A converted loft is the room in a British house most likely to be unbearable in summer, and the reason is simple: it is directly under the roof, its insulation is usually at rafter level rather than at ceiling level, and warm air from the whole house rises into it. It is also the room where cooling is easiest to install during the conversion and hardest afterwards.
Why it overheats
Heat arrives through the roof all day and keeps arriving into the evening as the structure gives it back. Rooflights add direct solar gain, and stack effect brings warm air up the stairwell from the rest of the house. A loft room can be several degrees above the floor below at bedtime, which is exactly when it is being used.
Specify it during the conversion
While the rafters are open, the pipe route, the drain, the electrical circuit and the mounting position can all be planned and hidden with almost no extra work. Once the plasterboard, insulation and decoration are in, the same route means lifting a new floor or chasing a finished wall. This is the single biggest cost difference in loft cooling and it is a timing decision.
Where things go under a roof
Sloping ceilings limit high wall mounting, so the usual positions are the gable wall, the dwarf wall, or a low level console. The outdoor unit normally goes at ground level with a long pipe run up the elevation, or on a flat roof section if there is one, and both need agreeing early because the run is what makes the job easy or hard.
Fix the fabric where you can
Insulation at rafter level, ventilation above it, and shading or a solar control glazing specification on the rooflights all reduce the load. A cooling system is a good answer to a room that overheats and a poor substitute for a roof that was insulated badly, because it will spend its life fighting a fault rather than a summer.
Questions people ask about loft air conditioning
Can air conditioning be added to a finished loft?
Yes, and it costs more and disturbs more. The work is finding a pipe and drain route from the room to an outdoor position through a finished house, which usually means lifting a section of floor or boxing a run. It is a normal job for an installer and a bigger one than it would have been.
Will one unit cool the loft and the landing?
Not reliably. Conditioned air does not travel round corners or down stairs, and a stairwell is the route warm air uses to come up. Cool the room; consider a door at the head of the stairs to stop the room feeding the rest of the house.
Does the outdoor unit have to go at ground level?
Not necessarily, but the alternatives need care. A unit on a flat roof needs a stand that does not transmit vibration into the structure below, and any high level position needs access equipment for installation and for every future service.
Is planning permission needed?
The indoor part is inside the house. The outdoor unit on an elevation, particularly a front elevation or in a conservation area, is the part that may need checking, along with the roof structure if any new opening is being formed for it.