The best loft insulation for most UK homes is mineral wool — either glass wool or rock wool — laid to the recommended 270 mm depth. It is cheap, widely available, straightforward to install, and consistently performs well in independent tests. That said, there are good reasons to consider alternatives, and the right choice depends on your budget, whether you want to board the loft, and how much you care about environmental credentials.
This article breaks down the main loft insulation types with honest figures on cost, thermal performance, and practical trade-offs.
Why Depth Matters More Than Material
Before comparing products, it is worth stressing that adequate depth is the most important variable of all. A 270 mm installation of standard mineral wool will outperform a 50 mm installation of any premium product. Current guidance from the Energy Saving Trust recommends:
- 100 mm between the joists (first layer)
- 170 mm cross-rolled on top (second layer, at right angles)
This combination reaches a U-value of approximately 0.16 W/m²K, which meets the threshold for Building Regulation Approved Document L. The government’s ECO4 and Great British Insulation Scheme grants only fund work that brings loft insulation to at least 270 mm; anything less does not qualify.
Mineral Wool (Glass Wool & Rock Wool)
Mineral wool is the dominant product in UK lofts for good reason. Both glass wool (spun from recycled glass) and rock wool (spun from volcanic basalt) are non-combustible, dimensionally stable, and effective.
Glass wool (brands: Knauf Earthwool, Isover, Superglass) is lighter and compresses for transport, which makes the rolls easier to handle in a tight loft. It is the most common choice for DIY installation.
Rock wool (Rockwool, Rocksilk) is denser, slightly better at sound absorption, and a little less itchy to handle. It also performs marginally better if it gets slightly damp, though a dry loft should always be the starting point.
Both achieve broadly similar thermal performance at a given depth. The difference in real-world U-value at 270 mm is negligible.
Cost: £5–£9 per m² for materials at 270 mm total (two layers). A 50 m² loft floor area costs roughly £250–£450 in materials.
Sheep Wool Insulation
Sheep wool insulation (brands: Thermafleece, Black Mountain, YesColours Comfy) is a natural, breathable alternative that handles moisture differently from mineral wool. It can absorb and release small amounts of water vapour without losing significant thermal performance — a property known as hygroscopicity.
This makes sheep wool well-suited to older, draughty houses where interstitial condensation is a concern, or to properties where the owners want to avoid synthetic materials. It is treated with a natural borax preservative to resist fire and insects.
The thermal conductivity of sheep wool (lambda value around 0.038–0.040 W/mK) is broadly comparable to mineral wool (around 0.033–0.044 W/mK depending on product), so the depth requirements are similar. You will still need 270 mm.
Cost: £10–£18 per m² at 270 mm depth. Roughly 2–3 times the material cost of mineral wool. However, it is less irritating to handle, so some DIYers factor in the comfort benefit when comparing costs.
Multifoil Insulation
Multifoil products — layers of reflective foil separated by wadding — are heavily marketed and frequently misunderstood. They work by reflecting radiant heat rather than resisting conductive heat flow, which means their performance depends heavily on installation conditions (air gaps, surface emissivity, temperature differential).
Independent testing by BRE and others has repeatedly found that multifoil products rarely achieve the equivalent thermal performance their marketing materials imply when installed in real-world conditions. A 40 mm multifoil is very unlikely to match 270 mm of mineral wool.
That said, multifoil does have legitimate uses: as a membrane layer in cold-roof constructions, under metal roofing, or as a vapour control layer alongside other insulation. It is not a standalone replacement for mineral wool in a standard loft.
Cost: £4–£8 per m² for the multifoil itself, but you will still need additional insulation underneath to meet U-value requirements.
Rigid Insulation Boards (PIR/EPS)
Rigid polyisocyanurate (PIR) boards (Kingspan, Celotex, Recticel) offer higher thermal performance per millimetre than any fibrous product — a lambda value of around 0.022 W/mK vs 0.033–0.040 W/mK for wool products. A 120–140 mm PIR board can match the performance of 270 mm mineral wool.
This makes rigid boards useful where headroom in the loft is limited, or where you want to install a raised boarding platform without the total thickness adding up to more than the joists can accept. They are also worth considering if you intend to board the loft directly on top, as they provide a firm, stable base.
The main downsides are cost and the environmental impact of the manufacturing process. PIR boards are a petroleum product with a high embodied carbon.
Cost: £18–£35 per m² at equivalent thermal performance. Installation is more fiddly than mineral wool rolls.
At-a-Glance Comparison
| Material | Typical Lambda (W/mK) | Depth for 0.16 U-value | Material Cost (per m²) | DIY-Friendly |
|---|---|---|---|---|
| Glass wool | 0.033–0.044 | 270 mm | £5–£9 | Yes |
| Rock wool | 0.033–0.040 | 270 mm | £6–£10 | Yes |
| Sheep wool | 0.038–0.040 | 270 mm | £10–£18 | Yes |
| Multifoil | — (radiant) | Not standalone | £4–£8 | Yes |
| PIR board | 0.022–0.025 | 120–140 mm | £18–£35 | Moderate |
Eco Options
If environmental impact is a priority, sheep wool and recycled glass wool are the most defensible choices. Some glass wool products (Knauf Earthwool, Isover) now contain 80–85% recycled glass content. Sheep wool is a renewable fibre, though long-distance shipping and chemical treatment add to its footprint.
Cellulose insulation — blown recycled newspaper — is common in the USA and Scandinavia but rarely used in UK lofts. It can be blown into difficult-to-reach areas and has good environmental credentials, but finding a UK installer for a domestic loft is difficult.
Which to Choose
For most UK homeowners doing a standard loft insulation job: glass wool at 270 mm, either DIY or through the Great British Insulation Scheme if your property qualifies. It performs well, costs little, and installation is well-documented.
If you are concerned about moisture management in a breathable old house, or want to avoid synthetic materials: sheep wool.
If you need to board over the insulation and space is tight, or you want the thinnest possible solution: PIR rigid board, accepting the higher cost.
Avoid: relying on multifoil as your only insulation. Use it as a supplement or membrane, not a replacement.