For most of the last century, building codes in Britain and much of the northern United States were written with one enemy in mind: the cold. Insulate the walls, seal the gaps, double up the glazing, keep the heat in. That work has saved a huge amount of energy and money.
Summers have changed the brief. The UK recorded 40°C for the first time in July 2022. In 2021 the Pacific Northwest heat dome pushed Portland and Seattle to temperatures their homes were never designed for, in a region where many households had no air conditioning. Further south, from Texas to Florida, long, humid heat waves are putting record loads on the grid.
The result is a new design problem. Homes built to hold heat in winter can turn into ovens in summer, and the answer has to work without simply adding more air conditioning to every room.
Why modern homes overheat
Several features of newer homes, especially apartments, make them prone to overheating.
Airtight, well-insulated construction keeps winter heat in, and it keeps summer heat in too once it has got inside. Large windows, popular for daylight and views, let in a lot of solar heat, especially on south- and west-facing walls. Many city apartments are single-aspect, with windows on one side only, so there’s no way to get a breeze through. Communal heating pipes running through corridors add heat all year round. And in noisy or busy streets, residents often can’t leave windows open at night, which is when cooler air could flush out the day’s heat.
Older homes have their own problems: poorly insulated loft conversions and top-floor flats under hot roofs are some of the worst-affected spaces in a heat wave.
The UK now regulates overheating in new homes
England introduced a new building regulation specifically for summer heat. Approved Document O, which applies to new residential buildings, sets standards for limiting unwanted solar gain and providing a way to remove excess heat.
Designers can show compliance through a simplified method, which limits glazing areas and sets minimum openings for ventilation, or through dynamic thermal modelling of how the home performs across a summer. The guidance also asks designers to consider whether residents can realistically use the cooling strategy. A window that can’t safely be left open at night because of noise, pollution, or security doesn’t count for much.
The US has no national equivalent, but energy codes, utility programmes, and some state and city rules increasingly address cooling performance, and California has included cool-roof requirements in its building energy code.
Keep the sun off the glass
Solar gain through windows is usually the biggest single source of summer heat in a home. External shading stops it before it gets inside.
Overhangs, balconies, and brise-soleil work well on south-facing walls, where the summer sun is high. East and west walls are harder, because the sun hits them at a low angle, so vertical fins, external blinds, shutters, or simply smaller windows work better. External shutters are common across mainland Europe and much of the American South, yet still rare in Britain, and they’re one of the most effective ways to keep a room cool.
Glass choice matters too. Solar control glazing cuts heat gain while still letting in daylight. Internal blinds help with glare but do far less for heat, because the sunlight has already passed through the glass.
Use cool nights while they last
In much of the UK and the northern US, summer nights are still cool even when days are hot. That’s a resource. A home with some thermal mass, such as concrete floors or masonry walls, can absorb heat during the day, then release it overnight if cool air is allowed to flush through.
Night ventilation only works if residents can open windows safely and quietly. Secure, lockable vents, ventilation louvres that can stay open with the windows shut, and layouts with windows on two sides of the home all help. In apartments, dual-aspect units perform far better than single-aspect ones.
Plan for humidity in the American South
Hot, humid regions like Florida, the Gulf Coast, and much of the Southeast have a different problem. Nights stay warm and muggy, so night cooling helps less, and moisture control becomes as important as temperature.
Here, homes depend more on mechanical cooling, but design still decides how much of it is needed. Reflective roofs, well-insulated and ventilated attics, deep porches and overhangs, and careful air sealing all reduce cooling loads. Air conditioning systems that are sized correctly and handle dehumidification well keep homes comfortable at higher thermostat settings and avoid the clammy, mould-prone conditions that oversized systems can create.
Ceiling fans and heat pumps
Moving air makes people feel cooler at the same temperature, which is why ceiling fans remain one of the cheapest cooling measures available. They use a small fraction of the energy of air conditioning and let people stay comfortable at a higher room temperature.
Where active cooling is needed, heat pumps are increasingly the default. Many air-source heat pumps can cool as well as heat, so a home that installs one for winter heating may get summer cooling at little extra cost. Powered by an increasingly clean grid, that’s a far better option than portable air conditioners running through an open window.
Retrofitting the homes we already have
Most of the homes that will exist in 2050 have already been built, so new regulations only cover part of the problem. For existing homes, the most effective measures are usually simple: external shading or shutters on the worst windows, reflective or well-insulated roofs, insulation and ventilation in lofts, ceiling fans, and secure night ventilation. When homes are insulated for winter, it’s worth checking how they’ll perform in summer at the same time, so a retrofit doesn’t make overheating worse.
Where to start
For anyone designing or upgrading a home in the US or UK, a few questions make a big difference. Which rooms take the afternoon sun, and how will they be shaded? Can the home be ventilated at night safely and quietly? Is there a path for air to move through? And if cooling equipment is needed, can it be a heat pump sized for a home that’s already been designed to stay cool?
Asked early, those questions produce homes that stay comfortable through heat waves without leaning entirely on air conditioning.
