U-value of a precast concrete sandwich panel

Two leaves of precast concrete with insulation cast between them: the U-value, and why the ties that hold the outer leaf matter.

A precast concrete sandwich panel — an 80mm outer leaf, 150mm of PIR insulation and a 150mm structural inner leaf, the leaves connected by glass-fibre composite ties — has a U-value of about 0.15 W/m²K, well inside Approved Document L Volume 2’s 0.26 for a wall and the 0.18 the NCM notional building takes for a side-lit building. The 230mm of concrete adds only 0.10 m²K/W; the insulation and the ties decide it. Ties below 1 W/mK, like glass-fibre composite connectors and the plastic ties FP McCann uses, need no correction (BR 443 §4.8.2), but steel connectors, which other precasters use, can matter a great deal: in a 2012 study of a similar panel, steel connectors took the measured U-value from 0.139 to 0.260.

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The build-up (outside → inside)

Ties taken as glass-fibre composite (λ below 1 W/mK), so no fixings correction (BS EN ISO 6946 Annex F; BR 443 §4.8.2); the PIR at BR 443’s air-gap level 1 for a single layer of boards. Opens in the England · non-domestic regime.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR≈ 0.220.217
120mm PIR≈ 0.180.184: misses the side-lit notional 0.18, unrounded
150mm PIR≈ 0.15As pre-filled (0.150)
200mm PIR≈ 0.120.116
150mm PIR at air-gap level 2≈ 0.180.178: a 2020 Estonian study recommends level 2 for sandwich panels with ties
150mm PIR at level 0 (two layers, staggered joints)≈ 0.140.141
200mm mineral wool slab (λ 0.038)≈ 0.19A non-combustible core (0.190)
150mm XPS (λ 0.034)≈ 0.220.222

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

The calculation itself is the plain BS EN ISO 6946 sum — both leaves and the insulation are continuous — and two things decide whether it holds. The ties. BR 443 (2019) §4.8.2 says ties with a conductivity below 1 W/mK “have very little effect on the calculated U-value”, and Ancon puts it plainly for its basalt-fibre wall ties: “Fixings with a lambda value of less than 1.0W/mK are excluded from u-value calculations”. Glass-fibre composite connectors (Thermomass, whose maker says steel conducts heat “at a rate over 50 times greater”) and Schöck’s Thermoanker (λ 0.7) are in that class, and FP McCann’s outer skins hang on “proprietary plastic ties”. Steel is another matter: a University College Dublin study (2012) of a 90 / 120 / 150mm phenolic-cored panel measured 0.139 W/m²K with no connectors, 0.150 with fibre-reinforced polymer ones and 0.260 with steel ones in a hot box, and a 2020 Estonian study of stainless steel diagonal ties found they added 8% with the insulation tight to them and 45–56% where 10mm air gaps formed beside them — against 4–39% by the ISO 6946 method — and recommended air-gap level 2 for such panels. With stainless anchors, get their χ from the maker or a BS EN ISO 10211 calculation. The leaves: Halfen gives 70mm as the minimum facing layer under the German national annex to Eurocode 2, with the load-bearing layer at least 50% thicker; UK projects have used 80mm outside, 200–450mm of insulation and 150–195mm inside (FP McCann’s University Locks panels are 80 / 250 or 450 / 195mm), and a precaster’s design guide for Wates gives 80 / 230 / 80mm at 0.145 with a stone wool core and steel connectors. That core is the choice where the fire rules for a building restrict combustible materials in its external walls, as PIR is combustible. The inner leaf also gives the room a lot of thermal mass: U-Monkey’s BS EN ISO 13786 figure for this panel is κ about 280 kJ/m²K.

Condensation risk in a precast sandwich panel

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in the pre-filled panel, and it stays that way with each assumed vapour value tested across its plausible range: the dense concrete inner leaf and the PIR keep vapour out of the core.

A mineral wool core is vapour-open: 200mm of it shows seasonal condensation against the back of the outer leaf in winter (a peak of 127 g/m²) that dries out in summer — a pass under BS EN ISO 13788, but water held against the concrete outer leaf, which a one-dimensional check cannot follow further.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer on the warm side, a more vapour-open layer on the cold side, more insulation to warm the inside surface — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 building'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 precast concrete sandwich panel?

A precast concrete sandwich panel — an 80mm outer leaf, 150mm of PIR insulation and a 150mm structural inner leaf, the leaves connected by glass-fibre composite ties — has a U-value of about 0.15 W/m²K, well inside Approved Document L Volume 2’s 0.26 for a wall and the 0.18 the NCM notional building takes for a side-lit building. The 230mm of concrete adds only 0.10 m²K/W; the insulation and the ties decide it. Ties below 1 W/mK, like glass-fibre composite connectors and the plastic ties FP McCann uses, need no correction (BR 443 §4.8.2), but steel connectors, which other precasters use, can matter a great deal: in a 2012 study of a similar panel, steel connectors took the measured U-value from 0.139 to 0.260.

Do the ties in a sandwich panel affect its U-value?

Plastic and glass-fibre composite ties barely do — below 1 W/mK BR 443 needs no correction for them. Steel can: one study measured a panel’s U-value rising from 0.139 to 0.260 W/m²K with steel connectors, and stainless diagonal ties added up to 56% where air gaps formed round them. With steel ties, enter their point thermal transmittance (χ) from the maker in the insulation layer’s fixings sheet, and consider air-gap level 2.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (Approved Document L Volume 2 for buildings other than dwellings, and the Welsh and Scottish equivalents), 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? For a building other than a dwelling, U-values feed the SBEM calculation that checks the building’s emission and primary energy rates against their targets (BER and BPER against TER and TPER) — and the non-domestic U-value page has the Volume 2 limits. 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