Compare Roof Insulation Types — Best Value in 2026 UK
Mineral wool at £25–£45/m² fitted (£/U-value champion for loft floors). PIR boards at £45–£80/m² (best for rafter-level where every mm of headroom counts). Spray foam at £45–£70/m² (fastest fit, but mortgage-flagged on 4 of 6 major lenders). Sheep's wool & wood fibre at £60–£110/m² (eco premium). Multifoil at £35–£60/m² (good for thin gaps, weak standalone). Full 2026 UK side-by-side below.
Which roof insulation is best value in 2026 UK?
Headline 2026 UK comparison — 50 m² unconverted loft to current Building Regs Part L (U-value 0.13 W/m²K target):
- Mineral wool quilt 270 mm @ floor: £1,250–£2,250 installed — the £/U-value winner
- PIR board 150 mm @ rafter level: £2,250–£4,000 installed — thinnest profile for headroom
- Spray foam 200 mm @ rafter: £2,250–£3,500 — fastest fit, but mortgage risk on 4 of 6 lenders
- Sheep's wool 270 mm @ floor: £3,000–£5,500 — best moisture-buffering and eco credentials
- Wood fibre board 140 mm @ rafter: £3,500–£6,000 — best long-term breathability
- Multifoil 25 mm: £1,750–£3,000 — only viable supplementing other insulation
Best for budget: mineral wool at loft floor level. Best for converted/converting loft: PIR between and under rafters. Best for eco / breathability: wood fibre. Avoid: open-cell spray foam if you're remortgaging or selling within 10 years — closed-cell is mortgageable but flagged.
Full 2026 UK side-by-side comparison
Headline numbers across cost, U-value, lifespan, mortgage and grant eligibility on a typical 50 m² UK loft.
Match the material to the job
Cold loft (unconverted, insulation at ceiling level)
Default answer: 270 mm of mineral wool quilt at the joist level. That hits the 2025 Part L target (U-value 0.13 W/m²K) with the cheapest material on the market and the easiest DIY install. Two layers — 100 mm between joists, then 170 mm laid perpendicular across the joists. Total fitted cost for 50 m²: £1,250–£2,250. Add a loft hatch insulator and weatherstrip (£45–£80) to avoid throwing 5–8% of the saving away through the access opening.
Warm roof / converted loft (insulation at rafter level)
Default answer: PIR between and under the rafters. PIR at λ=0.022 gets you to U-value 0.13 W/m²K in just 150 mm of total thickness — mineral wool at the same target needs 350 mm and won't fit between 100 mm rafters with a 50 mm air gap. Typical detail: 100 mm PIR between rafters + 50 mm PIR taped over the rafter underside as a continuous layer to break the cold bridge. Sheep's wool or wood fibre is the eco-equivalent at £60–£120/m² with similar thickness.
Flat roof (warm-deck)
Default answer: PIR board over the deck with a single-ply or EPDM membrane over the top. 120–140 mm of PIR hits U-value 0.13 W/m²K with room for a fall to outlets. Avoid cold-deck flat roofs in 2026 retrofit — most fail Approved Doc C condensation checks. Sheep's wool and mineral wool are unsuitable at flat-roof level due to moisture-pinning risk.
Why spray foam needs careful thought in 2026
Spray foam is fast (a typical loft sprays in 1–2 days) and well-priced — but as of late 2025, Lloyds, Halifax, Nationwide and Santander all flag spray-foamed properties and require a roof-timber moisture survey before mortgage approval. Open-cell foam is now refused outright by 4 of 6 major lenders. Closed-cell foam is acceptable with a building surveyor's report (typically £450–£900). If you're selling or remortgaging within 10 years, the resale drag plus the survey cost typically outweighs the install saving — most BestBuilders-vetted insulation specialists now steer homeowners towards rafter-level PIR or wood fibre instead.
Real 2026 Leeds 52 m² loft — three quotes compared
A real homeowner project we reviewed in February 2026: 1960s 3-bed semi in Leeds LS18, cold loft, 52 m² floor area, existing 100 mm rockwool to be topped up to current Part L target. Owner planning a loft conversion in 2028, so future warm-roof compatibility mattered.
Given the planned 2028 loft conversion, the owner chose PIR at rafter level: £3,420 total, U-value 0.13. Mineral wool at joist level would have been £1,800 cheaper today but would have needed to be lifted and rebuilt in 2028 (£1,400 sunk cost). The rafter PIR future-proofs the loft conversion and stays in place. Spray foam was rejected because the owner plans to remortgage in 2027 — the £450–£900 survey cost plus 3–7% resale drag tipped the calculation against it.
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Insulation costs compared
Where this job sits against the other general builders jobs, on 2026 UK prices. Your own quote depends on size, access and spec.
Common Questions
How we sourced these figures
- Building Regulations 2010 (Part L 2025 update) — U-value targets for new and renovated roofs
- Ofgem — ECO4 scheme — grant eligibility and measure list
- RICS BCIS — 2026 UK labour rates for insulation install
- UK Fire Protection Association — fire performance ratings for foam and board products
Methodology note: Cost ranges combine RICS BCIS rates, manufacturer pricing and our internal dataset of 880+ UK insulation quotes reviewed in the 12 months to 30 April 2026. Mortgage-flagging behaviour is from BestBuilders' lender-policy survey conducted November 2025. Last fact-checked: .
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