The room under the roof: cooling a loft conversion in Europe
Contents (6)
The converted loft is Europe’s favourite extra room and its worst summer room. From the terraced streets of Manchester and Amsterdam to the zinc roofs of Paris and Brussels, the top floor runs several degrees above the rest of the house on a July evening — often still climbing after sunset. Owners buy an air conditioner, find the usual sizing rules leave them short, and conclude the machine is faulty. It is not: the roof is a heat source the rest of the house does not have.
Why the top room behaves differently
- The roof surface gets extremely hot. Dark tiles, slate or zinc in full sun routinely exceed 60 °C, far above anything a wall reaches. That temperature difference drives heat through the roof build-up all afternoon.
- The buffer has been removed. An unconverted attic is a thermal air lock between the roof and the living space. Converting it turns that buffer into the room itself, so what used to be absorbed by an empty void now arrives at head height.
- The insulation is thinner than it looks. Modern national requirements for a converted roof commonly sit around 0.15–0.20 W/m²K, which needs the equivalent of 200–300 mm of mineral wool — more than the 100–150 mm a rafter depth allows once the ventilation gap above is preserved. Insulating between rafters alone therefore falls structurally short.
- Rooflights face the sky. A sloping or near-horizontal window intercepts far more solar radiation at midday than a vertical one, and a square metre of unshaded glazing in sun admits several hundred watts — a small electric heater per window, left on.
- The house sends its own heat upstairs. Warm air rises through stairwells all day, so the loft receives the ground floor’s gains on top of its own.
The order of works, cheapest first
Cooling a loft with a compressor before dealing with the fabric means paying for the same heat every summer for fifteen years. The sequence that pays back, in order:
- Shade the rooflights from outside. External awning blinds or shutters intercept the sun before it crosses the glass and re-radiate the energy to open air. An internal blind stops the light but not the heat — the radiation has already entered the room and the blind simply becomes a warm radiating panel. For a loft with two unshaded rooflights this single measure often changes the room’s character more than any machine.
- Check the insulation and the ventilation gap. Over-rafter insulation boards, fitted the next time the roof is stripped, are the definitive fix; between-and-under-rafter insulation is the retrofit compromise. Either way the ventilated gap above the insulation must survive the works, because a blocked gap traps both heat and moisture.
- Flush the room at night. A loft has an advantage here: opening a rooflight at the top and a window low in the house creates a genuine stack effect, and the taller the height difference the stronger the draught. Two or three hours of night flushing removes heat stored in plasterboard and screed that would otherwise start the next day already warm.
- Then cool mechanically, sized for what remains. The full passive sequence for a European home is in our guide to cooling without air conditioning.
Sizing a room under the roof
The comfortable European default of 100 W/m² does not apply here. A converted loft belongs in the hard-case band of our guide to sizing in kW — 130 W/m² — and unshaded rooflights should then be counted separately, because glazing that faces the sky is a load in its own right rather than a modifier of floor area.
| Loft room | Insulated roof, shaded glazing (100 W/m²) | Typical conversion (130 W/m²) | Plus unshaded rooflights |
|---|---|---|---|
| 12 m² | 1.2 kW | 1.6 kW | add ~0.3–0.5 kW per m² of sunlit glass |
| 16 m² | 1.6 kW | 2.1 kW | same allowance |
| 20 m² | 2.0 kW | 2.6 kW | same allowance |
| 30 m² | 3.0 kW | 3.9 kW | same allowance |
These are planning figures for a conversation with an installer, not a substitute for a room-by-room calculation. The useful discipline is that shading the glass usually saves a whole capacity step: a 20 m² loft with two shaded rooflights sits comfortably on a 2.5 kW split, while the same room with the sun landing on bare glass pushes towards 3.5 kW, a bigger outdoor unit and a heavier bill every summer thereafter.
Which machine fits under a slope
The geometry of a loft rules out some equipment and favours others:
- High wall split — needs a flat section of wall, with clearance above and free discharge in front. Mounted on a gable wall it works well; squeezed under a slope with its outlet a few centimetres from the plasterboard, it recirculates its own air and disappoints.
- Floor console — sits at the knee wall where the ceiling is lowest, which is dead space anyway. It also heats better in winter, blowing warm air along the floor rather than into the highest, warmest part of the room.
- Slimline ducted — hidden in the flat central ceiling or an eaves cupboard, with short ducts to one or two grilles. The tidiest answer in a loft with a dormer, and the only one that discreetly serves two rooms.
Each format’s strengths are compared in our guide to wall, console, cassette or ducted units, and the European equipment families themselves are set out on our split air conditioner pages.
Getting pipes, condensate and power up there
A top-floor room is the hardest place to serve. Three constraints decide the design:
- Pipe run and height difference. Domestic split datasheets typically permit a total refrigerant line of the order of 15–25 m with a vertical separation of 8–15 m; a three-storey house with the outdoor unit at ground level is close to those limits, so the model has to be chosen for its piping envelope, not only its capacity. Beyond the factory pre-charge — commonly a few metres — the installer must add refrigerant by weight and record it.
- Condensate cannot always fall. Where gravity drainage is impossible, a small condensate pump is fitted; specify a quiet one and place it away from a bed or desk.
- Power and permission. A dedicated circuit sized by the installer is the norm, as explained in our guide to the electrical side of an air conditioner. Fixing an outdoor unit high on a street-facing wall, on a roof or in a conservation area is where planning rules bite — the national picture is in our guide to permission to install air conditioning.
What it costs to run up there
Running hours, not tariffs, are what make a loft expensive. The same 2.5 kW machine that idles gently in a first-floor bedroom can spend a July afternoon near full output under an under-insulated roof, and the difference over a season is easily a factor of two in kilowatt-hours. That is the honest argument for spending on shading and insulation first: they cut the hours, and the hours are the bill. The method for estimating your own is in our guide to air conditioning running costs in Europe.
Frequently asked questions
Why is my loft hotter at eight in the evening than at two in the afternoon?
Because of thermal lag. Heat entering the roof at midday takes hours to travel through tiles, battens, insulation and plasterboard, so the peak indoor temperature arrives long after the peak outdoor one. Heavier, better-insulated roof build-ups delay and flatten it; thin ones let it through faster and higher. It is also why night flushing matters: the room has to be emptied of stored heat before the next cycle begins.
Do internal blinds on rooflights help at all?
A little, and much less than people expect. Once the sun has passed the glass, the energy is inside the room; the blind absorbs it and re-emits it as heat. Reflective internal blinds designed for the purpose do better than a fabric curtain, but external awnings and shutters remain in a different league because they stop the radiation outdoors.
Is it cheaper to insulate the roof or to fit an air conditioner?
They answer different questions. Insulation and shading reduce the load permanently and improve winter comfort as well; a machine removes whatever load is left, quickly and on demand. In a loft that overheats badly the arithmetic almost always favours doing the fabric work first, then buying a smaller machine — the smaller unit costs less to install and less to run for its whole life.
Can one unit cool a loft with two rooms?
Only with ductwork or a second indoor unit. Air will not turn corners through doorways in useful quantities, so a single wall split leaves the far room warm while short-cycling in the near one. A slimline ducted unit with two grilles, or a multi-split with a small head in each room, is the arrangement that works — see our guide to multi-split systems.
Will a portable air conditioner cope in a loft?
It will take the edge off a small loft room for an hour or two, no more. The hose has to leave through a rooflight or dormer window, which is awkward to seal and often points at the sky; single-hose machines also expel indoor air, drawing warm air up the stairwell to replace it. In the hottest room of the house those handicaps are magnified — the detail is in our guide to portable versus split, and the sleeping-comfort side in our guide to air conditioning in a bedroom.