Why new European homes overheat, and the rules meant to stop it

Updated September 9, 2026 · Claude AC · 2 sources cited

There is a conversation that repeats every July in every European city with cranes on the skyline. A household moves into a flat completed in the last few years — triple glazing, a certificate covered in the best letters, heating bills a fraction of the old place — and discovers that by four in the afternoon the living room is at 30 °C and will not come down. The old, draughty, badly rated apartment they left behind was, in August, more bearable. Nobody warned them, and their instinct is that something must be broken.

Nothing is broken. A modern European dwelling is a winter machine built to an energy standard written almost entirely around heating, and the qualities that make it excellent in February — a thick insulated envelope, near-airtight construction, generous glazing to harvest free solar gain — are precisely the qualities that make it a heat trap once the sun turns. Regulators worked this out roughly a decade ago, and the past few years have produced a genuinely new layer of building rules across Europe: summer comfort is now a pass-or-fail item on a building permit in several member states. Almost no buyer knows those rules exist, or that they can ask to see the results.

What makes a new home overheat

The causes stack, and they are structural rather than accidental.

• An envelope designed to keep heat in. Insulation is direction-blind. Once the internal gains of a summer day are inside — sun, bodies, cooking, appliances — the same fabric that holds warmth through a January night holds it through an August one.
• Glazing sized for daylight and for winter gain. An unshaded south or west-facing window admits several hundred watts of solar energy per square metre at peak. Four square metres of west glass in a small living room is a substantial electric heater running through the hottest hours of the day.
• Airtightness without a summer plan. Old buildings leaked, which flushed heat at night for free. New ones do not, so night cooling has to be designed and operated deliberately.
• Windows that cannot really be opened. Single-aspect flats have no cross-ventilation path at all. Add acoustic constraints on a busy street, fall-protection restrictors above ground level, security concerns and pollution, and the ventilation the designer assumed becomes theoretical.
• Mechanical ventilation that ventilates but does not cool. A heat-recovery system is not air conditioning; if its summer bypass is missing, wrongly commissioned or left disabled, it can hand the day’s heat straight back into the supply air.
• Communal heat. Hot water risers and heat-network pipework running through insulated shafts and corridors are a documented cause of chronic overheating in apartment blocks, and it affects circulation spaces and hallway-adjacent rooms year-round.

The rules, country by country

There is no single European overheating standard. What exists is a fast-growing set of national instruments, each measuring something slightly different, and all of them producing a document that sits in the building file for your home.

Two features of this landscape matter more than the numbers. First, every one of these regimes tells the designer to solve summer passively before reaching for a machine: English guidance is explicit that mechanical cooling should only be relied on where excess heat cannot reasonably be removed without it, and the French penalty band makes a cooled design measurably more expensive in the energy calculation. Second — and this is the part that surprises people — the Dutch overheating check applies to dwellings without active cooling. Specify cooling and the overheating test is answered by the equipment rather than by the building. That is an entirely rational rule which nonetheless creates a quiet incentive to sell homes with a split unit and a thin summer design behind it.

What to ask before you sign

All of this generates paperwork you are entitled to ask about, and a developer’s sales office is rarely expecting the question. Four things are worth having in writing.

• The overheating assessment itself — the Part O compliance route used, the DH value in °C.h for your specific dwelling, or the TOjuli figure. Ask for the number, not the reassurance. A DH of 340 and a DH of 1 200 are both compliant in France and describe entirely different summers.
• What shading was assumed. Overheating models routinely assume external shutters, brise-soleil, solar-control glass or deep reveals. External shading is also the first item value-engineered out of a project between planning and completion. If the model assumed it, it must exist on the façade you are buying.
• Whether the assumed window opening is available to you. If the calculation relies on openings at night, check the restrictors, the acoustic situation and whether the top-floor flat with a single aspect can actually create a cross-draught.
• The ventilation strategy in summer, including whether the heat-recovery unit has a functioning summer bypass and who commissioned it.

Orientation and level do the rest of the work. A single-aspect west-facing flat on the top floor of a lightweight building is the hardest case in European housing, for the reasons set out in our guide to cooling a room under the roof. National context matters too: the same design behaves very differently in France and in the Netherlands, and the trajectory of the summers those buildings will actually meet is set out in our record of Europe’s heatwaves.

If the home already overheats

The order of intervention is not negotiable, because each step changes the size of the next one. Stop the sun at the glass first: external blinds, shutters or awnings intercept solar energy before it reaches the room, while internal blinds absorb it once it is already inside and achieve a fraction as much. Then make night ventilation real — secure openings, a cross-draught path, and the discipline to close everything at breakfast. Check the ventilation unit’s summer bypass. Add air movement, which buys perceived comfort for tens of watts rather than hundreds. The full sequence, with what each step is worth, is in our guide to cooling a European home without air conditioning.

Only then does a machine become the right answer, and it will be a smaller machine than it would have been first. A well-shaded new dwelling has a genuinely low cooling load — the fabric works in both directions once the gains are controlled — so sizing should be calculated rather than assumed, using the method in our guide to sizing an air conditioner in kW. Fitting fixed cooling also changes the energy assessment of the dwelling, which matters at resale and is covered in our guide to air conditioning and the energy performance certificate.

Frequently asked questions

Is a new home really more likely to overheat than an old one?

In summer, frequently yes, and studies of overheating in recently built dwellings across northern Europe report exactly that pattern. Heavy masonry buildings have thermal mass that absorbs the day’s heat and gives it back at night, and older buildings leak air, which flushes it. A lightweight, airtight, highly glazed new dwelling has neither buffer, so it responds fast in both directions. It will be far cheaper to heat and, without shading and night ventilation, hotter to live in during a heat episode.

Do these rules ban air conditioning in a new home?

No. They make it a designed choice rather than a default. English guidance requires that excess heat be removed by passive means where that is reasonably possible, with mechanical cooling as the remaining step; the French DH framework allows a cooled dwelling but charges the energy calculation for it; the Dutch check simply steps aside where active cooling is installed. Nothing prevents an owner from installing a unit afterwards, subject to the usual consent and noise rules.

My new flat overheats but the developer says it complies. What can I do?

Compliance and comfort are different tests, and a dwelling can pass a calculation and still be unpleasant. The useful move is to ask for the overheating assessment and check it against reality: whether the external shading in the model was built, whether the assumed window openings exist and can be used, and whether the ventilation bypass works. A gap between the modelled design and the delivered building is a defect conversation with the developer; a compliant design that is merely close to the limit is not, and the practical route is shading and night ventilation.

Which rooms overheat first?

Top-floor rooms, rooms with west-facing glazing, single-aspect rooms with no cross-ventilation path, rooms adjacent to heat-network risers, and small bedrooms whose windows must stay shut for noise. Bedrooms are the ones that matter most, because night-time temperature drives sleep loss rather than daytime peaks.

Does thermal mass still help in a modern building?

It helps if it is exposed and if there is a way to discharge it at night. A concrete soffit left visible, coupled with secure night ventilation, flattens the daily peak usefully. The same mass behind a plasterboard ceiling and a closed window does very little, which is why mass and night ventilation are one strategy rather than two.

Read next

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• The room under the roof: cooling a loft conversion in Europe
• Air conditioning maintenance in Europe: rules, rhythm and real jobs
• Air conditioning and the energy performance certificate of your home in Europe
• Cooling a shop, café or small office in Europe

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