Keeping a house cool in summer is mainly about one thing: stopping solar heat from getting in. Heat that is already inside takes far more effort and energy to remove. Passive cooling is the collective name for measures that prevent that without a cooling machine.
The order is nearly always the same: block it, then purge it, and only then cool actively. In England, Approved Document O has set overheating requirements for new dwellings since 2022.
1. Keep solar heat out
- External shading is by far the most effective. Screens, awnings, shutters and roller blinds stop the radiation before it passes the glass; internal blinds only meet the heat once it is already in the room
- Watch the orientation. East and west are often harder than south in practice: the low sun shines straight into the glass, whereas an overhang works well on a south face
- Solar control glass has a lower total solar energy transmittance (g-value): ordinary insulating glass passes roughly half to sixty percent of solar energy, solar control glass clearly less — but it works in winter too, when you want that free heat
- Rooflights and dormers are the biggest heat source in a loft; external shading there gives the most effect per pound spent
- Planting — a deciduous tree or a pergola on the sunny side gives shade in summer and lets light through in winter
2. Purge the heat
- Night ventilation is free cooling: open windows on two sides at night and early morning while it is cooler outside than in, and close everything as soon as the outside air becomes warmer
- Create a flow path — a single open window barely cools; windows on two elevations, or low plus a rooflight high up, create a draught
- Think about security — a lockable restrictor or vent position is what makes night ventilation realistic at ground level
3. Let the structure work
- Mass buffers heat — a heavy floor or wall absorbs heat during the day and releases it at night, but only if you actually ventilate at night
- Insulation works both ways — roof insulation keeps heat out in summer, not just in during winter; poorly insulated roofs are the main reason loft rooms become unusable
- Limit internal gains — fridge, tumble dryer, lighting and equipment all release heat inside the envelope
Indicative costs
- Electric external screens: in the order of 800 to 2,000 euros per window installed, strongly dependent on size and type
- Split air conditioning with one indoor unit: roughly 1,500 to 3,000 euros installed, plus running cost and servicing
Air conditioning only makes sense once the measures above are in place — otherwise you are cooling against an open heat source.
Related terms
- Solar shading
- Night ventilation
- G-value
- Insulation
- Balanced ventilation
- Indoor climate
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