When cooler air is not worth letting in
The free question is whether it is cooler outside. The next question is whether that cooler air is actually worth opening for.
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.
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.
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.
Sticky air. Relative humidity is a percentage. Dew point 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 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.
In a city, the outdoor air you are judging is also shaped by the urban heat island. 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.
In this guide
Dew point, not humidity, tells you when to open
Relative humidity is a percentage. Dew point is the temperature that makes air feel sticky. Use it to decide between a short opening and throwing every window wide.
Dirty outdoor air versus indoor heat
Poor air quality is a reason to keep windows mostly closed. Indoor heat during a heatwave is also a health risk. Temperature still leads when the room is already very hot.
Pollen and whether to open the windows
Pollen only changes window 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.
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. Close the sunny side; shade-side windows can still follow the temperature.
Common questions
- Should I keep the windows shut when air quality is bad?
- Often yes, but not always. Poor outdoor air is a reason to keep openings small. If the room is already very hot, around 28 °C or more, and the outdoor air is still cooler, a short opening can still dump heat. Only very poor air fully closes windows when indoor heat is not severe.
- Does pollen mean I should never open?
- Only if you are pollen-sensitive, and only on high or very high days. If you are not bothered by pollen, temperature still leads. On high pollen days, sensitive people get a smaller opening. On very high days, windows stay closed unless the room is already dangerously hot.
- Why close the sunny side when UV is high?
- A UV index of 6 or more usually arrives with strong sun on the glass. That sun dumps heat into the room. Close the sunny-side windows. Shade-side windows can still follow the temperature if outdoor air is cooler.
- Is humidity the same as dew point?
- No. Relative humidity is a percentage and it swings when the thermometer swings. Dew point is the temperature that makes air feel sticky. High dew point means a short opening is usually better than throwing every window wide.
These guides are informational and are not medical advice. If heat is making you unwell, follow your local health agency’s guidance.
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