Understand why UK homes overheat in summer and whi... | Blog

Updated: Jun 17, 2026

For most of the year, a well-insulated, airtight home is exactly what you want. Then a few hot weeks arrive and the same qualities that keep heat in during winter start working against you. Rooms become uncomfortable, bedrooms don't cool down at night, and no amount of opening windows at 2pm seems to help.

Overheating in UK homes is becoming more common, and it's now taken seriously enough that new-build homes must be assessed for it under Part O of the Building Regulations. But you don't need to be building a new house to do something about it. Understanding where the heat comes from tells you exactly where to focus - and which fixes are worth the money.

Where the heat actually comes from

A glass-roofed conservatory heating up in direct summer sun

On a hot day, the warmth in your home arrives from three main sources, and they are not equal.

The biggest by far is solar gain - sunlight coming through glass. Windows, patio doors, conservatories and rooflights act like a one-way valve: short-wave sunlight passes straight through the glazing, hits your floors, walls and furniture, and is re-radiated as heat that the glass then traps inside. A south or west-facing room with large windows can gain an enormous amount of heat this way in a single afternoon.

The second source is conducted heat - warmth soaking through the building fabric (walls and roof) when the outside air is hot. In a reasonably insulated UK home this is usually a smaller contributor than solar gain, and it's slow.

The third is internal gains - heat produced inside the home by people, cooking, lighting, ovens, fridges, computers and other appliances. Individually small, but they add up in an occupied house and they run around the clock.

The reason this matters: if most of your unwanted heat is sunlight coming through glass, then the most effective thing you can do is stop that sunlight before it gets through the glass. Everything else is secondary.

How hot is "too hot"?

It's worth having a benchmark. The Chartered Institution of Building Services Engineers (CIBSE) uses an indoor operative temperature of around 26°C as the point beyond which bedrooms start to compromise sleep - its TM59 overheating standard tests bedrooms against this threshold during sleeping hours (10pm-7am). Living spaces use a more flexible "adaptive comfort" measure, but 26°C is a reasonable line in the sand for a bedroom you actually want to sleep in.

For context on how far a room can drift past that: in a monitored study of a London apartment, an unshaded room reached 47.5°C of operative temperature. That's not a typo - direct sun through glass, with limited ventilation, in a real flat.

The order of operations that actually works

There's a logical hierarchy to keeping a home cool, and following it in order saves you money and effort.

1. Stop heat getting in - shade the glass. Because solar gain is the dominant source, blocking sunlight before it reaches the window is the single highest-impact step. Shading on the outside of the glass is dramatically more effective than anything on the inside, for reasons we cover in the next article in this series - but in short, once sunlight has passed through the glass, the heat is already in the room. External shading (awnings, external blinds, shutters, or even a well-placed tree or brise soleil) intercepts the energy before it ever gets in.

2. Get heat out - ventilate at the right time. Shading reduces how much heat enters. Ventilation removes the heat that's already there. The trick is timing: open windows overnight and in the early morning when outdoor air is cooler than indoor air, and keep them closed during the hottest part of the day when opening them just lets hot air in. Cross-ventilation (windows open on opposite sides of the home) and the "stack effect" (warm air escaping through high openings, drawing cooler air in low down) move far more air than a single open window.

3. Reduce what you generate - manage internal gains. Cook in the evening, use the oven less on hot days, switch to LED lighting (which runs far cooler than halogen), and turn off devices that aren't in use. None of these is transformative on its own, but together they take the edge off, especially overnight.

4. Improve the fabric - glazing and insulation. Insulation and modern glazing are genuinely valuable, but mostly for evening out temperature swings and for winter performance. They are the slowest and most expensive lever to pull for summer comfort, and - importantly - they don't replace shading. A common and costly misconception is that energy-efficient glazing solves overheating. It doesn't, and we'll explain why in article two.

A whole-home approach, not a single gadget

A retractable awning shading a patio and patio doors from the sun

The reason this is framed as a hierarchy rather than a shopping list is that no single product "fixes" overheating. A home that is well shaded but never ventilated will still trap heat. A home that's ventilated but has unshaded south-facing glass will keep overheating faster than you can clear it. The homes that stay genuinely comfortable in a heatwave get the basics working together: keep the sun off the glass, flush the heat out overnight, and don't pour fuel on the fire indoors.

For most existing homes, the highest-value upgrade - the one that addresses the dominant heat source directly - is external shading on the windows and doors that catch the most sun. That's where awnings, external blinds and shutters earn their place: not as a luxury, but as the part of the building that does the heavy lifting on a hot day.

Where to start

Walk around your home on a sunny afternoon and notice which rooms get unbearable and when. South and west-facing rooms with large areas of glass are almost always the culprits, and west-facing rooms tend to suffer worst in the early evening just as you want to use them. Those are the windows worth shading first.

In the next two articles we look at the evidence behind external versus internal shading (and why the difference is bigger than most people expect), and then at a practical, room-by-room routine for getting a home through a heatwave without air conditioning.

GDCG supplies and installs awnings and external shading across Lincolnshire, Nottinghamshire, and Leicestershire. If overheating in a conservatory, patio room or south-facing living space is making summer uncomfortable, we can talk through what would actually make a difference for your home. Contact us for a free survey, or visit our Grantham showroom.