Why does my home overheat in summer?
Overheating is a growing problem in UK homes, and London — as an urban heat island — is particularly affected. It happens when a home gains more heat than it can lose. Heat comes in through sun-facing glazing (solar gain), through poorly insulated roofs and walls, and from appliances and occupants. If the home cannot then lose that heat — because ventilation is poor or night-time cooling isn't possible — the temperature climbs and stays high, especially in bedrooms and top-floor rooms. Modern airtight homes and flats can be surprisingly prone to it.
Overheating is not just uncomfortable; it disrupts sleep and can be a genuine health risk during heatwaves. An overheating investigation identifies why your home traps heat and how to keep it cool without resorting to energy-hungry air conditioning.
Solar gain and glazing
The biggest cause of overheating is usually solar gain through glass. South- and west-facing windows, large glazed extensions, rooflights and loft-conversion windows can pour heat into a room on a sunny day. Once that heat is in, it is hard to remove. Shading — external where possible — and appropriate glazing make a substantial difference. We assess where your solar gains come from and how best to control them.
Ventilation and night cooling
A home that cannot lose heat overnight will stay hot. Effective ventilation — particularly secure night-time ventilation and, in some homes, mechanical ventilation — lets cooler night air flush out the day's heat. Where ventilation is poor or windows cannot safely be left open, heat accumulates day after day during a warm spell. We assess how well your home can purge heat and how to improve it.
Insulation, thermal mass and the roof
Insulation works both ways: a well-insulated roof keeps summer heat out as well as winter warmth in, which is why poorly insulated loft rooms overheat so badly. Thermal mass — the ability of heavy materials to absorb and slowly release heat — can smooth out temperature swings when combined with night cooling. We consider how your fabric contributes to overheating and how to improve its summer performance alongside its winter performance.
How Retrofit IQ investigates overheating
We assess the sources of heat gain — glazing orientation and size, roof and wall insulation, shading — and how the home loses heat, including ventilation and night-cooling potential. Thermal imaging and temperature assessment help build the picture, and where relevant we consider airtightness and ventilation together. You receive a clear diagnosis of why your home overheats and a prioritised, passive-first strategy to keep it comfortable.
Part of a full Building Performance Investigation
Overheating is the summer face of building performance, and it shares its causes — fabric, glazing, ventilation — with winter comfort. An overheating investigation is most valuable as part of a full Building Performance Investigation, which addresses year-round comfort in one prioritised plan.
Which rooms overheat, and why
Overheating is rarely uniform across a home; it concentrates in predictable places. Top-floor and loft rooms suffer because heat rises and the roof gains solar heat all day. South- and west-facing rooms bake in the afternoon sun. Rooms with large glazed areas — extensions, bay windows, rooflights — gain heat quickly and struggle to lose it. Small rooms with little ventilation trap heat overnight. Identifying which of your rooms overheats, and which of these factors is responsible, is the first step to keeping them comfortable, and it is exactly what our assessment establishes.
Shading options compared
Shading is the most effective single defence against solar overheating, and external shading beats internal every time — because it stops the heat before it passes through the glass. External options include shutters, brise-soleil, awnings, deep reveals and planting, all of which cut the solar gain reaching the room. Internal blinds and curtains help a little but, like curtains against a cold window, they trap heat that has already entered. Appropriate glazing with a lower solar-gain coefficient also helps. Our assessment recommends the shading approach that suits your home's orientation and appearance.
Overheating, future climate and regulations
Overheating is a growing concern rather than a passing one. UK summers are warming, heatwaves are becoming more frequent, and building regulations now pay explicit attention to limiting overheating in new homes. For existing homes, designing in passive cooling is an investment in future comfort and resilience as much as present relief. Addressing it thoughtfully now — before reaching for energy-hungry cooling — positions your home well for the summers ahead, and our assessment is framed with that longer view in mind.
Solving summer and winter together
The most satisfying outcome of an overheating investigation is that many of the measures improve winter comfort too. Better roof insulation keeps summer heat out and winter warmth in. Good ventilation flushes summer heat and, correctly designed, removes winter moisture. Shading reduces summer gain without harming winter performance. By assessing overheating as part of the wider building performance picture, we design for year-round comfort — a home that is neither too hot in July nor too cold in January, from one coherent plan.
What you receive
London, overheating and the urban heat island
London is particularly prone to overheating because of the urban heat island effect: the density of buildings, hard surfaces and human activity keeps the city several degrees warmer than the surrounding countryside, especially at night. That matters because the main defence against overheating — flushing the day's heat out with cooler night air — is less effective when the night air itself is warm. It also means London homes cannot rely on the same passive cooling that might work elsewhere, and need their overheating risk assessed specifically for an urban setting. Our investigation takes this local reality into account.
The health risks of a home that overheats
Overheating is not just uncomfortable; during hot spells it can be a genuine health risk, disrupting sleep and posing particular dangers to older people, young children and those with certain medical conditions. A bedroom that stays hot into the night makes proper rest almost impossible, with knock-on effects for wellbeing. As summers grow warmer, designing homes that stay cool without energy-hungry cooling is increasingly important — and it starts with understanding why a particular home traps heat, which is exactly what an overheating investigation provides.
Passive cooling measures, in the right order
The most effective and lowest-cost defences against overheating are passive, and there is a logical order to them. First, keep the heat out: control solar gain with external shading, appropriate glazing and, where relevant, better roof insulation. Second, manage internal gains from appliances. Third, get the heat out: maximise secure night-time and cross-ventilation so cooler air can flush the building. Only when these have been exhausted should mechanical cooling be considered. Our report sets out this hierarchy for your home, so you invest in the measures that work before reaching for the ones that cost most to run.
Why air conditioning is a last resort
Air conditioning is tempting during a heatwave, but it is energy-hungry, adds to running costs and, at city scale, contributes to the very urban heat island that makes overheating worse. In many homes, a combination of shading, night ventilation and improved roof insulation keeps temperatures comfortable without it. Where some mechanical assistance is genuinely warranted, understanding the building first ensures it is modest and well-targeted rather than a brute-force fix for a problem that passive measures could largely have solved. The investigation is what tells you which camp your home is in.
- A clear diagnosis of why your home overheats
- An assessment of solar gain and glazing
- A review of ventilation and night-cooling potential
- Consideration of roof and wall insulation and thermal mass
- A prioritised, passive-first strategy to keep the home cool
A real-world investigation example
A family in a top-floor London flat with large west-facing windows found the living room and main bedroom became unbearable on summer afternoons and stayed hot well into the night. Assessment showed heavy solar gain through the unshaded west glazing, a poorly insulated flat roof adding to the heat, and no secure way to ventilate at night, so the day's heat was never flushed out. Rather than fit energy-hungry air conditioning, the plan prioritised external shading to the worst windows, improved roof insulation and secure night-purge ventilation. Together these passive measures kept the rooms comfortable through subsequent heatwaves — and improved winter warmth as a bonus.
Common homeowner mistakes
The most common mistake is reaching straight for portable air conditioning, which is expensive to run, treats the symptom and, city-wide, worsens the urban heat island. Another is relying on internal blinds and curtains, which trap heat that has already passed through the glass — external shading is far more effective. Homeowners sometimes blame insulation for summer heat, when good roof insulation actually helps keep heat out. And overlooking secure night-time ventilation means the home never gets a chance to purge the day's accumulated heat.