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.
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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