Air conditioning for a loft conversion

The loft conversion sits directly under the largest solar surface in the house, was often built with only 100 mm of PIR between the rafters, and has small dormer windows that were sized to a planning constraint rather than to summer ventilation. In a mild British summer the room is comfortable; in the increasingly common warm week, the same room runs six or seven degrees above the ground floor by mid-evening. This is the sharpest edge of the wider UK cooling picture; a loft bedroom is the room where a fixed split most often earns its place.

Retrofit measures reduce that gap. External roller blinds on the dormer, thickened insulation at the ridge, and open windows on both gables for a night purge together typically pull the peak temperature back by three to four degrees. Where that is enough the fabric route is finished; where it is not, a fixed split completes the job.

A typical 2.5 to 3.5 kW single-split installation in a converted loft lands between roughly £1,800 and £3,200 in 2026, all-in, including the indoor cassette, the outdoor condenser, a short refrigerant pipe run, condensate drainage and F-Gas commissioning. The variance is driven mainly by pipe-run length, the ease of routing the condensate and whether scaffold access is required for the condenser.

Multi-split systems that serve two or three rooms from a single outdoor unit typically start around £4,500 for two indoor heads and rise from there. They tend to make sense when the loft conversion is one of several rooms to be treated, and rarely for the loft alone.

Size to the peak cooling load, not to the peak heating load. A well-insulated loft conversion typically needs 2.5 to 3.5 kW of cooling capacity; oversizing gives short cycles, poor humidity control and shorter equipment life. A design that quotes the smaller unit is usually the better design.

Choose an inverter-driven unit with a modern refrigerant. R32 is the current UK default and passes F-Gas comfortably; the older R410A units should be avoided on both regulatory and running-cost grounds. Confirm the installer holds a current F-Gas company certificate before the quote is signed.

Design the condenser placement before quoting. A rear elevation with a bracket at ridge height is the neatest solution for many houses; a ground-level position on the side elevation is quieter for the neighbour and easier to service. The choice is architectural before it is mechanical.

A well-sized single-split loft unit runs at 300 to 600 W for most of a warm afternoon and cycles down after dusk. Over a typical British summer that translates to roughly £30 to £80 of extra electricity across the whole cooling season, and the same unit provides useful shoulder-season heating that offsets part of the winter heating bill.

The bigger cost story is what the loft install does not solve. If the underlying reason the loft is uncomfortable is a mediocre insulation package, the split will run harder every summer than it needs to. Doing the shading and ridge insulation alongside the split install typically halves the running cost across the unit's life.