The summer of 2026 has once again put the way UK homes cope with hot weather firmly under the spotlight. Traditionally, we think about energy efficiency in terms of keeping our homes warm and cutting winter heating bills. But as UK summers get warmer, more and more homeowners are asking the opposite question: how do I keep my house cool?
It’s a fair thing to wonder about. If insulation stops heat escaping in winter, does it also stop heat getting in during a heatwave? Or does all that insulation just trap the warmth and make things worse?
The short answer is that a well-insulated home is generally easier to keep comfortable all year round, but there’s a bit of nuance worth understanding, and insulation is only one part of the picture. This isn’t just a one-off, either. The Met Office notes that UK summers are getting warmer over the long term, with several of the hottest summers on record occurring in recent years. So thinking about energy efficiency as a year-round issue, rather than just a winter one, is only going to become more relevant.
Let’s break it down.
Why do UK homes get so hot?
Most UK housing was designed with one goal in mind: keeping the heat in. That makes a lot of sense given our climate, but it does mean our homes aren’t always well set up for prolonged hot spells. Heat builds up in a home through a few different routes:
- Solar gain through windows. This is usually the biggest culprit. Sunlight streaming through glass heats up rooms quickly, especially south- and west-facing ones, and once that heat is inside it tends to linger.
- Poorly managed ventilation. Opening the windows at the wrong time, say, in the middle of a scorching afternoon, actually lets hot air in. Timing matters enormously, and a lot of overheating comes down to ventilating at the wrong moment.
- Dark roofs and surfaces. Dark materials absorb far more solar radiation than light ones. A dark roof baking in direct sun can transfer a surprising amount of heat into the rooms below, particularly in the loft and top-floor bedrooms.
- Internal heat gains. Ovens, tumble dryers, computers, lighting and even our own bodies all give off heat. On a hot day, these small sources add up.
- Heat entering through the building fabric. The walls, roof and floor themselves conduct heat. When the outside air is hotter than the inside, warmth gradually works its way through the fabric of the building and this is where insulation comes in.
Can insulation keep a house cool?
Here’s the key thing to understand: insulation doesn’t create cold, and it doesn’t create heat. What it does is slow down the transfer of heat from one side of a wall, roof or floor to the other.
In winter, that means insulation slows the rate at which the warmth you’ve paid to generate escapes to the colder outside. Less heat loss means your heating doesn’t have to work as hard, which is exactly why insulation is so closely tied to lower energy bills.
In summer, the same principle works in reverse. When it’s hotter outside than in, insulation slows the rate at which that outdoor heat makes its way in through the building fabric. It won’t single-handedly keep a house cool during a heatwave, remember, most of the heat gain comes through windows and ventilation, but it does help keep the fabric of the building from steadily warming up your rooms.
This is the really useful shift in thinking. Insulation isn’t just a winter measure you install to reduce the gas bill. It’s a year-round building-performance improvement that makes your home easier to keep at a comfortable temperature whatever the season.
Does external wall insulation help in summer?
For many homes, particularly solid-wall properties built before the 1920s, the walls are one of the weakest points in the thermal performance of the building. Solid walls have no cavity to fill, so they let heat pass through relatively easily in both directions.
External wall insulation (EWI) tackles this by fitting a layer of insulation to the outside of the walls, which is then covered with a reinforced, weatherproof render finish. It changes the thermal performance of the walls quite dramatically, improving their U-value (the measure of how readily heat passes through a building element, the lower the number, the better the insulation).
Depending on the property and the finish you’re after, the insulation layer might be EPS (expanded polystyrene), mineral wool, or wood fibre, each with slightly different properties around breathability, thermal performance and how they handle moisture. On top sits the render system, which both protects the insulation and gives the house its new external appearance.
The winter benefit is well established. The Energy Saving Trust estimates that around a third of the heat lost from an uninsulated home escapes through its walls, so insulating them properly makes a real dent in heat loss and heating costs.
But the same wall that’s leaking heat in January is also letting summer heat in during July. By slowing heat transfer in both directions, EWI contributes to steadier internal temperatures and better thermal comfort year-round. So the message is simple. EWI isn’t just about getting through the winter of 2026. It’s about creating a home that’s easier to keep comfortable throughout the year.
Can plastering help keep your home cool?
This is where a common misconception is worth clearing up, because “plaster” and “insulation” often get lumped together and they’re not the same thing.
Standard plaster primarily finishes and protects the internal face of a wall. It gives you that smooth surface ready for decorating, but it isn’t designed to provide meaningful thermal resistance. A skim of ordinary plaster won’t keep your home noticeably cooler.
Insulating plaster is a different product. Depending on its composition and thickness, it can offer some thermal benefit, lime plasters and specialist insulating plasters behave quite differently from a standard gypsum finish, particularly in older, breathable buildings.
Insulated plasterboard (and internal wall insulation more broadly) combines a layer of insulation with an internal finish in one system, fitted to the inside face of external walls. It’s often used where external insulation isn’t practical.
External wall insulation and render takes the opposite approach, the insulation goes on the outside and is covered with a reinforced render system, as described above.
So if you’re wondering whether re-plastering a room will keep it cool, the honest answer is: not on its own. But if you’re genuinely trying to improve how your walls perform, one of the insulating options above is what you’re actually after.
How to keep your house cool during a UK heatwave
Retrofit measures are the long game. When the mercury is already climbing, there’s plenty you can do straight away:
- Shade your windows. Close curtains or blinds on windows where direct sun is hitting them. Blocking the sun before it enters the room is far more effective than trying to cool the room afterwards.
- Ventilate strategically. Open windows overnight and early in the morning when the outside air is cooler, then close them (and the curtains) as the day heats up to keep the cooler air in.
- Reduce internal heat sources. Avoid using the oven, tumble dryer or other heat-generating appliances during the hottest part of the day, and switch off electronics you’re not using.
- Improve insulation where suitable. Loft insulation in particular helps keep top-floor rooms from overheating under a hot roof.
- Consider longer-term retrofit measures. Things like external wall insulation, better glazing and external shading make a lasting difference to how your home copes with heat.
The UK Health Security Agency’s (UKHSA) current hot-weather guidance echoes much of this, with specific advice on preparing homes for hot weather and reducing the risk of overheating, well worth a read before the next hot spell arrives.
What about winter?
Here’s the reassuring part: almost everything we’ve talked about isn’t only about surviving a July heatwave.
Properly designed insulation reduces winter heat loss just as it slows summer heat gain, which means less energy is needed to keep your home at a comfortable temperature when the weather turns. External wall insulation, loft insulation and the various internal options all pull double duty, cooler in summer, warmer and cheaper to heat in winter.
So whether you’ve found this article during a heatwave or you’re reading it on a cold January evening in 2027, the takeaway is the same. A well-insulated, well-managed home is simply a more comfortable, more efficient place to live, all year round.
