Air conditioning for a loft conversion: what works?

Why do loft conversions get so hot?

A 2019 study by Li, Taylor and Symonds modelled a London semi and found a converted loft has a substantially higher risk of overheating than conventional bedrooms. The roof takes the full sun, and warm air rises from the floors below. Converted lofts are found in 5.8% of English homes, so this is not a rare problem. The authors concluded passive measures would not be enough by the 2080s. Source: Li, Taylor and Symonds.

Do building regulations deal with this?

Not directly. Part O of the Building Regulations took effect on 15 June 2022, and the Planning Portal says its guidance applies to new residential buildings only. Elmhurst Energy notes that it does not extend to existing homes with extensions. So a new loft conversion may be perfectly compliant and still too hot to sleep in. Sources: Planning Portal; Elmhurst Energy.

What should you try first?

In that same study, external shutters cut overheating by 96%, and night-time ventilation by 89%, with ventilation the most cost-effective. External blinds on a roof window are easy to add, and they do not need the planning thought that a shutter on a Regency front would. If you can open the windows safely at night, try that before anything else. Air conditioning makes sense when these fixes are not enough, not possible or too noisy. Source: Li, Taylor and Symonds.

Where does the indoor unit go under a sloping roof?

Wall units need a flat, solid area high enough for air to circulate. Gable walls are the obvious place, followed by a dormer cheek or a partition wall strong enough to carry the weight. Avoid knee walls that are too low to let the air move. In a very low room, a floor-standing unit may be better. We check headroom at the survey.

How does the pipe get down to the outdoor unit?

That is usually the hard part. Pipes can run through the eaves storage, then down the outside of a rear wall in neat trunking, or down through a cupboard. A pitched roof is not permitted development for the outdoor unit, so plan on ground level or a flat roof, at least a metre from its edge. In a Leamington conservation area, keep it to the rear of the house. Source: Planning Portal.

What size does a loft room need?

Larger than floor area suggests. iHeat notes that loft rooms often get hotter than the rest of the house, so they may need more than a similar room elsewhere. Roof windows also add to the load. We work this out from the room itself, including the insulation that went in with the conversion. Source: iHeat. For a first estimate, see the size guide.