U-value of a SIP wall (structural insulated panels)

A brick-clad wall of 142mm structural insulated panels — 112mm of polyurethane between two 15mm OSB skins — with a service void and plasterboard inside has a U-value of about 0.20 W/m²K with 4% of the core taken up by timber, and about 0.18 with none. That clears the 0.26 limiting value but not the 0.18 the notional dwelling uses for walls. The 172mm panel gives about 0.16. BR 443 sets no default timber fraction for SIPs: it asks for the bridging to be worked out from the panel’s own splines, plates and openings, so the timber is the number to check.

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The build-up (outside → inside)

102.5mm brick outer leaf
50mm cavity (the wall ties cross it empty: no correction)
Breather membrane
15mm OSB/3 skin
112mm polyurethane core (λ 0.024), 4% timber at the joints and plates
15mm OSB/3 skin
25mm service void on battens (11.8% timber, BR 443 §4.6.2)
12.5mm plasterboard

Panel as in Kingspan’s TEK specification: 15mm OSB/3 skins and a 112mm polyurethane core at λ 0.024. Its BBA certificate (02/S029, 2010 issue) gave 0.19–0.20 W/m²K for this wall for 0% to 4% timber, with the core then declared at λ 0.023 — about 0.01–0.02 above what U-Monkey calculates for that λ; the certificate does not show its working. The pre-filled build-up takes 4%. The SIP core takes air-gap level 0 (BR 443 §4.8.1).

Typical U-values for this construction

Specification	U-value (W/m²K)	Notes
142mm panel, no timber counted	≈ 0.18	What Kingspan’s TEK sheet quotes with brick
142mm panel, 4% timber in the core	≈ 0.20	As pre-filled (0.198)
142mm panel, 7.5% timber (solid timber splines)	≈ 0.21
142mm panel + 25mm PIR in the service zone, 4% timber	≈ 0.16	Meets the 0.18 notional value
172mm panel (142mm core), 4% timber	≈ 0.16	Meets 0.18 (0.163); Kingspan’s sheet quotes 0.14
172mm panel, 7.5% timber	≈ 0.18	Meets 0.18, just (0.178)
Another maker’s 150mm panel: 128mm PUR core (λ 0.028), 11mm OSB, 4%	≈ 0.20	Thinner skins, weaker core

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

Two things set the figure: the core and the timber. A polyurethane core (λ 0.024 here) beats an EPS one (about λ 0.030), so an EPS-cored panel has to be about a quarter thicker for the same result. The timber is the variable to watch: splines at the panel joints, the sole and head plates and the framing round openings all put wood (λ 0.13) where the core would be. BR 443 (2019) asks for the bridging fraction to be worked out from the construction of the panel, using the same guidelines as a timber-frame wall (§4.4.1(iii)): leave out timbers outside the heat-loss wall area and a zone round windows and doors, and count every timber with no insulation behind it. Insulated splines keep it low; solid timber splines at every joint push it towards 7–8%. The core is foamed or bonded to its skins with sealed joints, so U-Monkey gives it BR 443’s air-gap level 0, and the ties holding the brick to the panel cross an empty cavity, so they need no correction.

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 SIP wall (structural insulated panels)?

Do SIPs meet the Future Homes Standard?

They can, but not always with the standard panel. The Future Homes Standard’s notional dwelling (SAP 10.3 Appendix R) uses 0.18 W/m²K for walls, the same as Part L 2021, and that is a reference for setting the dwelling’s targets, not a limit: the limiting value for a wall stays at 0.26. A 142mm panel with brick comes out at about 0.20 once its timber is counted, so other parts of the house have to make up the difference; the 172mm panel (about 0.16), or 25mm of PIR in the service zone (about 0.16), gets under 0.18 on its own.

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.