How do I cool a loft conversion that overheats?

People with loft conversions try everything from the general cooling guides and find it only half works. That is not a failure of the advice; it is a different construction. The heat is not arriving through a ceiling from a ventilated void, it is coming through a shallow insulated slope directly above the reader's head, and the room usually has rooflights facing the sky rather than windows facing a wall.

Safety

Step by step

1. Understand what is different about the construction.In a standard house the insulation sits at ceiling joist level and a ventilated cold loft separates you from the roof covering. In a conversion the insulation moves to the rafters, so the room is directly against the roof plane. The depth available between rafters is limited, so the insulation is often thinner than a modern loft floor would have, and there is no air buffer at all.
2. Check that the ventilation gap above the insulation is clear.A rafter-level insulated roof normally needs a continuous air gap between the insulation and the underside of the roof covering, ventilated at eaves and ridge, so moisture can escape. If that gap has been squashed or blocked, the roof runs hotter and you have a condensation and rot risk in winter. This is a job to look at rather than to improvise on — if it looks blocked, get a competent roofer or surveyor to assess it.
3. Shade the rooflights from outside if at all possible.Rooflights face upwards, so they receive strong radiation for far more of the day than a vertical window. External roller blinds or awnings made for the rooflight are the single most effective change, and manufacturers make them for the common brands. External beats internal by a wide margin here for the same reason it does everywhere else.
4. Use internal blackout blinds as the fallback.If external is not practical or affordable, a reflective-backed internal blind fitted tight to the frame is the next best thing. A blind with gaps around the edge lets a convection loop run behind it and loses much of the benefit.
5. Get a low-in, high-out draught working.A loft room often has a rooflight at the top of the slope and a window or door lower down, or a stairwell opening. Opening both creates a strong stack draught, because the height difference in a loft room is unusually large for its size. One opening on its own does very little.
6. Purge it hard as soon as the outside air is cooler.Loft rooms have little thermal mass, which cuts both ways — they heat fast and they also cool fast once you flush them. Opening everything the moment the outdoor temperature drops below the indoor one shifts a loft room faster than it shifts a masonry room downstairs.
7. Close it up early in the morning.Because it heats so fast, a loft room left open into a sunny morning is warm again by nine. Shut and shade it before the sun is on the roof, not after.
8. Consider whether more insulation is possible.Where headroom allows, adding an insulated board layer under the rafters improves both summer and winter performance. It reduces ceiling height, and it must not be achieved by filling the ventilation gap above. Worth pricing alongside external rooflight blinds rather than instead of them.
9. Plan for the worst nights rather than pretending.Some loft rooms cannot be made comfortable in a genuine heatwave. Deciding in advance that the sleeper moves downstairs on those nights is a better plan than three fans and a bad week.

Tips

• External rooflight blinds are sold by the rooflight manufacturers and fit the frame properly. Generic solutions fixed to the roof surface are hard to make secure and are a wind risk.
• A pale-coloured external blind reflects more than a dark one; a dark internal blind is close to the worst combination available.
• If the conversion has a dormer, the dormer's flat roof is another large sunlit surface directly above the room — worth checking its insulation depth separately.
• Fitting a fan high in the room and running it to exhaust through an open rooflight at night uses the height difference to your advantage.
• A loft room that also overheats badly in winter mornings with the heating off is telling you the insulation is thin rather than the shading being wrong.

Common mistakes

• Stuffing extra insulation into the rafter void to fix the heat — It blocks the ventilation gap that keeps the roof structure dry, which causes condensation and eventually rot in the timbers. The gap is not spare space.
• Relying on an internal blind alone on a large rooflight — The glass is already hot and the blind traps a layer of hot air against it. It helps with glare far more than with temperature.
• Leaving the rooflight open through a sunny morning for 'air' — Once the outdoor air is warmer than the room, an open rooflight is an inlet for heat as well as a skylight for direct sun.
• Treating a hot loft bedroom as purely a comfort problem — For an infant, an older person or someone with a heart or lung condition it is a health risk, and moving them downstairs is the correct response rather than a defeat.

Questions people ask