Best Loft Insulation UK 2026: Materials, Thickness & DIY Guide

The Short Answer

For most lofts, you need Knauf Loft Roll 44 in 170mm thickness, laid across the top of your joists as a second layer. If you want to board part of the loft for storage, add Loft Legs so you don't squash the insulation. And whatever else you do, insulate the loft hatch — it's a 10-minute job that's absurdly effective for the price.

Why This Is the Best Money You'll Spend on Your House

A quarter of your home's heat goes straight through the roof if it's not properly insulated. That's not marketing — it's physics. Hot air rises, hits your ceiling, and if there's nothing stopping it, it carries on through the loft and out through the tiles. You're literally heating the sky.

The Energy Saving Trust reckons topping up from 120mm to the recommended 270mm saves a typical semi £200–£350 per year. The materials cost £150–£250. That's a payback of under two years — and then it keeps saving you money for the rest of the time you live there. There's genuinely no other home improvement that comes close to this return.

Even better, it's a proper DIY job. No specialist tools, no training, no need to hire anyone. A Saturday afternoon and you're done.

How Much Do You Need?

Building Regulations say 270mm of mineral wool. Most UK homes built before 2000 have less — often 100mm or even 50mm if it was done decades ago and never topped up.

The standard approach:

• First layer (100mm) — between the joists, filling the gaps
• Second layer (170mm) — laid across the top of the joists at right angles, covering them completely

If you already have 100mm between the joists, you just need the 170mm cross-layer on top. If you're starting from scratch, you need both.

What to Buy

The Insulation: Knauf Loft Roll 44 — 170mm

This is the standard choice for the cross-layer, and it's standard for a reason. It's a glass mineral wool roll at exactly 170mm — designed for this job. Each roll covers about 5.93m². For a typical three-bed semi (30–40m² of loft), you'll need 6–7 rolls.

It's non-combustible, moisture-resistant, and doesn't degrade over time. Cut it with a long-bladed knife, unroll it in position, and it springs to full thickness. Not glamorous, but it works perfectly and it'll outlast the house.

For Topping Up Thin Layers: Superglass Multi-Roll 100mm

If your existing insulation between the joists is thin (50–80mm), the Superglass 100mm is useful for bringing it up to joist height before laying the Knauf cross-layer on top. Each roll covers a generous 10.64m², and it's a bit lighter and easier to handle in tight spaces than Knauf. Not essential if your between-joist insulation is already at 100mm, but worth it if it's patchy or thin.

If You Want Loft Storage: Loft Legs

This is important: don't board directly onto the joists. If you do, you'll compress the insulation underneath, and compressed insulation loses 50% or more of its thermal performance. You might as well not have bothered.

Loft Legs are raised stilts that sit on the joists and hold boarding above the full 270mm of insulation. A 12-pack covers about 1.7m² of boarded area. For a decent storage space (4–6m²), you'll need 3–4 packs plus some boarding. It's a bit more work, but it means you get storage without wrecking your insulation.

Don't Skip This: Loft Hatch Insulation Kit

Here's a secret that's barely a secret: your loft hatch is probably the single worst thermal weak point in your house. An uninsulated, poorly sealed hatch is like leaving a small window open 24/7. A hatch insulation kit — basically a pre-cut pad and some draught strips — costs £15–£25 and takes 10 minutes to fit.

If you only do one thing from this entire guide, do this. The return on investment is almost comical.

Shopping List

For a typical three-bedroom semi, topping up from 100mm to 270mm:

Item	Quantity	Approximate Cost
Knauf Loft Roll 170mm	6–7 rolls	£120–£210
Loft Hatch Insulation Kit	1	£15–£25
Loft Legs (if boarding)	3–4 packs	£60–£140
Dust mask (FFP2) and gloves	1 set	£5–£10
Total (insulation only)		£140–£245
Total (with boarding)		£200–£385

Against annual savings of £200–£350, even the boarding option pays for itself in under two years.

DIY Tips (From Mistakes People Actually Make)

• Wear a proper dust mask, gloves, and long sleeves. Mineral wool fibres are itchy and irritating. An FFP2 mask is ideal. You'll thank yourself.
• Never step between joists. The plasterboard ceiling below will not hold your weight. Lay boards across the joists to work from, and move them as you go.
• Don't block the eaves. There should be a gap at the edges of the roof where the eaves meet the loft. This is deliberate ventilation that prevents condensation. You can buy eaves ventilator trays that hold insulation back from the edges.
• Don't insulate under your cold water tank. The tank needs some warmth from below to avoid freezing in winter. Insulate over and around it, but leave the bottom open to the heated space below.
• Move electrical cables on top of the insulation. Cables buried under insulation can overheat. Clip them to joist tops or lay them on the insulation surface.

For costs and savings of all insulation types, see our home insulation costs guide. While you're in the loft, don't forget to insulate any exposed pipes — it takes 20 minutes and saves £30–£60 per year. If you're insulating before getting a heat pump, our insulate-first guide explains the right order.