The standard loft build-up — 100mm of mineral wool between the joists plus 170mm laid over (270mm total) — achieves a U-value of about 0.15 W/m²K with λ 0.040 quilt, inside the 0.16 limiting value. Check the roll’s lambda: with λ 0.044 loft roll the same 270mm comes out at about 0.17, just over, and needs around 300mm. The 0.11 notional needs roughly 380mm of λ 0.040 quilt, 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), λ 0.040 | ≈ 0.15 | The standard recommendation |
| 270mm total of λ 0.044 loft roll | ≈ 0.17 | Just over the 0.16 limit — the roll’s lambda matters |
| 300mm total (100 + 200 over), λ 0.040 | ≈ 0.14 | Common top-up |
| ~380mm total, λ 0.040 | ≈ 0.11 | New-build notional territory |
Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and 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.
Condensation risk in a cold loft
Insulating at ceiling level leaves the loft as a cold, ventilated void, so the question is not only the U-value but where the moisture from the rooms below ends up. BS 5250:2021 is clear that a condensation calculation is not the tool for this: it models vapour diffusing through the ceiling, but most of the moisture that reaches a cold loft arrives on air leaking through it, so the standard relies on prescriptive rules instead. U-Monkey’s free BS EN ISO 13788 check follows it. As pre-filled, the ceiling itself shows no condensation in any month, and 400mm of quilt makes no difference, but the check reports a cold pitched roof as outside the scope of a calculation rather than as a pass.
Boarding is still worth modelling, because the check can see it. Lay chipboard straight onto the insulation and it predicts condensate building up under the boards year on year, a fail; raise them clear on legs with a ventilated air gap and that clears. What decides a cold loft in practice is air leakage, so seal round the hatch, pipes and downlights until the ceiling works as an air and vapour control layer, and keep the eaves ventilation clear when you top up.
Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included.
Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.
It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.
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 a junction):
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 0.15 W/m²K with λ 0.040 quilt, inside the 0.16 limiting value. Check the roll’s lambda: with λ 0.044 loft roll the same 270mm comes out at about 0.17, just over, and needs around 300mm. The 0.11 notional needs roughly 380mm of λ 0.040 quilt, or a board upgrade.
Is 270mm of loft insulation still enough?
With λ 0.040 quilt, yes: it comes out at about 0.15, inside the Part L limiting value of 0.16, and it remains the standard advice for existing homes. Check the roll’s lambda, though: at λ 0.044 the same 270mm is about 0.17, just over, and needs around 300mm. New-build notional performance (0.11) needs substantially more — around 380mm of λ 0.040 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), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) 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, mortar joints in blockwork 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 one-and-a-half-brick (317.5mm) solid wall
- 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
- Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16
- U-value of a cold flat roof (cold deck)
- U-value of an inverted flat roof
- 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, and why two calculators can give different answers for the same wall.