to dump heat. That is when this trade-off is sharpest. ## The air-and-heat checklist - Temperature still leads: cooler outdoor air can help a hot room - Prefer a short burst when air is poor - Keep closed when air is very poor and the room is not dangerously hot This is not medical advice. Follow your local health agency if you have asthma, heart or lung conditions, or if someone is showing heat illness. --- ## Pollen and whether to open the windows Pollen only changes window advice if you said you are **sensitive**. If you are not, temperature still leads. That is deliberate. Closing for pollen on a 30 °C indoor evening, for someone who is not bothered by it, would trap heat for no gain. The app asks during setup, and you can change the answer later in Your home. ## How the advice changes On **high** pollen days, sensitive people get a **smaller opening**. On **very high** days, windows stay closed unless the room is already dangerously hot. That trade-off is the same idea as [dirty air versus indoor heat](/guides/air-worth-letting-in/dirty-air-versus-indoor-heat). Pollen maps in Ventilate cover **Europe** (CAMS). Elsewhere the pollen row may stay empty. Missing data does not close your windows. Grass, birch, and ragweed peaks do not land on the same week in Paris, Madrid, and Berlin. The rule is the same: only override temperature if you asked the app to. See [heatwave playbooks by city](/guides/city-heatwaves). ## The pollen checklist - Set pollen sensitivity in onboarding or Your home - High pollen: crack a window rather than open wide - Very high pollen: keep closed unless indoor heat is severe This is not medical advice. If you have asthma or a diagnosed allergy, follow your clinician and your local pollen alerts. --- ## High UV and windows on the sunny side A UV index of **6 or more** is high. Those hours are also when sun on glass dumps heat into the room. UV itself is about skin and eyes outdoors. Indoors, the ventilation problem is the heat through the pane. The two often arrive together: clear sky, high sun, hot glass. Ventilate Pro closes windows on the **sunny side** when UV is 6 or higher. Shade-side windows can still open if outdoor air is cooler. That is the same idea as [protecting your windows from the sun](/guides/ventilating-your-home/protect-your-windows-from-the-sun): you do not treat every façade the same. A [top-floor flat](/guides/apartment-layouts/top-floor-heat) feels this first. So does a street façade with no trees, which is often the hot side of a [heat island](/guides/heat-and-cities/urban-heat-islands). ## Shade the glass if you can Exterior shutters beat interior curtains. Once sunlight has passed through the glass, most of its energy is already in the room. Blocking it outside is the bigger win. UV advice in the app is about which windows to keep shut, not a substitute for sunscreen. ## The sunny-side checklist - Close sun-facing windows when UV is 6 or higher - Leave shade-side windows free to follow the temperature advice - Block sun on the glass before it reaches the room This is not medical advice. For time outdoors, follow your local UV and heat guidance. --- # Your comfort --- ## How to stay cool indoors without air conditioning You can do everything right with your windows and still be miserable. Body temperature, hydration, and the small habits of a hot week matter as much as the building does, and they are the part you control minute to minute. ## Cooling a body is not the same as cooling a room Your body sheds heat mainly by evaporating sweat. That is why **moving air feels cooler** even when the air temperature has not changed, and why humidity makes the same temperature feel far worse: when the air is already saturated, sweat has nowhere to go. This is the whole reason a fan helps a person but not an empty room. Point it at yourself and you speed up evaporation from your skin. Leave it running in an empty room and you have just added a small heater. See [how to cool your body down](/guides/staying-cool-indoors/how-to-cool-down). ## Eat and drink for the week, not the moment Hydration during a heatwave is a steady, boring discipline rather than a rescue action. Sipping water through the day works. Drinking a litre when you already feel unwell is catching up too late. Heavy meals and alcohol both add to the load your body is carrying. See [what to drink and eat](/guides/staying-cool-indoors/what-to-drink-and-eat). ## The ones who cannot tell you Pets overheat faster than people and have far fewer ways to shed heat. Dogs cannot sweat through their skin. Cats will find the coolest tile in the flat but can still get into trouble. Plants wilt visibly and are easy to over-correct with midday watering that mostly evaporates. See [keeping pets safe in the heat](/guides/staying-cool-indoors/keeping-pets-safe) and [watering plants and floors](/guides/staying-cool-indoors/watering-plants-and-floors). ## Watch the people around you Older neighbours, very young children, anyone on medication that affects temperature regulation or hydration, and anyone living alone are all at higher risk than a healthy adult. A heatwave is a good week to knock on a door. ## Safety first These are comfort measures, not medical advice. Confusion, a headache that will not shift, a stopped ability to sweat, or a body temperature that keeps climbing are emergencies. Follow your local health agency's guidance and seek help. --- ## How to cool your body down in a heatwave When a room cannot be made cooler, you can still make **yourself** cooler. The two are separate problems, and the second one is often easier to solve. ## Work with evaporation Your body's main cooling mechanism is evaporating sweat off your skin. Every gram that evaporates takes real heat with it. Anything that speeds up evaporation makes you feel cooler, even when the air temperature has not moved at all. That is why a fan works on a person but not on an empty room, and why a humid 30 °C is so much harder to bear than a dry 30 °C. In saturated air, sweat sits on your skin and does nothing. ## The cool yourself checklist - **Use a fan on oscillating mode**, not aimed straight at your face. A constant jet on one spot dries eyes and airways and tends to leave you stiff after a few hours - **Wear light, loose clothing** in natural fibres. Loose matters more than thin: it lets air circulate against your skin - **Mist water on your skin**, particularly on the face, neck, wrists, and forearms. A fine spray plus moving air is the cheapest cooling you have - **Cool your pulse points**: a damp cloth on the back of the neck, or running cool water over the inside of your wrists, works quickly - **Cool drinks help, but very cold drinks can upset your stomach** when you are overheated ## A cool shower, not a cold one A lukewarm or cool shower lowers your skin temperature and leaves you damp, which keeps evaporation going for a while afterwards. It works well before bed. An ice-cold shower feels more decisive but is less effective: the cold triggers your blood vessels to constrict, which reduces heat transfer from your core to your skin, and you can end up rebounding warmer. ## Slow down Muscle activity produces heat. During the hottest hours, the useful move is to postpone anything strenuous to early morning or late evening. Cooking, exercise, and moving furniture all add heat to both you and the room. A heatwave is a legitimate reason to do less. ## Sleeping through it - Ventilate the bedroom hard during the cool hours, and shade it aggressively during the day - A damp towel or light sheet instead of a duvet keeps evaporation working overnight - A fan across the bed, not pointed at your head, at low speed - Cool your feet and wrists before lying down. Both are efficient at shedding heat ## Know the warning signs Feeling uncomfortable is normal in a heatwave. These are not: - Confusion, disorientation, or slurred speech - A headache that will not shift - Nausea, vomiting, or dizziness on standing - Skin that has stopped sweating and turned hot and dry - A racing pulse that does not settle with rest Those are signs of heat exhaustion or heatstroke, and heatstroke is a medical emergency. What matters is your safety: always follow your local health agency for further recommendations. --- ## What to drink and eat during a heatwave Food and drink matter more than usual when your body is already working hard to stay cool. Sweating costs you water and salts continuously, and the meals that feel normal in spring can add real load in the middle of a hot week. ## The hydration checklist - **Sip water regularly through the day**, before you feel thirsty. Thirst lags behind actual need, especially in older adults - **Avoid heavy hot meals and alcohol** when it is very warm. Both make your body work harder at the worst moment - **Prefer cool drinks over ice-cold ones.** Very cold liquids can cause stomach cramps when you are overheated - **Light salads and fruit are easier on your body**, and much of your fluid intake can come from what you eat - **Replace salts, not just water**, if you have been sweating heavily for days ## Why steady beats catching up Drinking a large volume at once mostly passes through you. Your body can only absorb so much at a time, and a litre downed in one go does less good than the same litre spread across an afternoon. A glass every hour is the boring answer, and it is the one that works. If you find that hard to remember, keep a filled bottle in whichever room you actually sit in. ## Foods that carry water Watermelon, cucumber, tomatoes, peaches, courgettes, and leafy salads are mostly water and require no cooking. Chilled soups do the same job. This is a week where cold plates, bread, cheese, and fruit are a reasonable dinner. ## The kitchen is a heat source An oven running for an hour can noticeably warm a small flat, and it does so during the exact hours when you are trying to keep heat out. So can a hob under a large pan, a dishwasher on a hot cycle, and a tumble dryer. Where you can, shift cooking to the cool hours, cook once for several meals, or skip it. This is the same logic as [phase one of the heat cycle](/guides/ventilating-your-home/phases-of-heat): during the day, everything that produces heat indoors works against you. ## About salt and sugar If you have been sweating for several days, plain water alone can leave you short on electrolytes. Ordinary food usually covers it: a salty snack, olives, cheese, or a normally salted meal will do. Sports drinks are not required for sitting out a heatwave at home, and their sugar content is not helping you. Alcohol deserves its own line. It is a diuretic, it disrupts sleep on hot nights, and it blunts your judgement about how you are coping. Coffee in normal amounts is less of a problem than it is often made out to be, but it is not a substitute for water. ## Who needs watching Older adults feel thirst less reliably. Young children dehydrate faster relative to their size. Anyone on diuretics or medication affecting fluid balance should follow their doctor's guidance rather than general advice. During a long heatwave, checking in on someone is worth more than any tip on this page. --- ## How to keep pets safe in a heatwave Pets overheat faster than people. They cannot tell you in words when it is too much, and several of the species we live with are poorly equipped for heat in the first place. Dogs cannot sweat through their skin, so they rely almost entirely on panting. Flat-faced breeds such as bulldogs, pugs, and Persian cats move air far less efficiently and are at much higher risk. Rabbits, guinea pigs, and small caged animals are extremely vulnerable and often kept in exactly the wrong spot: a hutch or cage in a sunny corner of a room. ## The pet care checklist - **Keep fresh water and shade available at all times**, and put out more than one bowl in case one is knocked over while you are out - **Let them rest on cool tile or stone floors.** Move a bed off carpet and give them access to the coolest room in the flat - **Never leave animals in a car**, even for a few minutes and even with a window cracked. Interiors reach lethal temperatures within minutes - **Walk dogs early or late**, not in the middle of the day - **Test the pavement with the back of your hand.** If you cannot hold it there for five seconds, it will burn paws - **Move cages and hutches** out of direct sun and away from windows and conservatories ## Cooling an animal safely Damp towels, a shallow tray of water for a dog to stand in, and a fan they can move away from all help. Frozen water bottles wrapped in a towel work well for rabbits and guinea pigs. Two things to avoid: do not force an animal into cold water, and do not aim a fan directly at a caged animal that cannot move out of the draught. Give them the option to approach or retreat from anything you set up. Cats will usually find the coolest surface in the home by themselves. The useful thing you can do is make sure the good spots are reachable, and that a cat shut in a hot room by a closed door is not a possibility while you are out. ## When to call a vet Call if you see: - Heavy panting that does not settle after moving to a cool place - Drooling that is thicker or heavier than usual - Vomiting or diarrhoea - Unsteadiness, stumbling, or a distant, unresponsive look - Bright red or very pale gums - Collapse Heatstroke in animals progresses fast and is a genuine emergency. Move them somewhere cool, wet their coat with cool, not ice-cold, water, offer a drink if they are alert enough, and phone the vet on the way rather than waiting to see whether it improves. ## Small animals and birds Rabbits and guinea pigs suffer badly above roughly 25 °C and can deteriorate quickly. Birds need shade and a shallow water dish, and their cages should never be near glass. If your home has one room that stays significantly cooler, that is where they should spend the hot days, which is one more reason to know [how your home's ventilation actually behaves](/guides/ventilating-your-home). --- ## Watering plants and floors to cool a room Damp a floor, hang a wet sheet, put out a bowl of water: these are old tricks and they do work. They also fail completely in the one condition where people most often try them, which is a closed room with still air. ## Evaporation is the mechanism Water turning into vapour absorbs heat from its surroundings. That is the whole effect. It is real and it is measurable, but it depends on two things: **surface area** and **airflow over that surface**. A bowl of water in a sealed room evaporates slowly, cools almost nothing, and quietly raises humidity. The same bowl in front of a fan, in a room with a window open to cooler air, does noticeably more. And in an already humid room, adding moisture makes conditions worse, because you are reducing your own body's ability to evaporate sweat. Rule of thumb: **if the air is not moving, do not add water.** ## The comfort checklist - **Lightly mop tile floors**, or place bowls of water near airflow. Damp, not wet - **Water plants in the evening**, not in the midday heat - **Avoid flooding.** Evaporation helps only when air is circulating, and standing water is a slip risk and a mould risk - **Hang a damp sheet in front of an open window** during the cool hours, so incoming air passes through it - **Skip it entirely in humid weather.** When the air is already saturated, more moisture is a net loss ## Why evening watering matters for plants Watering at midday wastes most of what you pour. It evaporates off the surface before roots can take it up, and droplets on leaves can scorch in strong sun. Water in the evening or early morning instead, and water deeply and less often rather than a little every day. Deep watering pushes roots downward where soil stays cooler and damper; frequent shallow watering keeps roots near the hot surface, which is where they are most exposed. Potted plants are the hardest case, especially on a balcony. The pot itself heats up, so roots cook from the sides as well as from above. Move pots into shade during the hot days, group them together so they shade each other, and stand them on a saucer or in a tray of gravel and water. Balconies deserve a note of their own: a dark tiled balcony above a sun-baked street is one of the hottest surfaces attached to your home, and a plant that thrives there in June can die in a week in August. ## Humidity and comfort There is a real trade-off here that a lot of advice skips. Cooling by evaporation adds water vapour to the air, and humid heat is harder for your body to cope with than dry heat because it blocks your own sweating. If the outdoor air is dry, evaporative tricks are helpful. If it is already muggy, prioritise **moving air** instead and leave the water out of it. See [how to cool your body down](/guides/staying-cool-indoors/how-to-cool-down) and [where to place a fan](/guides/ventilating-your-home/where-to-place-a-fan). --- # Your city --- ## Heatwaves in European cities, and why nights stay hot A heatwave on a weather map is one blob. A heatwave at the window is local. Cities run **several degrees warmer** than the fields around them, especially after sunset. Dark streets store the day and give it back when you were counting on a cool night. That is an [urban heat island](/guides/heat-and-cities/urban-heat-islands). It is why night ventilation, the only free cooling most flats get, starts later and ends earlier than a regional forecast suggests. The physics does not change from Paris to Lisbon. The **street** does: stone versus asphalt, courtyards versus traffic, dry heat versus sticky Atlantic air, a park at the end of the block or none. ## What this changes for you In a neighbourhood that cools properly, the strategy is simple: shut by day, flush after dark. In a heat island that stays warm until midnight: - Daytime shading has to be better, because you will recover less overnight - The flush [starts later](/guides/apartment-layouts/night-flush-ventilation) - Courtyard and green façades matter more as intake - Top floors feel the lag first How hot your neighbourhood gets sets the floor for how cool your home can get. That floor is a public choice as well as a personal one: [heatwaves and public choices](/guides/heat-and-cities/heatwaves-and-public-choices). ## Five cities, same rules, different nights - [Paris](/guides/city-heatwaves/paris): dense arrondissements, courtyards, late cooling - [Madrid](/guides/city-heatwaves/madrid): dry heat, hot days, nights that can still drop if the street lets them - [Rome](/guides/city-heatwaves/rome): stone mass, a historic core that holds the afternoon - [Berlin](/guides/city-heatwaves/berlin): shorter spikes, more parks, still a real island in the centre - [Lisbon](/guides/city-heatwaves/lisbon): Atlantic humidity, hills, a river, sticky evenings These pages are not forecasts. They are how the same ventilation rules meet a local heat island. For timing on *your* windows, the app still needs your indoor temperature and today's local weather. --- ## Heatwaves in Paris, and the night the street stays warm Paris is the example the toolkit already uses, for a reason. On canicule nights the city can sit **several degrees above** nearby countryside. Asphalt, stone, zinc roofs, and traffic store the day. After sunset they keep radiating. That is a textbook [urban heat island](/guides/heat-and-cities/urban-heat-islands), and it is why a 22 °C forecast for the region can still feel like 26 °C at a street window in a dense arrondissement. ## What the island does to ventilation - The useful night window **starts late**. Opening at 8 pm onto a still-hot boulevard is how people heat the flat - It **ends early**. East-facing Haussmann rooms take morning sun fast. Shutters down before the crossover - **Courtyards** are often the cooler intake. Street façades are exhaust, noise, and stored heat - **Top floors under zinc** are a special case. Treat them as [top-floor heat](/guides/apartment-layouts/top-floor-heat) A planted cour intérieure and a balcony on a sun-baked avenue are not the same climate. Map them as [courtyard versus street](/guides/apartment-layouts/courtyard-or-street-windows). ## Practical Paris night - Keep volets closed on the sunny side all day. This is the single biggest indoor win in a typical immeuble - Start the [night flush](/guides/apartment-layouts/night-flush-ventilation) only when outdoor air is cooler than indoor air, even if that is after 11 pm - If you have two façades, courtyard wide, street a crack, fan toward the street - On poor-air or high-pollen days, keep openings short: [air worth letting in](/guides/air-worth-letting-in) Trees, lighter courtyards, and less bare paving are the public half. Those are votes and local plans: [public choices](/guides/heat-and-cities/heatwaves-and-public-choices). Inside the flat, timing and shutters are what you control. --- ## Heatwaves in Madrid, dry heat and a street that holds the day Madrid heat is often **dry and fierce by day**. Sweat evaporates, so a fan on your body works. The trap is thinking the night will automatically save you. The centre still has an [urban heat island](/guides/heat-and-cities/urban-heat-islands): dark paving and traffic hold the afternoon and radiate it after dark. A patio interior or a tree-lined street can be several degrees kinder than a wide, bare avenue at 11 pm. ## Dry air is not the same as cool air Low humidity helps your body. It does not lower the thermometer on a west façade at 5 pm. Shutters (persianas) and exterior shade are still Phase 1. Dew point is often more forgiving here than in Lisbon, so when the street finally drops below indoor temperature you can often open **wider** than you would on a sticky Atlantic night. Still check: [dew point](/guides/air-worth-letting-in/dew-point-best-moment-to-open). Pollen (olive, grasses) is a real overlay for sensitive people in late spring and early summer: [pollen and windows](/guides/air-worth-letting-in/pollen-and-open-windows). ## Practical Madrid night - Persianas down on sun-facing glass from late morning. A top-floor piso under a hot roof needs this more than a shaded bajo - Do not open onto the calle at 8 pm because the clock says evening. Wait until outdoor air is cooler than indoor air - Patio or interior façade as intake when you have it. Street as exhaust - UV on clear summer days is routinely high. Close the sunny side: [high UV](/guides/air-worth-letting-in/high-uv-and-sunny-windows) The regional meseta can cool more than Paris once the sun is gone, **if** your street is not a heat battery. Layout decides which of those two nights you get: [apartment layouts](/guides/apartment-layouts). --- ## Heatwaves in Rome, stone that keeps the afternoon Rome's centre is stone, narrow streets, and dark paving: a heat island with **thermal mass**. Thick walls delay the indoor peak, which feels like a gift at 4 pm. They also delay the cool-down. By 10 pm the street canyon may still be giving back the afternoon, which is the same [urban heat island](/guides/heat-and-cities/urban-heat-islands) pattern as Paris, with more stored stone. ## Mass cuts both ways - Old palazzi and thick tuff walls: cooler through the worst of the day, slower to dump heat at night. You need a **longer** flush once outdoor air is cooler - Top floors and dark roofs: still the hot stack. See [top-floor flats](/guides/apartment-layouts/top-floor-heat) - Cortili and inner courtyards: often the only sane intake in a dense block - West-facing terrazze: shade first, open last A single-sided flat onto a tourist street has almost no through-draught and a noisy, hot exhaust. Fan on the sill, patience, and no 7 pm "fresh air" reflex: [single-sided layouts](/guides/apartment-layouts/single-sided-or-cross-ventilation). ## Practical Rome night - Persiane and scuri closed on the sunny side all day - Start [night flush](/guides/apartment-layouts/night-flush-ventilation) when the outdoor air is cooler than the room, which in a tight vicolo can be late - If you have two sides, cortile wide, strada a crack - Poor air from traffic sits on the same streets that hold heat. Keep openings short when AQI is poor: [dirty air versus heat](/guides/air-worth-letting-in/dirty-air-versus-indoor-heat) Trees and unpaved courtyards are scarce in parts of the historic core. That is a public-choice problem as much as a window problem: [what cools a city](/guides/heat-and-cities). --- ## Heatwaves in Berlin, shorter spikes and a real centre island Berlin is greener than many European capitals, and heatwaves here are often **shorter spikes** rather than weeks of 40 °C. That can trick people into leaving windows open all afternoon "because it is only a few days." The physics does not care. A mineral courtyard in Mitte or a west façade in a Gründerzeit block still runs as a [heat island](/guides/heat-and-cities/urban-heat-islands) after sunset, while a garden in a Dorf a few kilometres out has already dropped. ## Parks are not the whole city - Streets that face a park or a planted Hinterhof are better intakes. Use them - Bare Höfe, dark brick, and traffic corridors still store the day - Altbau with thick walls: delayed peak, delayed night dump. Plan a real [night flush](/guides/apartment-layouts/night-flush-ventilation) - Plattenbau and top floors under a hot roof: shade early, flush as soon as outdoor air is cooler Birch and grass pollen can matter in late spring. Sensitive people should not treat a mild 27 °C evening as automatically wide-open: [pollen](/guides/air-worth-letting-in/pollen-and-open-windows). ## Practical Berlin night - Rollläden or blinds down on the sunny side once the first hot day is forecast, not after the flat has already cooked - Hinterhof or green side wide, street a crack, if you have both - The outdoor drop often comes earlier than in Paris, **except** on a dense, unshaded block. Measure against indoor temperature, not against Brandenburg - After a short spike, keep night ventilation going a day or two: walls are still loaded. See [after the heatwave](/guides/ventilating-your-home/after-the-heatwave) More trees and less sealed courtyard paving are exactly the public measures that make Berlin's milder reputation true on *your* street: [public choices](/guides/heat-and-cities/heatwaves-and-public-choices). --- ## Heatwaves in Lisbon, Atlantic air that feels heavier than it reads Lisbon's summers are not always the hottest in Europe on paper. They can **feel** worse, because Atlantic air is often humid and the city is a bowl of hills, stone, and traffic above the Tejo. A modest overnight drop in degrees does not always mean air that cools you. That is dew point talking, on top of a [heat island](/guides/heat-and-cities/urban-heat-islands) that keeps streets warm after dark. ## Sticky plus stored heat - Check [dew point](/guides/air-worth-letting-in/dew-point-best-moment-to-open), not only the evening temperature. High dew point: short openings, not all windows wide all night - River and hillside exposures can be better intakes when the breeze is real. Tight, mineral streets inland hold heat like any capital core - West-facing façades on a hill take a long, hard afternoon. Shade first - Portas de janela and tall windows are balcony-door cousins: great at night, radiators by day. See [balcony doors](/guides/apartment-layouts/balcony-doors-and-small-windows) Pollen is less of a winter story here than in Madrid, but poor air on still, hot days still argues for a smaller opening: [dirty air versus heat](/guides/air-worth-letting-in/dirty-air-versus-indoor-heat). ## Practical Lisbon night - Estores down on the sunny side through the afternoon, especially on top floors - Wait until outdoor air is cooler *and* not oppressive. A 26 °C sticky night can be a worse deal than a slightly warmer dry one - If you have two sides, prefer the more open, breezier, greener façade as intake - A fan on the body still helps when the flush has to stay small. Layout: [single-sided or cross](/guides/apartment-layouts/single-sided-or-cross-ventilation) Hills and the river do not cancel the island. They just move it. Trees, shade on stone, and less bare paving still set how long your street stays hot: [why cities get hotter](/guides/heat-and-cities). --- ## Why cities get hotter, and what actually cools them Everything else in these guides is about the space you control. This one is about the part you do not: the street outside, the material it is made of, and the decisions that put it there. It matters practically, not just politically. **How hot your neighbourhood gets sets the floor for how cool your home can get.** Night ventilation only works if the night air is actually cooler, and in a dense, unshaded, heavily paved area, it often is not cool enough for long enough. ## The physics is not complicated Dark, dense, hard surfaces absorb solar energy during the day and release it slowly afterwards. Vegetation does the opposite: it shades the ground and transpires water, which cools the surrounding air. A city is mostly the first kind of surface and not much of the second. Add waste heat from traffic, buildings, and air conditioning units, and reduce the wind that would otherwise mix the air, and you get a built-up area that runs several degrees warmer than the countryside around it. The gap is usually widest at night, exactly when you were counting on cooler air. See [what an urban heat island is](/guides/heat-and-cities/urban-heat-islands) for how this plays out street by street. For how that lag feels in Paris, Madrid, Rome, Berlin, or Lisbon, see [heatwaves in European cities](/guides/city-heatwaves). ## What actually helps The measures with the strongest evidence behind them are unglamorous and slow: **more trees**, especially mature ones with real canopy; **less bare asphalt**, replaced with shade, permeable ground, and small parks; **lighter surfaces** on roofs and sunny façades; and **green roofs and planted courtyards** instead of sealed ones. None of these are things an individual installs over a weekend. They are budget lines and planning rules, decided locally, over years. See [heatwaves and public choices](/guides/heat-and-cities/heatwaves-and-public-choices). ## Why it belongs in a ventilation app Because the advice changes. In a neighbourhood that cools properly at night, the strategy is simple: shut during the day, flush hard after dark. In a heat island that stays warm until 2 am, the useful window is narrower and later, fan placement matters more, and the daytime shading has to be better because you will recover less overnight. Staying cool at home is personal. Cooler cities are a collective choice, and it is worth knowing which one you are living in. --- ## What is an urban heat island? Cities like Paris, France can run **several degrees hotter** than the countryside on summer nights. Asphalt, concrete, and dark roofs store heat by day and release it after sunset. That pattern is called an **urban heat island**, and it is the reason your home may stay warm even when rural areas nearby have cooled. ## Why cities hold heat Four things stack up, and none of them are avoidable in a dense built environment. **Dark, dense surfaces.** Asphalt and concrete absorb a lot of solar energy and hold onto it. A road surface can reach well over 50 °C in the afternoon and is still radiating that heat back at midnight. **Very little vegetation.** Plants shade the ground and release water vapour as they transpire, which cools the air around them. A street of bare paving does neither. **Waste heat.** Traffic, industry, buildings, and air conditioning units all discharge heat into the same streets. Air conditioning is a particularly awkward case: it makes one flat cooler by pushing that heat, plus the energy used to move it, out into the street where everyone else is walking. **Blocked airflow.** Tall buildings on both sides of a narrow street trap radiated heat and slow the wind that would otherwise mix warm air upwards. Planners call it an urban canyon, and it is very effective at holding heat in place. ## The night is where it hurts Daytime temperature differences between city and countryside are often modest. The gap opens up **after sunset**, when open land radiates its heat away quickly to a clear sky and the city cannot, because its surfaces are still discharging what they absorbed. That is exactly the wrong time for anyone relying on night ventilation. Your cooling strategy depends on outdoor air dropping below indoor air for a long enough stretch, and a heat island shortens that window from both ends: it starts later and ends earlier. ## Where you feel it in your own home - **Balconies above sun-baked streets** stay warm well into the evening, so the "cool" air you are pulling in through that window is not as cool as the forecast suggests - **Courtyards with greenery** are usually several degrees cooler than the street façade of the same building, which makes them the better intake side - **Top floors and unshaded roofs** take the full load of the day and radiate downwards into the rooms below all night - **Ground floors** on shaded streets often stay noticeably cooler than the flats above them The practical consequence: when you have windows on more than one side, the greener, shadier, more sheltered side is usually the right intake. See [where to place a fan](/guides/ventilating-your-home/where-to-place-a-fan). ## What cools a city, and your block - More **trees** and planting along streets, squares, and schoolyards - **Less bare concrete**: shade, permeable ground, and pocket parks - **Lighter surfaces** and reflective or shaded façades on sunny sides - **Green roofs** and planted courtyards instead of heat-trapping asphalt This app helps inside your home. Your street and neighbourhood matter too, and those are decided elsewhere: see [heatwaves and public choices](/guides/heat-and-cities/heatwaves-and-public-choices). How that island shows up in Paris, Madrid, Rome, Berlin, or Lisbon is in [city heatwave playbooks](/guides/city-heatwaves). --- ## Heatwaves and public choices Heatwaves are getting **longer and more frequent**. How hot your city feels is not only the forecast. It is also shaped by **public choices**, local and global. ## Locally Zoning and building rules affect your daily life more directly than they sound: tree cover, street layout, parking versus green space, roof materials, and whether new blocks leave room for airflow. A coherent development plan can require shade, greenery, and cool materials. Or it can add more heat-trapping concrete. The decisions are usually made years before anyone feels the consequence, in documents almost nobody reads, at consultations almost nobody attends. Some of the levers that matter most: - **Tree planting and maintenance budgets.** A street tree takes fifteen to twenty years to deliver meaningful canopy, so this year's budget is really about the 2040s. Watering young trees through their first summers is the unglamorous half of the job, and it is where planting programmes most often fail - **Paving rules.** Permeable surfaces, unsealed ground, and courtyards that are planted instead of tarmacked - **Roof and façade materials.** Lighter and reflective surfaces on sun-facing sides, and green roofs where the structure allows - **Building layout.** Whether new blocks are oriented and spaced to let air move, or built to maximise floor area with no through-ventilation. This decides whether the flats inside can be ventilated at all, which is the difference between [cross-ventilation and a single-sided flat](/guides/ventilating-your-home/where-to-place-a-fan) - **Cool public refuges.** Air-conditioned libraries, shaded squares, drinking fountains, and pools matter enormously to people whose homes cannot be made safe ## Globally Emissions cuts and climate policy influence how often extreme heat returns. No single vote solves everything, but **policy direction** matters, and the frequency of the heatwaves you will be ventilating against in twenty years is being set now. ## What to look for when you vote - Plans that **prioritise shade, trees, and cool surfaces** over bare concrete - **Heat adaptation** budgets: young trees, cool public refuges, insulation help - Rules that **limit unnecessary paving** and favour green courtyards in new buildings - Candidates with a **clear heat and climate adaptation** programme ## Adaptation is not only about comfort Heat is among the deadliest weather risks in Europe, and the deaths are unevenly distributed. Top-floor flats, buildings without shutters, streets without trees, and households that cannot afford to cool a room are where the harm lands. That is what makes this a public question rather than a private one. Individual advice, including everything else on this site, helps people who have a home that can be ventilated. Making sure more homes are in that category is a collective decision. ## Between now and then Vote, attend the consultation, ask what the tree budget is. And in the meantime, work with the building you have: [shade the glass](/guides/ventilating-your-home/protect-your-windows-from-the-sun), [flush the heat at night](/guides/ventilating-your-home/phases-of-heat), and [look after your body](/guides/staying-cool-indoors). Staying cool at home is personal. **Cooler, fairer cities** are a collective choice worth voting for.