Every figure checked against its source on 7 November 2026
Cooling a loft conversion: the options that work
A converted loft is the hottest room in most Dartford houses: it sits under the roof, takes the sun from both sides and has no wall thick enough to slow the heat. This guide covers why, what the free measures achieve, and how a wall unit goes in on a dormer or a gable with the outdoor unit two floors down.
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Why does a loft room overheat more than a bedroom below it?
Because the roof is its wall. A 2019 study by researchers at UCL found converted lofts in about 10.8% of London dwellings and in 5.8% of dwellings across England, and found that they carry a substantially higher risk of overheating than the rooms beneath. Heat comes through the roof covering and the dormer cheeks all afternoon, the Velux windows add solar gain, and warm air from the rest of the house rises up the stairs. Source: Li, Taylor and Symonds.
What should you try before buying anything?
Shade and night air. The UCL loft study measured a 96% reduction in overheating from external shutters and 89% from opening windows at night, and once capital and running costs were weighed, night ventilation gave the best value of anything tested. External blinds on roof windows, a reflective film, and opening the loft and a ground-floor window after dark so the house breathes, all cost a fraction of a unit. Where they are not enough, or where the windows cannot be left open, air conditioning takes over. Source: Li, Taylor and Symonds.
Where does the indoor unit go in a loft?
On the gable end if there is one, or on a dormer cheek, as high as the sloping ceiling allows. The unit needs a few centimetres of clearance above it and a flat vertical surface behind it, which rules out most of the sloping ceiling. A floor-standing console on a knee wall is the alternative in a room with no gable and no dormer. The condensate drain needs a fall, which is usually easy from a loft.
Where does the outdoor unit go, and how do the pipes reach it?
At the back, two storeys down. The pipes run down the rear wall in trunking, or inside a boxed-in corner of a bedroom below, to an outdoor unit on the ground or on a bracket at first-floor height. A long vertical run is not a problem for the refrigerant, within the limits of the unit, but it is labour, and the price reflects it. On a semi or a terrace the outdoor unit is the house's one permitted unit, so a family wanting the loft and a bedroom done usually ends up with a multi-split. Source: Planning Portal.
Does a loft unit need any extra permission?
No more than any other: the permitted development conditions apply to the outdoor unit, not to where the indoor one is. Two things are worth checking, though. A loft conversion done under a planning permission may carry conditions, and in one of Dartford's conservation areas the outdoor unit still has to stay off the road-facing wall and roof. Our air conditioning in Dartford page lists the areas. Sources: Planning Portal; Dartford Borough Council.
Questions people ask
Will it heat the loft in winter too?
Yes, a reversible unit will, which is useful in a loft at the far end of the radiator circuit.
What size for a loft bedroom?
Start from the size guide and add for the roof and the glass. A loft of 15 square metres often needs what a 20 or 25 square metre bedroom would.
Can the pipes go through the eaves instead?
Sometimes, into an outdoor unit on a flat roof below, at least a metre in from the edge. The route is settled at the survey.
Sources we checked
Worth knowing before you ring
With these we can usually tell you on the first call whether the job is simple.
- The postcode
- Whether it is a house, a flat or business premises, and roughly its age
- If you rent or lease, who owns the external wall
- Which room, how big, and the direction its window faces
- Any management company, conservation area or listing that applies
Decided? Our air conditioning in Dartford page covers what we install, what we maintain and where we go.
The services this guide leads to
Home air conditioning
The back bedroom, the new flat with the glass wall, the loft nobody can sleep in: cooled in summer, warmed in winter.
Multi-split air conditioning
Several rooms, one box in the garden. The design that fits inside a semi's single permitted outdoor unit.
Air conditioning installation
The whole job from the first measurement to the handover of the remote, with every consent checked on the way.
Other guides
What does air conditioning cost to fit and to run?
Where the money goes: the unit, the labour, the wiring, the electricity, and the extras a flat or a new estate brings.
Air conditioning and planning permission: the rules for England
Every permitted development condition in one table, then the twists Dartford's new estates and conservation areas add.
Air conditioning with no outdoor unit: how monoblocs work
The through-the-wall units for flats and blocks that cannot take an outdoor box.
Air conditioning in a flat: who has to agree, and in what order
Lease, freeholder, management company and planning, in the order to tackle them.
Bedroom air conditioning: what it does for sleep, and what it costs
The sleep threshold, noise at night, sizing and the price of a quiet bedroom.
What size of air conditioner does a room need?
Kilowatts and BTU by room size, with the adjustments that matter.
How to check an air conditioning installer on the F-gas register
The company certificate, the engineer's category and where to look them up.
Portable or fixed air conditioning: which is right for you?
Price, power, noise and running cost, and when a portable is the right call.
Want us to ring you?
A name, a number and a postcode is enough. Say which room if you can. Or ring 01322 952910.