U-value of 270mm loft insulation

Quilt between and over the joists — the classic ceiling-level loft job.

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 is 0.162, just over, and needs 300mm. The 0.11 notional needs about 365mm of λ 0.040 quilt, or a board upgrade.

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The build-up (outside → inside)

Joists at BR 443’s default 12.8% (§4.5.1), and everything above the insulation — the ventilated loft, the roof timbers and the tiles — as one roof-space resistance, R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). RdSAP’s own table, which also allows for the joists and the roof space, gives 0.16 for 270mm of mineral wool.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
270mm total (100 between + 170 over), λ 0.040≈ 0.15The standard recommendation
270mm total of λ 0.044 loft roll≈ 0.16Just over the 0.16 limit (0.162) — the roll’s lambda matters
300mm total (100 + 200 over), λ 0.040≈ 0.13Common top-up
~365mm total, λ 0.040≈ 0.11New-build notional territory

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) 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; if you need storage, raise the boards on legs tall enough to clear the full depth of quilt with an air gap above it.

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.

Check this build-up for condensation — free

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. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer, a vapour-open underlay, a ventilated void, a warm deck — and applies the one you pick in a tap, with Undo.

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 is 0.162, just over, and needs 300mm. The 0.11 notional needs about 365mm 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 0.162, just over, and needs 300mm. New-build notional performance (0.11) needs substantially more — around 365mm 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 in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Roofs

Walls

Floors

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.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

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