U-value of 100mm loft insulation (and what a top-up achieves)

The thin layer between the joists that many lofts still have, and the case for topping it up.

100mm of mineral wool quilt between the ceiling joists gives a U-value of about 0.43 W/m²K with λ 0.040 quilt, or 0.45 with older λ 0.044 roll. That is worse than the 0.35 at which Approved Document L counts a roof as an element to upgrade. Topping it up with 170mm laid over the joists (270mm in all) brings it to about 0.15, and 200mm over (300mm) to about 0.13. RdSAP assumes 0.40 for 100mm, and uses the same figure for an older loft whose insulation is unknown, noting that most lofts have at least 100mm.

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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). The pre-filled link opens as an existing roof, judged against Approved Document L’s 0.35 threshold (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
50mm quilt between the joists≈ 0.72
75mm quilt between the joists≈ 0.53
100mm quilt, λ 0.040≈ 0.43As pre-filled
100mm loft roll, λ 0.044≈ 0.45
100mm, joists at 600mm centres (35mm trussed rafters, 6.8%)≈ 0.39Fewer, thinner joists
+ 170mm over the joists (270mm)≈ 0.15Meets the 0.16 improved value
+ 200mm over the joists (300mm)≈ 0.13
+ 300mm over the joists (400mm)≈ 0.10See the 400mm page

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

While all the insulation is between the joists, the joists matter: BR 443’s default is 47mm timbers at 400mm centres plus 1% for other timbers, 12.8% of the layer, and a trussed-rafter roof at 600mm centres has about half that. Once quilt is laid over the joists the timber is covered and the top layer works at full value, which is why a top-up does so much more than its thickness suggests, and why BR 443 drops the air-gap correction to level 0 with a second layer in place. Lay the top-up across the joists, not along them; do not squash it under boards — raise them on legs tall enough to clear the quilt with an air gap above it — and keep it back from the eaves vents.

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 100mm loft insulation (and what a top-up achieves)?

100mm of mineral wool quilt between the ceiling joists gives a U-value of about 0.43 W/m²K with λ 0.040 quilt, or 0.45 with older λ 0.044 roll. That is worse than the 0.35 at which Approved Document L counts a roof as an element to upgrade. Topping it up with 170mm laid over the joists (270mm in all) brings it to about 0.15, and 200mm over (300mm) to about 0.13. RdSAP assumes 0.40 for 100mm, and uses the same figure for an older loft whose insulation is unknown, noting that most lofts have at least 100mm.

Is it worth topping up 100mm of loft insulation?

Yes: going from 100mm to 270mm cuts the heat through the ceiling by about two-thirds, from about 0.43 to 0.15 W/m²K. For a 50m² loft at a 20°C difference that is about 430 W down to 150 W, all the time the heating is on. The Energy Saving Trust gives 270mm of mineral wool as the minimum to aim for.

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