Overheating & Summer Heat
Homes overheat when more solar gain enters by day than can be purged at night. Shading, roof improvements and night ventilation usually cool a home without air conditioning.
Why does my house overheat in summer?
A house overheats in summer when more solar heat enters during the day than can be removed at night. Sun through unshaded glazing and a hot roof pours energy in; if the home is then unable to purge that heat overnight — because windows stay shut, ventilation is limited or the structure holds onto warmth — temperatures climb day after day. Overheating is a building-physics balance of solar gain in versus heat out, and it is increasingly common in modern, well-insulated and glazed homes.
Read the guideWhy is my bedroom too hot at night?
A bedroom that is too hot to sleep in usually combines three things: it gains a lot of heat during the day, it is high in the house where heat collects, and it cannot purge that heat at night. Being upstairs, often under a sun-heated roof, with windows kept shut for security or noise, the room stores the day's warmth and holds it through the night. Cooling it means cutting the daytime gain and enabling secure overnight ventilation.
Read the guideHow do I keep my house cool without air conditioning?
You can keep a home comfortable in summer without air conditioning by working with building physics rather than against it: stop the heat getting in during the day, and purge the heat that does build up at night. In practice that means shading the windows that gain the most sun, keeping the fabric (especially the roof) insulated, closing up against the hot daytime air, and ventilating hard once the outside cools in the evening. Done in the right order, these passive measures keep most UK homes comfortable through all but the most extreme heat.
Read the guideDo I need external shading, or will internal blinds do?
For controlling overheating, external shading is far more effective than internal blinds, because it stops the sun before it passes through the glass, whereas an internal blind only intercepts the heat once it is already inside the room. Internal blinds help with glare and privacy and offer some benefit, but they cannot match external shutters, blinds or overhangs for keeping a room cool. So if your aim is to stop a room overheating in summer, the honest answer is that external shading is what really works.
Read the guideWhy is my top-floor flat or loft room so hot in summer?
Top-floor flats and loft rooms overheat in summer because they sit directly beneath the roof, which is the surface of the building exposed to the most intense solar heat — and they usually have the least insulation and the poorest night-time cooling above them. The roof absorbs a great deal of heat through the day and radiates it down into the room; rooflights and dormer windows add direct solar gain; and the heat that builds up has nowhere to escape because hot air collects at the top of the building and there is no purge route. It is a combination of solar gain through the roof and glazing, weak insulation, and a lack of night ventilation — all of which can be addressed once the dominant cause is identified.
Read the guideWill better insulation make my house hotter in summer?
A common worry is that insulating a home will make it hotter in summer by 'trapping heat' — but this misunderstands how insulation and overheating work. Summer overheating is driven by solar gain (sunlight entering through windows and heating the roof) and by an inability to get rid of that heat through ventilation, not by insulation. Insulation actually slows heat from passing through the fabric in both directions, so a well-insulated home gains heat from the hot outdoors more slowly and, once cooled, stays cool longer. The real causes of a hot house are unshaded glazing, a sun-baked roof and a lack of night ventilation — and insulation, far from causing the problem, is part of the solution.
Read the guideWhy does my bedroom stay hot all night?
A bedroom that stays hot all night has absorbed more heat during the day than it can release after dark, and cannot purge it because night ventilation is inadequate. Bedrooms — especially upstairs and under the roof — gain heat through sunlit windows and a hot roof during the day, and store it in the walls, ceiling and contents; if the room is closed up and there is little cool night air flushing through, that stored heat keeps radiating into the room overnight, so it never cools enough to sleep comfortably. The fix is to keep the heat out by day and purge it by night, not to add air conditioning.
Read the guideHow do I stop my loft conversion overheating?
Loft conversions overheat in summer because they combine the worst gains in the house: a large roof surface baking in the sun above thin insulation, roof windows (rooflights) that admit intense overhead sunlight, and a position at the top of the house where heat collects. The result is a room that becomes uncomfortably hot and stays hot. The way to cool it without air conditioning is to cut those gains — improve the roof insulation, shade the rooflights externally — and to purge the stored heat with night ventilation, so the conversion becomes comfortable in summer as well as warm in winter.
Read the guide