Loft conversion too hot in summer? Why, and how to fix it

A loft conversion is often the nicest room in the house for nine months of the year, and the hardest to sleep in for the other three. The reasons are built into how lofts are converted, which means the fixes are fairly predictable.

Why loft rooms overheat

• The roof is right above you. On a sunny day roof coverings get far hotter than the air, and that heat conducts inwards.
• Roof windows face the sky, so they collect sun for more of the day than an upright window.
• Lightweight timber and plasterboard have little thermal mass, so the room heats up quickly.
• Warm air from the rest of the house rises and collects at the top.
• Older conversions often have less insulation than modern rules would require.

Roof windows are the biggest single source

An upright window on a south wall catches the sun for part of the day, and less of it at midday when the sun is high. A roof window tilted towards the sky catches it for far longer, including the strong midday sun, and there is nothing overhead to shade it. That is why a loft room with two or three roof windows can heat up faster than any other room in the house, even when the roof itself is well insulated.

The same windows are also the best way out for hot air, because they sit at the highest point of the house. Used well, they turn from the problem into the solution: shaded by day, open at night.

Fixes, from quickest to biggest job

1. 1

   Ventilate the whole house at night

   Open a low window downstairs and the roof window at the top to pull air up through the house.
2. 2

   Blinds on the roof windows

   Reflective or blackout blinds made for your window model, closed before the sun reaches them.
3. 3

   External awning blinds

   Fitted on the outside of roof windows, they stop the heat before it reaches the glass and make the largest single difference.
4. 4

   Check the insulation

   More insulation between and under the rafters, with a ventilated gap above, cuts the heat coming through the roof.
5. 5

   Add cooling

   A small wall unit sized for the room. Loft rooms usually need a size up from a room of the same area downstairs.

Why building rules did not prevent it

Part O of the Building Regulations, in force since 15 June 2022, requires new homes in England to be designed to limit overheating. It applies to new residential buildings, not to a loft converted within an existing house, so there is no overheating test for a loft room. Conversions are checked for things like fire safety, structure, stairs and insulation, and more insulation keeps heat out in summer as well as in during winter, but nothing tests how hot the room gets in July.

Cooling a loft room: sizing and planning

A loft room usually needs a bigger unit than a room of the same floor area lower down, because of the roof and the roof windows. On a house, an outdoor unit for an air-to-air heat pump can often be permitted development within the size, siting and noise limits, while a cooling-only air conditioner needs planning permission in England. Both are indicative; confirm with your local planning authority before installing.