U-value of a cold flat roof converted to a warm roof

Re-roofing a cold deck flat roof: insulation above the deck, the old wool kept or taken out, and the one-third rule.

A cold flat roof with 100mm of mineral wool between the joists and a ventilated void has a U-value of about 0.45 W/m²K. Converting it to a warm roof — PIR above the deck under new waterproofing, the vents sealed — with 120mm of PIR and the old wool kept takes it to about 0.13, well inside the 0.16 Approved Document L asks of a renovated roof. Keeping the wool makes it a hybrid roof, and the deck stays warm only while enough of the insulation is above it: BS 5250’s rule of thumb is no more than a third of the thermal resistance under the vapour control layer, which takes about 110mm of PIR over 100mm of wool, counting the insulation alone. Take the wool out and 150mm of PIR above the deck gives about 0.14.

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The build-up (outside → inside)

Joists at 12%, bridging both the old wool and the sealed void above it. The “before” row has the void open to outside air, so the deck and waterproofing above it are not counted (BS EN ISO 6946 6.9.4). The one-third rule is as BS 5250:2021 Table 4 footnote B is quoted by Rob Firman in AT Journal 148 (Winter 2023), reproduced on Designing Buildings (“Hybrid flat roofs”); Premier Guarantee’s rule is from its technical manual (version 12). The pre-filled link opens as an upgraded existing roof, judged against Approved Document L’s 0.16 (2021 edition, Table 4.3).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Before: cold roof, 100mm mineral wool between the joists, ventilated void≈ 0.45Everything above the void is ignored
Hybrid: 70mm PIR above the deck, the wool kept≈ 0.18Too little above: U-Monkey flags the deck as cold
Hybrid: 90mm PIR above≈ 0.16The deck stays warm, but misses the one-third rule
Hybrid: 110mm PIR above≈ 0.14One-third rule just met
Hybrid: 120mm PIR above≈ 0.13As pre-filled
Hybrid: 140mm PIR above≈ 0.12
Warm roof, wool taken out: 140mm PIR above the deck≈ 0.15Meets 0.16: 0.153
Warm roof, wool taken out: 150mm PIR above the deck≈ 0.14

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

Once the deck is warm, the insulation above it does the work, and the wool below adds a little if it can stay. BS 5250:2021 calls for a condensation risk calculation whenever insulation is split above and below the deck, and gives a rule of thumb that no more than a third of the thermal resistance should be on the warm side of the vapour control layer (as quoted in industry guidance on hybrid roofs); Premier Guarantee’s technical manual asks for at least as much resistance above the deck as below, with a condensation calculation. U-Monkey checks the deck itself: with 70mm of PIR over the wool the underside of the deck falls to about 11.4 °C in February, below the room air’s 11.9 °C dew point, so it reports a cold deck; with 110mm or more the deck stays more than a degree above it (13.3 °C). Bauder advises that the existing insulation may need to come out, and that the ventilation under the deck has to be sealed off — a void left vented under a warm deck puts the new insulation outside the ventilation, where it does nothing for the rooms below. Height and load are the usual limits: where the frame can’t carry the load or the upstands aren’t high enough, Approved Document L allows a lesser standard for a flat roof (Table 4.3, note 4).

Condensation risk in a converted flat roof

Putting insulation above the deck moves the deck to the warm side, which is the point of the conversion — but only if enough of the insulation is above it. U-Monkey’s free BS EN ISO 13788 check on the pre-filled roof finds a few grams of seasonal condensation under the new waterproofing (about 6.5 g/m² in March) that evaporates in summer, and gives a conditional pass that depends on three assumed values: the membrane’s vapour resistance, the PIR’s foil facings and the vapour control layer as installed. It also notes that the deck stays above the room air’s dew point (13.7 °C against 11.9 °C in February) only while the insulation above it is at least as modelled.

With 70mm of PIR over the wool, the check reports a cold deck instead, outside the scope of a calculation: room air leaking into the joist zone would condense on the underside of the deck, and the calculation, which can’t see air leakage, already finds about 280 g/m² there in February. The same applies to a vented void left open under the new deck. Seal the old ventilation, put the vapour control layer on the deck under the new insulation, and don’t thin the insulation above the deck on site.

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 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 cold flat roof converted to a warm roof?

A cold flat roof with 100mm of mineral wool between the joists and a ventilated void has a U-value of about 0.45 W/m²K. Converting it to a warm roof — PIR above the deck under new waterproofing, the vents sealed — with 120mm of PIR and the old wool kept takes it to about 0.13, well inside the 0.16 Approved Document L asks of a renovated roof. Keeping the wool makes it a hybrid roof, and the deck stays warm only while enough of the insulation is above it: BS 5250’s rule of thumb is no more than a third of the thermal resistance under the vapour control layer, which takes about 110mm of PIR over 100mm of wool, counting the insulation alone. Take the wool out and 150mm of PIR above the deck gives about 0.14.

Should I convert my cold flat roof to a warm roof?

Usually, when it needs re-covering. BS 6229:2018, the flat roofing code of practice (revised in 2025), noted that cold deck roofs are not now recommended, and BS 5250:2021 says they should not be used where the void spans more than 5m, can’t be cross-ventilated from both sides or can’t have a sealed vapour control layer — so a longer span being refurbished should be converted to a warm or inverted roof. In England, replacing the waterproofing over more than half the roof is renovating a thermal element (regulations 2 and 23), which brings in the 0.16 W/m²K of Approved Document L’s Table 4.3 where it is technically, functionally and economically feasible. A warm conversion meets that with about 140mm of PIR above the deck, or less if the wool below can stay as a properly checked hybrid.

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

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Floors

Different build-up? See the full flat roof U-value calculator, or start from scratch in the main U-value calculator.

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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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