Sheep’s wool vs mineral wool insulation: is the premium worth it

Quick answer: Sheep’s wool insulation costs around five times more than glass mineral wool for slightly better lambda. Thermafleece CosyWool at 100mm is about £15.58 per square metre inc VAT, against roughly £2.78 for a Knauf loft roll. Wool is 0.038 W/mK against 0.044, and Euroclass E against A1. Prices checked 27 July 2026.

The numbers side by side

We haven’t tested either material. This compares published data sheets and merchant pricing, checked 27 July 2026.

Read that price line carefully. It’s the whole argument. You’re paying about 460% more per square metre for a 14% improvement in lambda. On thermal performance per pound, sheep’s wool loses, and it isn’t close.

Where the premium actually goes

Three things, and only one of them is thermal.

Moisture handling. Wool fibre absorbs and releases water vapour without losing much thermal performance. That’s what “buffering humidity” means in practice. In a breathable build-up on a solid-walled cottage it’s genuinely useful, and it’s the strongest technical case for the material. Our guide to comparing insulation materials covers where breathability matters.

Handling. No glass fibres, no itch, no dust mask required. If you’re working in a cramped loft over a weekend, that isn’t a trivial difference.

Embodied carbon. Wool is a by-product of an industry that exists anyway, processed at low temperature. Glass wool is melted at well over 1,000 degrees. If your specification counts embodied carbon rather than only operational energy, wool scores well. That’s a real reason to choose it, and it’s a different question from cost per U-value.

Fire, which is the honest weakness

Thermafleece CosyWool is Euroclass E and passes the EN ISO 11925-2 ignitability test. Glass and stone mineral wool are Euroclass A1, the top of the scale. A1 means no contribution to a fire at all.

Euroclass E isn’t a ban. But it’s a long way down from A1, and it’s why wool doesn’t appear in cavity barriers, fire stopping or anywhere a regulation asks for non-combustible material. In a cold loft above a plasterboard ceiling the practical risk is low. But if you’ve got unsleeved downlighters or old cabling up there, an A1 material is the safer specification. Read insulation fire safety for UK homeowners before deciding.

Moths

The question everyone asks. Thermafleece’s data sheet lists CosyWool as a pass for moth and beetle proofing to ISO 3998, and a pass for mould resistance. What the data sheet doesn’t do is name the treatment that gets it there.

So ask. Not all sheep’s wool insulation is treated the same way, and the chemistry in this corner of the market has changed more than once as approvals have come and gone. Ask the supplier which treatment their product uses and which test it passed, and don’t assume two wool products are equivalent on this.

Where sheep’s wool earns its price

• Solid stone or brick walls being lined internally, where a breathable build-up is the right approach and a vapour-tight foam board would be the wrong one.
• Between rafters in an old roof with no membrane, where the construction needs to dry inwards.
• Small jobs where the total material cost is a few hundred pounds and the handling comfort is worth the premium.
• Projects where embodied carbon is a stated target rather than an afterthought.

And where it doesn’t. A standard cold loft in a 1970s semi with a felt underlay and 250m² of ceiling to cover. At £15.58 per square metre against £2.78, wool would add well over £3,000 to a job a mineral wool roll does for a few hundred. And the 14% better lambda? You can buy that back by laying about 15mm more of the cheap stuff. That’s the position, and it isn’t close.

If you want the thickness maths behind that, see U-values explained and the loft insulation thickness guide. For a modelled cost of a whole insulation job, the calculator and the insulation product guide work from a £1,498 mid estimate, with the method in how we calculate costs.

FAQs

Is sheep’s wool insulation better than mineral wool?

It’s slightly better thermally, at 0.038 W/mK against 0.044, and much better on handling and embodied carbon. It’s worse on fire, at Euroclass E against A1, and roughly five times the price. For a standard loft, mineral wool is the better buy.

Does sheep’s wool insulation get eaten by moths?

Treated products are tested against it. Thermafleece’s data sheet lists CosyWool as a pass for moth and beetle proofing to ISO 3998. Products vary, so ask the supplier which treatment is used and which test it passed before you buy.

Can sheep’s wool insulation be used in a loft?

Yes, laid between and over the joists in the same way as a mineral wool roll, and it comes in matching thicknesses from 25mm to 150mm. The barrier is cost, not suitability: covering a typical loft in wool runs into four figures where mineral wool runs into three.