How Much Loft Insulation Do I Need? UK Guide

Current UK guidance points to 270 mm of mineral wool loft insulation. That's the depth that gets a standard timber-joisted loft down to a sensible U-value with ordinary glass or rock mineral wool, the stuff sold in rolls at every builders' merchant. It wasn't always that. Homes from the 1990s and 2000s were often done to 100 mm or 200 mm, and genuinely old lofts that have never been touched might have 25 mm or nothing at all. So the honest version of "how much do I need" is: enough to get you to 270 mm from wherever you are now.

That gap matters more than the target. Going from a bare loft to 270 mm is transformational. Going from an existing 100 mm to 270 mm is a single top-up layer and still worth doing, because it roughly halves what's left of the heat loss. But it means you should never just buy 270 mm's worth of rolls — you buy the difference. Takeaway: aim for 270 mm, but subtract what's already up there before you order anything.

Here's the bit that catches people out when they're counting rolls. UK loft insulation is almost always laid in two layers, and it's not for luck. The joists in most lofts are around 100 mm deep. The first layer goes between the joists and fills that gap. The second layer goes across the joists, at right angles, covering the timber.

That second layer is doing real work. Timber conducts heat far better than insulation, so exposed joists act as a row of thermal bridges — little motorways for heat straight through your carefully insulated loft. Laying the top layer across them shuts that down. It also means that if you're going from bare to 270 mm with 170 mm rolls, you're laying two passes, so you need close to double the roll count you'd guess from area alone. A top-up from 100 mm, by contrast, is usually just that single cross-joist layer. Takeaway: two layers isn't optional gold-plating — the cross-joist layer is where a big chunk of the saving comes from, and it's why roll counts aren't just area ÷ coverage.

Rolls depend on four things: your loft area, the target depth, what's already there, and the coverage printed on the pack. A common 170 mm mineral-wool roll covers somewhere around 5.4 m² per pack, but this varies enough between products that you should always read your own pack. The method is: work out how many layers you need (shortfall ÷ roll thickness, rounded up), multiply your area by that, add about 10% for waste around joists, pipes and offcuts, then divide by the pack coverage and round up.

As rough anchors for a 50 m² loft with 170 mm rolls: bare to 270 mm is around 22–25 rolls; a top-up from 100 mm to 270 mm is around 11 rolls. Scale roughly with your own area. The 10% waste isn't padding — lofts are full of awkward cuts around trusses, cables and the loft hatch, and running one roll short means a second merchant trip for the sake of a fiver. Takeaway: count layers first, then area, then add 10% — a bare 50 m² loft is roughly 24 rolls, a top-up roughly half that.

Take a three-bed semi in Sheffield with a 50 m² loft that already has the old 100 mm between the joists. The target is 270 mm, using 170 mm rolls that cover 5.44 m² each, bought at £22 a roll.

Most online loft calculators quote you the full 270 mm and stop. Two problems with that. First, they over-sell rolls to anyone topping up, because they ignore what's already there. Second, and more important, they say nothing about the things that turn a well-insulated loft into a damp one.

You should not insulate directly under a cold-water storage tank — leave that patch clear so a little warmth rises and stops it freezing. You should not cover downlighters, extractor fan motors or transformers without the correct clearances, because buried fittings overheat. And you must not stuff insulation hard into the eaves, because that blocks the ventilation path that lets the roof space breathe. A loft that's been crammed wall-to-wall, 270 mm deep, right over the light fittings and into the eaves, is a condensation and overheating problem dressed up as a good job. The contrarian truth is that a slightly less "complete" looking loft — clearances left, eaves kept open — performs better and lasts longer than a perfectly packed one. Takeaway: the depth is the easy part; the clearances and eaves ventilation are what separate a warm loft from a mouldy one.

For most people the answer is standard mineral wool roll — glass or rock — from a merchant like Jewson, Travis Perkins or a shed like B&Q or Wickes. It's cheap, it's easy to lay, and 270 mm of it hits the guidance. Sheep's wool and recycled options exist and are pleasant to handle but cost considerably more for the same U-value. Rigid PIR boards (Celotex, Kingspan) hit the target in less depth but are far dearer per square metre and are usually reserved for warm-roof loft conversions, where the insulation sits at the rafter line rather than on the floor. If you're converting the loft into a room, this floor-insulation guide isn't your job — see the conversion route instead. Takeaway: roll mineral wool is the default for a floor-insulated loft; save the rigid boards for conversions where depth is tight.

Every loft is a slightly different area and starting depth, so the roll count and cost are worth calculating for yours specifically rather than working off the anchors above. The Loft Insulation Calculator UK takes your area, existing depth and target, and returns the layers, rolls, cost and finished U-value band. If you want the full layer-by-layer U-value for the roof build-up, the U-Value Calculator UK does that to BS EN ISO 6946, and the Dew Point Calculator UK checks condensation risk once you know your build-up.