Why New Build Houses Overheat

It is one of the more surprising complaints we hear. People move out of a draughty Victorian terrace into a brand new house, spend a winter delighted with how warm and cheap it is, and then discover they cannot sleep in it from the middle of May.

It is not a fault. It is the predictable result of how modern houses are built.

Built to keep heat in — which works both ways

A new house is airtight and heavily insulated. That is the whole point: it holds onto the heat you pay for instead of leaking it into the garden. The trouble is that insulation does not know which direction you want it to work in. Once summer heat is inside a modern house, through the windows and through the roof, the same fabric that keeps January out keeps July in.

Three things make it worse. New homes have large glazed areas, often deliberately south-facing so they gain free heat in winter. Lightweight modern construction has little thermal mass, so it heats up quickly rather than soaking up heat the way thick solid walls do. And a modern house does not have the accidental ventilation an older one does — no draughty sash windows, no open chimney, no gaps under the doors.

Mechanical ventilation moves air, it does not cool it

Many new homes have mechanical ventilation with heat recovery, and a lot of owners reasonably assume it will deal with summer. It will not. It exchanges stale indoor air for fresh outdoor air, and a good one has a summer bypass so it stops recovering heat when you do not want it. What it cannot do is deliver air colder than the air outside. On a 30-degree afternoon it is moving 30-degree air around your house.

What the overheating regulations changed

Since June 2022, new residential buildings in England have had to address overheating under Approved Document O of the Building Regulations. It is a genuine improvement: it pushes designers to limit excessive glazing, provide shading and make sure windows can actually be opened far enough to clear heat.

There are two limits to it, though. It is an assessment carried out at design stage against a model, which is not the same as a guarantee during a real heatwave. And it applies only to homes designed after it came in — a house completed in 2021 had no such requirement at all.

The problem with “just open a window”

Opening windows is the answer the regulations lean on hardest, and in a lot of real homes it does not work.

• On an estate near a main road or a flight path, open windows mean noise all night.
• Nobody wants ground floor windows open while they sleep.
• Hay fever season and the hottest months are the same months.
• Many new layouts have no cross-ventilation — windows on one elevation only, so nothing actually flows through.
• And when the outside air is 24 degrees at midnight, an open window is not cooling anything.

What actually deals with it

External shutters and brise soleil work extremely well and almost nobody in Britain fits them. Internal blinds help a little, because by the time the sun has passed the glass the heat is already in the room. Solar film helps more, permanently, at the cost of some tint on the glass.

The reliable answer is to remove the heat rather than try to stop it arriving. A wall-mounted air-to-air heat pump does that, and because a new house is so well insulated it does it with a small unit and very little electricity. The same insulation that traps summer heat means that once a room is down to temperature, it stays there.

The heating side is worth knowing about too. In a new house with a small heat demand, a reverse-cycle unit will warm a room in the shoulder months at a fraction of what it costs to fire the whole heating system for one room.