U-value of an insulated composite roof panel

Trapezoidal steel-faced roof panels with a PIR or stone wool core: declared U-values by thickness, and what Part L needs.

An insulated composite roof panel with a 115mm PIR core — Kingspan’s QuadCore KS1000RW trapezoidal panel, λ 0.018 — has a declared U-value of 0.16 W/m²K. That meets both roof limits in Approved Document L Volume 2 (2021): 0.18 for a flat roof, which the 100mm panel meets at its declared 0.18, and 0.16 for a pitched one; the 2026 edition, from 24 March 2027, sets 0.18 for every roof. The thickness is the core at the troughs — the 115mm panel is 146mm over the crowns — and the U-value is the maker’s: BR 443 needs a numerical analysis of the joints, and a layer-by-layer sum at the core thickness reads low: 0.176 for the 100mm core (declared 0.18) and 0.42 for the 40mm one (declared 0.47), the thinner the panel the more the joints count.

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The build-up (outside → inside)

The pre-filled link opens in the England · non-domestic regime with the panel as one layer, “Kingspan QuadCore KS1000RW roof panel (115mm core)”, at its declared U-value: U-Monkey takes the panel’s resistance from the U you give it, R = 1/U − (Rsi + Rse), here 1/0.16 − 0.14 = 6.11 m²K/W. Pick another core thickness in its row and U-Monkey fills in Kingspan’s declared U for it; Tata Steel’s Trisomet and Joris Ide’s panels are in the library too, and for any other maker’s panel add the generic one and type its declared U. Declared values are rounded to two places, and the result is no more precise than that.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
KS1000RW, 80mm core0.23Fails 0.18
KS1000RW, 91mm core0.20
KS1000RW, 100mm core0.18Meets the flat-roof 0.18 at its declared value
KS1000RW, 115mm core0.16As pre-filled: meets the 2021 pitched-roof 0.16
KS1000RW, 120mm core0.15Meets the side-lit notional roof, 0.15
KS1000RW, 150mm core0.12
Tata Steel Trisomet, 120mm core0.16Datasheet (2025); 100mm: 0.20
Joris Ide JI Roof 1000, 120mm PIR0.18Stated basis “EN 14509:2013”; 100mm: 0.21
Stone wool core: Joris Ide JI Vulcasteel Roof, 240mm0.18A2-s1,d0 in all but two of its coatings; 200mm: 0.21
Layer by layer: steel, 100mm core at λ 0.018, steel≈ 0.18Reads low even here (0.176; air gaps level 0, as a foamed core has none)
Layer by layer at 40mm≈ 0.42Reads low: declared 0.47

The panel rows are the makers’ declared U-values, which the pre-filled calculator turns into the panel’s resistance (R = 1/U − (Rsi + Rse), upward heat flow) and gives back unchanged; the layer-by-layer rows are the BS EN ISO 6946 combined method with BR 443 conventions at the core thickness and air-gap level 0 (a foamed core has no gaps), shown for comparison only. Declared values are rounded to two places. Joris Ide and Tata Steel figures are from their UK product pages and data sheets, read 13 October 2026.

What changes the number

As for a wall panel, BR 443 (2019) says a metal-faced composite roof panel cannot be summed layer by layer: its roof table asks for numerical analysis “to take account of joints between panels, if it is thinner at joints or if metal fully or partly penetrates insulation, and to take account of profiled sheets”. Panel makers declare U-values to BS EN 14509 — whose Annex A.10 adds the longitudinal joint, and in its detailed method the effect of the profiled faces, which its simplified method ignores — and many model them to BS EN ISO 10211; Kingspan says its KS1000RW values are “calculated using the method required by the Building Regulations Part L2”, without saying which. A layer-by-layer sum at the core thickness leaves the joints and through-fixings out and reads low: 0.176 for a 100mm core declared at 0.18, and 0.42 for a 40mm one declared at 0.47 — on a thin panel the joints count for more. Compare panels on their declared values by core thickness: at 120mm KS1000RW (λ 0.018) declares 0.15, Tata Steel’s Trisomet 0.16 and Joris Ide’s JI Roof 1000 0.18. A stone wool core (λ about 0.040–0.045), for A2-s1,d0 fire performance, needs about twice the depth — Joris Ide’s Vulcasteel roof panel reaches 0.18 at 240mm. Approved Document L Volume 2 (2021) sets 0.18 for a flat roof and 0.16 for a pitched one, and its Table 4.1 gives no dividing pitch; U-Monkey treats a low-pitched panel roof as flat, so check which your building control body applies. The 2026 edition, which takes effect on 24 March 2027 (24 September 2027 for higher-risk building work, with transitional arrangements), has one 0.18 for every roof. For a new building the test is the whole-building calculation — the BER and BPER against the TER and TPER — with the NCM notional roof at 0.15 for a side-lit building and 0.18 for a top-lit one. Rooflights are a separate element (2.2 in Table 4.1), and over-roofing an old roof is renovation, with 0.18 the improved value (Table 4.2).

Condensation in composite roof panels

U-Monkey’s free BS EN ISO 13788 check finds no interstitial condensation in the panel: with steel on both faces nothing diffuses in. What it cannot see is the joints, where vapour and air get through if anywhere; the side-lap seal belongs on the liner (warm) side, with the outer seals for weather.

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, a vapour-open underlay, a ventilated void, a warm deck — 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 an insulated composite roof panel?

An insulated composite roof panel with a 115mm PIR core — Kingspan’s QuadCore KS1000RW trapezoidal panel, λ 0.018 — has a declared U-value of 0.16 W/m²K. That meets both roof limits in Approved Document L Volume 2 (2021): 0.18 for a flat roof, which the 100mm panel meets at its declared 0.18, and 0.16 for a pitched one; the 2026 edition, from 24 March 2027, sets 0.18 for every roof. The thickness is the core at the troughs — the 115mm panel is 146mm over the crowns — and the U-value is the maker’s: BR 443 needs a numerical analysis of the joints, and a layer-by-layer sum at the core thickness reads low: 0.176 for the 100mm core (declared 0.18) and 0.42 for the 40mm one (declared 0.47), the thinner the panel the more the joints count.

Is a roof panel’s U-value quoted at the core or the overall thickness?

At the core thickness, measured at the troughs: Kingspan’s KS1000RW table runs from a 40mm core (71mm overall) to 150mm (181mm), the crowns adding 31mm, and Tata Steel’s Trisomet table is by core thickness too. Use the declared U for your core thickness: a layer-by-layer sum at the core thickness leaves the joints out and reads low (0.176 for a 100mm core declared at 0.18, 0.42 against 0.47 at 40mm). In U-Monkey, pick the maker’s panel and its core thickness and the declared U fills itself in (Kingspan’s KS1000RW, Tata Steel’s Trisomet and Joris Ide’s roof panels are in the materials); for any other panel, add “Insulated composite panel (the maker’s declared U-value)” and type the declared U for your core thickness.

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 — U-Monkey takes a factory-made composite panel at the maker’s declared U-value, as BR 443 (2019) §4.9 requires a numerical analysis of its joints and fixings (BS EN 14509); everything else uses the combined method of BS EN ISO 6946:2017 with BR 443 conventions, and BS EN ISO 13788 for the condensation check.

Other common build-ups

Roofs

Walls

Floors

Different build-up? See the full flat roof 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.

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