U-value of 400mm loft insulation

New-build levels of loft insulation, and where the returns run out.

400mm of mineral wool quilt — 100mm between the joists and 300mm over them — gives a U-value of about 0.10 W/m²K with λ 0.040 quilt, better than the 0.11 of the new-build notional roof. With λ 0.044 roll it is 0.109, just inside 0.11. 350mm gives 0.11 when rounded but is 0.114, so it misses the notional figure; λ 0.040 quilt needs about 365mm to meet it. Each 100mm added saves less than the last: 200 to 300mm saves twice as much heat as 300 to 400mm.

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The build-up (outside → inside)

Joists at BR 443’s default 12.8% (§4.5.1) and the roof space above as R 0.2 (BS EN ISO 6946 Table 9, BR 443 §8). A second layer covers the joists, so the air-gap correction is level 0.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
270mm (100 between + 170 over)≈ 0.15See the 270mm page
300mm (100 + 200)≈ 0.13
350mm (100 + 250)≈ 0.11Misses the 0.11 notional: 0.114
400mm (100 + 300), λ 0.040≈ 0.10As pre-filled
400mm of λ 0.044 loft roll≈ 0.11Meets 0.11 (0.109)
450mm (100 + 350)≈ 0.09
500mm (100 + 400)≈ 0.08

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

Past about 300mm the quilt over the joists does nearly all the work and the U-value falls slowly: each extra 50mm of λ 0.040 quilt saves less than 0.02 W/m²K. What spoils deep loft insulation in practice is compression (boards laid straight on it), gaps at the eaves and around the hatch, and quilt pushed into the eaves so that it blocks the ventilation; BS 5250 wants the roof space ventilated, and the condensation check in U-Monkey reports a cold loft as outside the scope of a calculation for that reason. RdSAP uses 0.11 for any loft with 400mm or more.

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 400mm loft insulation?

400mm of mineral wool quilt — 100mm between the joists and 300mm over them — gives a U-value of about 0.10 W/m²K with λ 0.040 quilt, better than the 0.11 of the new-build notional roof. With λ 0.044 roll it is 0.109, just inside 0.11. 350mm gives 0.11 when rounded but is 0.114, so it misses the notional figure; λ 0.040 quilt needs about 365mm to meet it. Each 100mm added saves less than the last: 200 to 300mm saves twice as much heat as 300 to 400mm.

Is 400mm of loft insulation too much?

Not thermally: it takes a loft to about 0.10 W/m²K, inside the 0.11 a new-build roof is modelled at, and there is no condensation penalty provided the loft stays ventilated and the ceiling is well sealed. The return per millimetre is small by then, though — going from 300 to 400mm saves about 0.03 W/m²K, against 0.28 for the first top-up from 100 to 270mm — so check the joists can take the weight of boarding raised above it, and that the hatch and eaves can be insulated and ventilated properly.

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.

Open this build-up pre-filled or call 01202 623236