A building standard developed in Germany around thirty five years ago. The idea is to hold the inside temperature steady by insulating heavily, removing thermal bridges, sealing the building against air leakage, fitting high performance glazing, and then ventilating mechanically with heat recovery so the air stays fresh. It is a performance standard rather than a product, and it applies in both directions: the same fabric that keeps winter heat in keeps summer heat out.

Does a Passivhaus need air conditioning?

Usually not, but it is not a cooling machine. The fabric holds whatever temperature it is given, so it relies on the outside air falling far enough overnight to be flushed through. In the case reported by the BBC the house tracked a night that stayed at 27 degrees and stayed at 27 degrees itself. On a run of nights like that the owner said she might use air conditioning briefly and then let the fabric hold the result.

Is retrofitting to this standard worth it financially?

On bills alone, no, not at these numbers. Around £80,000 of work returning a saving the owner estimates at nearly £750 a year does not pay back inside a normal ownership. The case for it is comfort, resilience in heat, and the fact that the work has to happen eventually on a house that will still be standing in 2050. Treat the bill saving as a rebate on that, not as the reason.

What should I do first if I cannot spend anything like that?

Shade the glass from the outside, and stop the draughts. Those are the two cheapest measures per degree in almost every British house, and neither requires the walls to come off. The incoming President Elect of RIBA put it to BBC Radio 4 that doing even half of these measures makes a significant difference to bills.

Is it cheaper to build to this standard than to retrofit to it?

Much. The Passivhaus Trust puts the extra construction cost at 2 per cent for flats rising to 5 per cent for a semi detached house, against tens of thousands to reach a comparable result on a house that is already standing. That gap is the whole argument for tightening new build standards, and it is why the Future Homes Standard coming into force in England on 24 March 2027 matters more than it sounds.

What it costs to make an old house cope with heat

On the hottest day of the fourth heatwave of this summer, a 1960s ex-council house in south London sat ten degrees below the outside air on a 35 degree day. It has no air conditioning. It also has around £80,000 of building work in it.

Both halves of that are the story, and most coverage keeps one and drops the other. The BBC visited the house in August and reported both, including the itemised bill, which is rare enough to be worth building an article around.

The house was built for a climate that has gone

This site stores the daily record back to 1950 for twenty five British towns. A house finished in the 1960s was finished into a country that had almost no hot weather in it.

Town	Days above 25C, 1960s	Days above 25C, 2020s
Cambridge	3.2	20.0
London	3.3	17.7
Reading	2.8	16.7
Oxford	1.9	14.7
Bristol	1.6	12.5
Birmingham	1.4	10.8
Sheffield	1.0	9.0
Manchester	0.6	7.0
Cardiff	0.5	7.7
Newcastle upon Tyne	0.1	3.2

Across all twenty five towns the average has gone from 1.1 days a year above 25 degrees in the 1960s to 8.4 this decade. Newcastle has more hot days now than London had then. The full series, by town and by decade, is at /towns.

That is the honest frame for a retrofit bill. Nobody in 1965 specified this house wrong. They specified it for a climate that has since been replaced, and the replacement arrived faster than the mortgage ran.

What the £80,000 bought

The owner, Marion Baeli, is an architect and a non executive director of the Passivhaus Trust, and she gave the BBC the costs. She did the work in stages over ten years because she could not pay for it in one go, which is how most of these projects actually happen.

Measure	Cost	What it is doing
Triple glazed windows	£30,000	Slowing heat moving through the weakest part of the wall, in both directions
Making the house airtight	£5,000	Stopping uncontrolled air movement, which is what carries heat in and out when nothing else does
Insulation to shared walls, floor and roof; removing thermal bridges; some cavity brick walls rebuilt as thinner super insulated timber frame; heat recovery ventilation	Not itemised	The bulk of the work, and the disruptive half, done first
Total	About £80,000, before architectural fees	Spread across ten years

Baeli estimates the energy demand of the house has fallen by 88 per cent, worth nearly £750 a year on her bills. In winter one radiator now heats four bedrooms over three floors.

The cheapest line in that table is the one that matters most

Airtightness cost £5,000. The windows cost six times that. Airtightness is also the measure that makes the rest of them work, because insulation only insulates still air, and a draught moves heat past it without asking.

This is the general shape of fabric work and it is worth internalising before you spend anything. The expensive, visible measures have the worst ratio of degrees to pounds. The cheap, invisible ones have the best. A house is not made comfortable by its windows, it is made comfortable by not leaking.

Baeli’s own analogy is a flask. You would not put holes in a flask and then argue about the thickness of the steel.

What it does not do

On one June evening this summer the outside minimum stayed at 27 degrees. The house stayed at 27 degrees too, the warmest reading she has ever taken in it.

That is not a failure of the work, it is what the work is. A well built fabric holds the temperature it is given. It does not manufacture cold. It needs a cool night to draw in and flush through, and on a genuinely tropical night there is no cool night to draw on. Baeli’s answer is that she might run air conditioning briefly on nights like that and then let the fabric hold the result, which is the correct use of a machine rather than an argument against the fabric.

This is the same mechanism that leaves an ordinary British bedroom hot at midnight, running in a house that is much better at it.

If you have £2,000 rather than £80,000

Most people do. Jay Morton, incoming President Elect of the Royal Institute of British Architects, told BBC Radio 4 that you will not insulate every wall, but that doing even the best half of these measures is significant for bills.

Baeli’s own advice for anyone not doing a full retrofit is a single measure: shade the windows. She hangs dust sheets over hers. It does not need to be sophisticated, and it is the highest return per pound available in a British house, which is why this site keeps saying to shade the glass from the outside.

After that, in rough order of pounds per degree: draught proofing, loft insulation, then everything expensive.

New build is where this is cheap, and old stock is where the problem is

The Passivhaus Trust puts the extra cost of building to the standard from the start at 2 per cent for flats, rising to 5 per cent for a semi detached house. Against a five figure retrofit, that is close to free.

England is moving that way. The Future Homes Standard comes into force on 24 March 2027 and pulls new build closer to this territory, and overheating in new homes is already covered by Part O. Around 1 per cent of new homes are currently built to the Passivhaus standard, and the Trust expects that to reach 10 per cent within a decade.

None of which touches the actual problem. Around 80 per cent of the housing stock that will exist in 2050 is already standing, and in England 20 per cent of owner occupied homes were built before 1919, against a total stock of 25.6 million dwellings, according to the English Housing Survey. Regulation for new homes fixes the small end of the problem. The large end gets fixed one house at a time, by the people who own them.

Before you spend anything, get the number

The reason to work out the running cost first is that it changes what the work is worth. A house facing a handful of hot spells a year does not justify the same spending as one facing a month of them, and the running cost calculator will give you the figure in pounds rather than in feelings.

Then spend in the order the physics suggests, not the order the showroom does: shading, draughts, insulation, ventilation, and glazing last.

The figures in this piece were computed by this site from daily weather readings it stores and processes. Underlying readings from the Open-Meteo historical archive, licensed CC BY 4.0.