The standard loft build-up — 100mm of mineral wool between the joists plus 170mm laid over (270mm total) — achieves a U-value of about 0.16–0.17 W/m²K, which meets the 0.16 limiting value. Topping up to 300mm reaches about 0.15; the 0.11 notional needs roughly 400mm or a board upgrade.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- 170–200mm mineral wool quilt laid over joists
- 100mm quilt between joists (~12% timber)
- Plasterboard ceiling
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 270mm total (100 between + 170 over) | ≈ 0.16–0.17 | The standard recommendation |
| 300mm total (100 + 200 over) | ≈ 0.15 | Common top-up |
| ~400mm total | ≈ 0.11 | New-build notional territory |
Figures computed with the BS EN ISO 6946 combined method (bridging included) using BR 443 conventional lambda values — indicative for the generic build-up shown. Your products, thicknesses and (for floors) footprint will move the number: that's what the calculator is for.
What changes the number
Only the layer between the joists is bridged — the continuous quilt over the top is the efficient part, which is why "more over the joists" beats "thicker between". Squashing quilt under boarding ruins it; use raised loft legs if you need storage.
Next: check the junctions
A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT):
Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.
Frequently asked questions
What is the U-value of 270mm loft insulation?
The standard loft build-up — 100mm of mineral wool between the joists plus 170mm laid over (270mm total) — achieves a U-value of about 0.16–0.17 W/m²K, which meets the 0.16 limiting value. Topping up to 300mm reaches about 0.15; the 0.11 notional needs roughly 400mm or a board upgrade.
Is 270mm of loft insulation still enough?
It meets the Part L limiting value (0.16) and remains the standard advice for existing homes. New-build notional performance (0.11) needs substantially more — around 400mm of quilt — or higher-performing boards at ceiling level.
Is this calculator free?
Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard) and the condensation-risk screening check are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.
Does it follow the official calculation method?
Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging and air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- U-value of a cavity wall with full-fill insulation
- U-value of a cavity wall with PIR partial-fill insulation
- U-value of a timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of insulation between rafters (loft conversion)
- U-value of a warm flat roof
- How much insulation does a flat roof need?
- U-value of an insulated concrete ground floor
- U-value of an insulated suspended timber floor
- U-value of a beam and block floor with PIR
Different build-up? See the full roof U-value calculator, or start from scratch in the main U-value calculator.
Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are.