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 and where to place a fan.