Sunlight through glass does most of the damage
Once sunlight has passed through a pane of glass, the heat is already inside the building. Getting it back out afterwards is much harder than stopping it in the first place.
This is why the two British standards that deal with overheating in homes, CIBSE TM59 and Approved Document O of the Building Regulations, both treat sunlight through windows as the main cause rather than a secondary one.
Upstairs windows take more of it than downstairs windows do. They are less shaded by fences, hedges and neighbouring buildings, so they collect direct sun for more of the day. West facing bedrooms get it in the late afternoon, when the fabric of the house is already warm from the morning.
The roof, and why the house will not cool down at night
A dark roof in direct sun runs well above the air temperature, and it holds that heat long after the sun has gone. The warmth conducts downwards into the loft, which becomes a reservoir of hot air sitting directly on top of your bedroom ceilings.
Loft insulation slows this down. It does not stop it. Insulation is a resistance rather than a barrier, which means it reduces the rate at which heat crosses the ceiling without preventing the crossing. A loft sitting at 45 degrees for eight hours still delivers a meaningful amount of heat downwards overnight.
There is also the matter of what bypasses the insulation altogether. Recessed spotlights, loft hatches and gaps at the eaves all provide a direct route. This is why a bedroom with a dozen downlighters in the ceiling often runs hotter than the one next door with a single pendant light.
Nowhere for the heat to go
A house built to hold heat through a British winter will struggle to shed it in a British summer: small opening windows, no route for air to cross from one side of the building to the other, no vents at the ridge of the roof.
One open window ventilates almost nothing, because air needs somewhere to leave as well as somewhere to enter. Two windows on opposite sides of the house move a great deal more air than one window does, and the difference is larger than most people expect.
Want this answered for your own address?
We read all the data available for your address and write up what it means, in plain English. From £9.50, and our team will get it to you within 5 working days. We take no money from anyone we recommend.
Why is my house so hot upstairs but cold downstairs?
The two complaints sound contradictory. They are the same mechanism, seen from different floors.
Where the ground floor is solid, it sits in contact with earth that stays at a fairly steady temperature all year and acts as a heat sink. The ground floor is also more shaded, and it usually contains more heavy material in its floors and internal walls, which is slow to warm up. In older houses with suspended timber floors over a ventilated void, that logic works differently, because the void underneath is close to outdoor temperature.
Upstairs has a hot roof above it, more window area for each square metre of floor, and lighter construction that warms up quickly.
On a hot day the two floors therefore drift apart by several degrees. In houses with a converted loft, or a bedroom underneath a flat roof, the gap is considerably wider.
Why is my house so hot at night?
Because the building is releasing the heat it took on during the day, and this becomes obvious once the air outside has cooled and the air inside has not.
This lag is why opening the windows at six in the evening often does not help. If the air outside is still warmer than the air inside, you are importing heat rather than losing it. The useful window tends to be late, from around ten at night through to early morning, and it only works properly if the air can move through the house rather than sit still in one room.
Humidity is the other half of the problem. Air holding a great deal of moisture suppresses the evaporation from your skin that would otherwise cool you down. A house that feels stuffy rather than simply hot often has a moisture problem as much as a temperature one.
Why do British homes get so hot?
Because they were built for a climate that no longer reliably turns up.
The housing stock in this country is among the oldest in Europe and it was designed around one problem, which was keeping warmth in through a long, damp winter. Hence small windows, and minimal shading, since shading costs you sunlight in February when you want every hour of it, and latterly a great deal of effort spent sealing out draughts.
None of it was a mistake. It was a sensible answer to the climate we then had.
The climate has moved. The Met Office recorded 2026 as the first year in the UK weather record to reach 35 degrees or higher in four consecutive months: May, June, July and August.
Meanwhile the standard home improvement advice of the last twenty years, which is more insulation and better airtightness, improves winter performance and can make summer overheating worse if it is done without any thought for ventilation or shading. Energy efficiency marketing tends to skip past that. Insulation is not a thermostat: it slows heat transfer in whichever direction the heat happens to be travelling.
What actually works, in order of cost
| Fix | Rough cost | What it does | Worth it when |
|---|---|---|---|
| Cross ventilation at night | Free | Flushes accumulated heat once the outside air is cooler | Always. Do this before spending anything at all. |
| External shading: blinds, awnings, shutters | Low to moderate | Stops sunlight before it reaches the glass | Blocking heat outside the window beats absorbing it inside. Substantially more effective than internal blinds. |
| Solar control film or glazing | Moderate | Cuts the heat coming through south and west facing glass | Bedrooms with large or west facing windows |
| Loft ventilation: soffit, ridge or gable vents | Low to moderate | Lets the loft vent instead of radiating downwards | The loft is noticeably hotter than outside on a summer evening |
| Sealing downlighters and loft hatches | Low | Closes the routes that bypass the insulation | Recessed spotlights in upstairs ceilings |
| Reflective or thicker loft insulation | Moderate | Reduces the rate of transfer in both directions | Insulation is thin or unevenly laid. Check what is up there before buying anything. |
| Mechanical ventilation with heat recovery | High | Does not cool the house. It manages air quality and moisture in an airtight home. Without a summer bypass it recovers heat into the incoming air and makes matters worse. | Part of a wider refurbishment, for air quality reasons, not as a remedy for overheating |
| Air conditioning | High to run | Actively removes heat | The cheaper options have been tried, or there is a medical reason |
Air conditioning sits at the bottom of that list deliberately. It works, and it is the only entry that removes heat rather than managing it. But it is the most expensive to run, and fixed external units can require planning permission depending on where they are sited and whether the property is listed or in a conservation area.
It is a reasonable answer once the cheap options have been exhausted, and an expensive one if the real problem is a loft with no vents in it.
Is it unhealthy to have a hot house?
Sustained indoor heat does carry real risk, and that risk is higher for older people, for very young children, for pregnant women, and for anybody with a heart or respiratory condition. Sleep quality falls off well before the temperature becomes dangerous.
If you are worried about a particular person, that is a question for a GP or for the NHS heat health guidance rather than for us. On the building itself, a bedroom that stays hotter than the rest of the house all night is usually fixable, and the cheap remedies deserve exhausting before the expensive ones.
Where most people go wrong
Not with the question, but with the order in which they answer it.
The common sequence is to buy a portable air conditioning unit in July, discover the running cost in August, and never address the reason the room became hot in the first place. The structural fixes sit unexamined at the bottom of the list, largely because nobody makes money by selling them and nobody telephones you about loft vents.
Which of them applies to your house depends on the orientation, the glazing, what shades the building through the day, how the loft is built and ventilated, and what the summer actually does at that address. Two houses on the same street can differ enormously. All of it is a matter of public record, and almost nobody has ever put it in front of the person who lives there.
Our Heat & energy module looks at sunlight, orientation, overheating risk and local temperature records for a specific address in England or Wales, and sets out which factors are driving it there. We do not sell, fit or recommend any product or installer, and we take no commission from anybody. It is not a survey, and we do not visit the property.