Guide

Loft Insulation: What Thickness Do You Need?

Loft insulation is one of the cheapest ways to cut heating bills, but only if it's the right depth. Here's what you need in a UK home.

The recommended depth

Current UK guidance is around 270mm of mineral wool insulation. Many older homes have far less — sometimes 100mm or none — so topping up to modern depth is a quick win that pays back through lower heating bills.

How it's built up

The recommended depth is usually achieved in two layers: the first laid between the ceiling joists, the second laid across them at right angles to cover the joists and eliminate cold bridges. If you already have a layer between the joists, you may only need the cross layer on top.

Working out how much

For two layers you effectively cover the loft area twice. Our loft insulation calculator doubles the area automatically when you choose the full 270mm depth and tells you how many rolls to buy.

Don't compress it

Insulation works by trapping air, so squashing it under loft boards or storage destroys its effectiveness. If you need storage, fit raised loft legs so the boards sit above the insulation. Keep insulation clear of the eaves for ventilation, and away from any downlights not rated to be covered.