The short answer
Loft rooms and top floors run hotter than the rest of the house because the roof heats up in the sun, warm air rises, and roof windows catch sunshine for much of the day. Shading, ventilation, insulation and well-planned cooling help.
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A loft conversion can be a pleasant room for most of the year and the hardest one to keep cool in hot weather. Top-floor flats and bedrooms under an unconverted loft have the same problem, often to a lesser degree. Several of the causes can be tackled, and the shape of the room affects what kind of cooling will fit.
Why the top of the house gets hotter
- The roof. Tiles and slates get very hot in strong sunshine, and the heat works its way through into the rooms below. In a loft room with sloping ceilings, only the roof structure and its insulation stand between you and that heat.
- Lightweight construction. Loft rooms are usually built from timber and plasterboard, which warm up quickly.
- Rising warm air. Heat from the rest of the house drifts up the stairs and collects at the top.
- Insulation. Older conversions may have less insulation in the roof than newer ones, and the depth of the rafters can limit how much was fitted.
In a bedroom below an unconverted loft, the loft space itself gets very hot on a sunny day. A good depth of insulation across the loft floor slows that heat getting into the rooms below, and keeps heat in during winter too.
Roof windows and dormers
A window set into a sloping roof faces the sky rather than the horizon. In summer, when the sun is high, more sunshine falls on it, for more of the day, than on an upright window of the same size, so a lot of heat comes in with the light. A dormer window is upright, and its small roof shades it a little when the sun is high.
Roof windows have one advantage: they're at the highest point of the room, where the warmest air collects. Opening them overnight, with a window open lower down in the house, draws cooler air up through the house and lets warm air out at the top. Many have a vent that lets a little air through while they're closed.
Keeping heat out
- Fit blinds to the outside of roof windows where you can. Blinds inside help, but by then the heat is already in the room.
- Close blinds before the sun reaches the glass, not once the room is hot.
- Ventilate overnight and early in the morning, and keep windows closed while it's warmer outside than in.
- Check the loft insulation above top-floor rooms, and make sure the loft hatch is insulated and closes well.
Adding insulation to the sloping ceilings of an existing loft room is a building job, and a builder or surveyor can tell you what's practical.
Cooling a loft room
Several kinds of system can cool the top of the house:
| System | Suits | Watch out for |
|---|---|---|
| Wall-mounted split | Rooms with an upright wall and space above the unit | Low knee walls and sloping ceilings rule some walls out |
| Floor-standing or console unit | Rooms where only a low knee wall is free | Takes up some floor or low wall space |
| Multi-split | Cooling a loft room and other rooms together | Pipes must reach every room from one outdoor unit |
| Ducted system | Rooms below an unconverted loft | A bigger installation |
| Portable unit | Short-term use | Usually louder; window kits may not suit roof windows |
Where to put the indoor unit explains what a wall needs, and floor-standing and console units covers the low-level option.
For a fixed system, a few things need planning:
- The pipe route. The refrigerant pipes often run down an outside wall to an outdoor unit on the ground or on a wall bracket. Every model has limits on pipe length and on the height between the two units, and longer runs may need refrigerant added on site, so the route is planned at the survey.
- Where the outdoor unit can't go. In England, under permitted development, the outdoor unit can't go on a pitched roof, must be at least 1 metre from the edge of a flat roof, and can't go on a wall facing a road above the ground floor. Conservation areas have stricter rules, listed buildings need listed building consent, and Scotland, Wales and Northern Ireland have their own rules. The planning permission guide has the detail.
- Drainage. Water from the indoor unit drains by gravity where the pipe can fall all the way, which is often easier from the top of the house. Where it can't, a small condensate pump is fitted.
- Size. Heat through the roof and roof windows means a loft room often needs more cooling capacity than a downstairs room with the same floor area. A proper assessment takes this into account.
Top-floor flats
A top-floor flat takes heat through the roof above, and can be hard to ventilate if all its windows are on one side. The same shading and night ventilation help.
Flats follow different planning rules from houses, so check with the local council before fitting air conditioning. If you own a leasehold flat, check your lease too, as work to the outside walls often needs the freeholder's consent. Flats and leasehold homes explains what to look for.
Related guides
- Why some rooms overheatA room overheats when heat gets in or builds up faster than it can escape. The usual causes are sunshine through large or west-facing windows, a roof directly above, heat from people and equipment, and too little ventilation.
- Sleeping in hot weatherBedrooms stay warm at night because the building gives back the heat it soaked up during the day. Keeping sun out by day, ventilating once it's cooler outside, and quiet cooling or a fan all make a warm night more comfortable.
- Where to put the indoor unitA wall-mounted indoor unit usually goes high on a wall with clear space around it, where its airflow can reach the whole room without blowing straight at people. The best spot also depends on the route for the pipes and where the condensate can drain.
- Flats and leasehold homesIn a flat or leasehold home, fitting air conditioning usually needs written consent from the freeholder under the lease, and the planning rules for flats differ from those for houses. Freeholder consent and planning are separate questions.