U-value of a knee wall in a loft conversion

A knee wall with 100mm of PIR between the studs and a continuous 50mm PIR layer inside them, finished in plasterboard, has a U-value of about 0.18 W/m²K taking the eaves space behind it as outside air — just inside the 0.18 Approved Document L sets for a new wall in an existing home (0.1796). 40mm inside gives 0.20 and misses on that basis. SAP 10.2 treats the eaves as an unheated space and adds a resistance of 0.5 m²K/W for it (Table 3.4), which takes the pre-filled wall to about 0.16 and the 40mm version to 0.179, inside 0.18; U-Monkey applies it when you pick the loft void under “On the other side”.

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The build-up (outside → inside)

(The ventilated eaves space behind: taken as outside air)
Breather membrane on the eaves face
100mm PIR between 100mm studs (15% timber)
50mm PIR across the inside of the studs (continuous)
12.5mm plasterboard

The pre-filled link opens as a new element in an existing home, judged against Approved Document L’s 0.18 (2021 edition, Table 4.2). The eaves are taken as outside air (Rse 0.04); SAP’s eaves allowance (SAP 10.2 and 10.3, Table 3.4) is the separate row, 1/(1/U + 0.5) — in the app, “On the other side”: the loft void behind the insulated wall of a room in roof.

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
Uninsulated: plasterboard on studs	≈ 2.27
50mm quilt between the studs	≈ 0.84	Many older rooms in roofs
100mm PIR between the studs only	≈ 0.35	The studs bridge it
100mm between + 25mm PIR inside	≈ 0.23
100mm between + 40mm PIR inside	≈ 0.20	Misses (0.197); 0.179 with SAP’s Ru
100mm between + 50mm PIR inside	≈ 0.18	As pre-filled: 0.1796
100mm mineral wool between + 50mm PIR inside	≈ 0.22
Pre-filled wall with SAP’s eaves allowance (Ru 0.5)	≈ 0.16	1/(1/0.1796 + 0.5) = 0.1648

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. Rows that give an Ru include the unheated space beyond the element as an extra resistance, U = 1/(1/Uo + Ru) (SAP 10.2 §3.3 formula (3); BR 443 §5), which the calculator applies under “On the other side”.

What changes the number

The studs bridge 15% of the insulation between them, so the continuous layer inside does more per millimetre than extra depth: 100mm between the studs plus 25mm across them (0.23) beats 150mm between deeper studs (0.24). Many loft conversions avoid the knee wall as a thermal line altogether by insulating the roof slope right down to the eaves, which brings the eaves space inside; the knee wall then needs nothing, but the slope has to meet the roof figure all the way down. Where the knee wall is the line, the floor of the eaves space behind it — the ceiling of the room below — has to be insulated too, and the junctions sealed, or cold air from the ventilated eaves washes through the insulation.

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 a knee wall in a loft conversion?

Can I count the eaves space behind a knee wall?

In a SAP calculation, yes: SAP 10.2 §3.3.5 gives an unheated-space resistance of 0.5 m²K/W (Table 3.4) for the insulated wall of a room in roof built into a roof insulated at ceiling level, for new dwellings and for conversions that create a room in roof, and BR 443 §5 says the effect of an unheated space can be incorporated into the U-value of the element. The pre-filled link takes the eaves as outside air, the cautious figure; pick “loft void behind the insulated wall of a room in roof” under “On the other side” and U-Monkey adds the 0.5. With Ru 0.5 the wall needs about 0.198 on its own to come out at 0.18, so ask your building control body which basis it will accept before thinning the lining.

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

Walls

Roofs

Floors

Different build-up? See the full wall 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.