How much loft insulation do I need?
Work out the loft area, see how much you need to top up to the recommended 270 mm, and get the number of rolls to buy. Topping up a thin, decades-old layer is one of the cheapest ways to cut a heating bill. Free, no sign-up.
A guide, not a guarantee. The first layer goes between the joists; the top-up layer is laid across them at right angles to cover the cold timber. Don't squash insulation in to fit — compressing it cuts its performance. Keep it clear of recessed downlights (unless they're fire-rated and IC-rated), leave the eaves vented to avoid condensation, and don't insulate under a cold-water tank. Loft boarding needs raised legs to keep the full depth. Nothing you type leaves your browser.
Area, the gap to your target, and whole rolls.
Start with the area. Length times width gives the loft floor area. Leave out a boarded storage zone or the hatch — you only insulate what's open. This is the area you need to cover with the top-up layer.
How much to add. The recommended depth for mineral wool is 270 mm. If you already have, say, 100 mm, you need a 170 mm top-up to reach it. If there's nothing there, you'll do it in two layers — typically 100 mm between the joists, then 170 mm across the top. We work out the top-up depth and flag if you're already at target.
Rolls cover less as they get thicker. A roll of loft insulation is a fixed length, so a 100 mm roll covers about 8 m², while a 200 mm roll of the same width covers only around 4.5 m². Enter the coverage for the thickness you're buying — it's on the pack — and we divide the area by it and round up.
Lay the top layer across the joists. The first layer fills the gaps between the joists. The top-up layer goes the other way, across the joists, so it covers the timber too — joists are a thermal bridge, letting heat leak through if they're left exposed.
What this leaves out. Loft boarding (which needs raised legs to keep the full depth under it), downlight covers, eaves vents to keep airflow, and a hatch insulator are all separate. If the loft is damp or poorly ventilated, fix that first — insulation traps moisture if the airflow's wrong.
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