Introduction
Homes are getting warmer
Overheating used to be a rare complaint. In a warming climate — and in airtight, well-insulated modern homes — it is now a design issue to plan for, not an afterthought.
Since June 2022, new homes in England must be assessed for overheating under Building Regulations Part O. But good design goes further than compliance: a comfortable home stays cool in summer without reaching for energy-hungry air-conditioning.
The idea in one line
Let in the light you want, keep out the heat you don't — then give the day's heat a way out at night.
How heat builds up
The physics, briefly
A home heats up when the gains outweigh the losses. Heat comes in through glazing (solar gain), through the fabric, and from people and appliances inside; it leaves through ventilation and, more slowly, through the fabric. Overheating is simply that balance tipping the wrong way — most often through too much unshaded glass.
Designing it out
Reduce gains first, then remove heat
The order matters. It is far easier, and cheaper, to keep heat out than to pump it away afterwards. We work down this hierarchy:
Orientation & glazing
Size and place windows to the sun path; go easy on large east and west glass.
Shading
Roof overhangs, external blinds, deep reveals and planting — shade works best outside.
Glazing
Solar-control glass that reflects some of the sun's heat, on the sunniest walls.
Thermal mass
Exposed masonry or concrete soaks up daytime heat, released at night.
Fresh-air cooling
Windows you can open for a through-draught, and a safe way to let heat out at night.
Mechanical help, last
MVHR set to a summer mode, and active cooling only where simple measures can't cope.
On glazing
Your window supplier can upgrade the glass itself — solar-control coatings that let daylight in but reflect much of the sun's heat. It's a simple, cost-effective step, and we specify it on the elevations that need it.
Meeting Part O
Two routes to compliance
- The simplified method — limits on glazing area by orientation, plus a minimum openable area for ventilation.
- A detailed computer assessment (the industry TM59 method) — for heavily glazed or higher-risk designs.
- Either route must show how you will ventilate safely at night: security, noise and air quality all count.
Common traps
Where homes go wrong
- Full-height south or west glazing with no external shading.
- Rooflights that bake the top floor in midsummer.
- Single-aspect rooms and flats that cannot cross-ventilate.
- Relying on windows you cannot safely leave open at night.
How we help
We check overheating from the very first sketch — orientation, glazing and shading worked through, and night-time ventilation resolved before you build. The traps above get designed out on the drawing board, not discovered on a hot afternoon.
Key terms
A few words worth knowing
Hover over or tap a card for more detail.
- Part O
- Since 2022, new homes in England must be assessed for overheating under the Building Regulations.
- Part O of the Building Regulations came into force in June 2022 and applies to new homes, not to extensions. It limits how much the sun can heat a home through its windows, and makes sure there's a safe, practical way to get rid of excess heat, particularly at night.
- Solar gain
- Heat from the sun coming in through the glass — the single biggest cause of overheating.
- Sunlight coming through glass warms everything it falls on, and that heat builds up inside. In winter it's free heating; in summer, through large south- and west-facing windows, it's the main reason rooms overheat. Shading, the choice of glass and the size of windows are how it's kept under control.
- MVHR
- Mechanical ventilation with heat recovery: quiet, filtered fresh air for an airtight home; a summer bypass mode stops it adding heat on warm days.
- An MVHR system quietly draws stale air out of kitchens and bathrooms and brings filtered fresh air into living spaces, passing the two through a heat exchanger so warmth isn't thrown away in winter. It suits airtight, well-insulated homes. In summer a bypass mode stops it warming the incoming air, but it isn't a substitute for air-conditioning.
- Night purge
- Ventilating on summer nights to flush the day's heat out of the building before morning.
- On hot days a house soaks up heat. Opening windows, high vents or rooflights overnight, when it's cooler outside, lets that heat escape so the home starts the next day cool. It works best with openings on opposite sides or at different heights that can be left open securely and quietly.
This guide is general information for projects in England and is not legal, planning or design advice. Every site is different — always check current rules with the relevant authority, or ask us.