U-value of an ICF wall (insulating concrete formwork)

Concrete cast between two layers of EPS or XPS, and what each size of form achieves.

An insulating concrete formwork wall with 100mm of white EPS either side of a 150mm concrete core, rendered outside and lined with plasterboard, has a U-value of about 0.16 W/m²K — the figure NSAI Agrément gives for that block (certificate 07/0277). Graphite (grey) EPS takes the same wall to 0.15, and 85mm grey forms round a 200mm core give 0.17. The concrete adds almost nothing: 150mm of it is worth about 0.07 m²K/W, against nearly 6 for the two layers of EPS. Thin forms are the trap: 50mm of XPS each side gives about 0.31, which misses even the 0.26 limiting value.

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The build-up (outside → inside)

Forms and λ from NSAI Agrément certificate 07/0277 (Amvic, revised April 2026), whose Table 1 gives 0.16 / 0.15 for the 350mm block in white / grey EPS and 0.19 / 0.17 for the 370mm block; the XPS rows use Polarwall’s forms (BBA 14/5127). ICF forms take air-gap level 0 (BR 443 §4.8.1: no air voids within the insulation).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
2 × 100mm white EPS, 150mm core≈ 0.16As pre-filled; NSAI 07/0277 gives 0.16
2 × 100mm grey EPS, 150mm core≈ 0.15NSAI 07/0277: 0.15
2 × 85mm grey EPS, 200mm core≈ 0.17NSAI 07/0277: 0.17
2 × 85mm white EPS, 200mm core≈ 0.19NSAI 07/0277: 0.19; misses 0.18
2 × 75mm grey EPS, 150mm core≈ 0.19NSAI 07/0277: 0.20, with its allowance for the plastic ties
2 × 50mm XPS, 150mm core≈ 0.31Misses the 0.26 limiting value
150mm + 50mm XPS, 150mm core≈ 0.17A thicker outer form only
2 × 100mm white EPS behind a brick outer leaf and 50mm cavity≈ 0.15

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

All the insulation is in the two forms, so their thickness and grade decide the result: graphite EPS (λ 0.031) is about a tenth better than white (λ 0.034). The forms interlock and the concrete is cast against them, so there are no air voids to correct for: U-Monkey gives ICF BR 443’s air-gap level 0. The plastic webs that tie the two forms together conduct very little, and BR 443 treats non-metal ties as negligible (§4.8.2); the NSAI certificate does make a small allowance for them, which puts its figures up to about 0.01 above U-Monkey’s for the thinner blocks. What the wall does not do is give the room thermal mass: the inner form insulates the concrete from it, so the heat capacity facing the room is about 12 kJ/m²K, against about 160 for a dense-block inner leaf with wet plaster.

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 an ICF wall (insulating concrete formwork)?

An insulating concrete formwork wall with 100mm of white EPS either side of a 150mm concrete core, rendered outside and lined with plasterboard, has a U-value of about 0.16 W/m²K — the figure NSAI Agrément gives for that block (certificate 07/0277). Graphite (grey) EPS takes the same wall to 0.15, and 85mm grey forms round a 200mm core give 0.17. The concrete adds almost nothing: 150mm of it is worth about 0.07 m²K/W, against nearly 6 for the two layers of EPS. Thin forms are the trap: 50mm of XPS each side gives about 0.31, which misses even the 0.26 limiting value.

Is an ICF wall good for thermal mass?

Not much, as the room sees it. The concrete core is heavy, but the inner EPS form separates it from the room, so very little of it takes part in the daily cycle: U-Monkey’s ISO 13786 figures give this wall an internal heat capacity of about 12 kJ/m²K, against about 160 for a brick-and-dense-block cavity wall with wet plaster. The mass still damps temperature swings through the wall itself — its decrement factor is very low — but for rooms that resist overheating, look to the floors and internal walls.

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.

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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