Loft insulation and boarding

The problem

Joists are about 100mm deep. Insulation needs 270mm. Boards screwed straight to the joists compress the wool to 100mm, and the loft loses most of its insulation value under the boarded area — exactly where the heat rises from the rooms below.

The fix: raised boarding

Plastic or timber loft legs fix to the joists and carry a raised deck 175mm or more above them, leaving the full insulation depth beneath with an air gap above it. Cost: +£25–£45 per m² of boarded area over the insulation itself. Board the area you will use — a walkway and a storage zone — not the whole loft.

The compromise: a thin layer over the joists

Where roof height makes raised boards impossible, a certified multifoil laid over the joists beneath the boards, with battens giving the air gap, adds performance without height. It does not match 270mm of wool. Price it as a compromise. Foil for lofts.

Also worth doing

• Insulate and draughtproof the hatch.
• Fit a proper ladder; a boarded loft gets used.
• Keep boards clear of the eaves so air still moves.

Questions homeowners ask