U-value of a garage conversion roof (flat roof insulated from below)

Upgrading a flat garage roof from inside when the garage becomes a room: between and under the joists, or a warm deck.

Insulating a flat garage roof from inside as part of a garage conversion — 100mm of PIR between 150mm joists, leaving a 50mm ventilated gap under the deck, and 72.5mm of PIR insulated plasterboard (60mm PIR) under the joists with its joints taped — gives a U-value of about 0.16 W/m²K: the 0.16 Approved Document L asks of a roof kept in a garage conversion (paragraph 4.12(b), Table 4.3), and the figure Celotex’s data sheet gives for it. 150mm of PIR between the joists alone, a common specification, reaches only 0.22, because the joists bridge it through its full depth. Building control guides usually prefer a warm deck where the roof is being re-covered anyway: 150mm of PIR over the deck gives 0.14.

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The build-up (outside → inside)

Joists at 400mm centres taken at 11.7% timber, the figure on Celotex’s Flat Roofing Application Data Sheet “Garage Conversion – Between and Under Joist” (November 2018); BR 443’s general rule, width over spacing plus 0.01 (12.75% for 47mm joists), gives 0.1605 for the pre-filled roof, a whisker over 0.16, so 65mm under the joists (0.155) is the specification with a margin; insulated plasterboard entered as PIR (λ 0.022) and 12.5mm plasterboard. Everything outside the ventilated gap is disregarded (BS EN ISO 6946 §6.9.4). Warm-deck rows: felt, PIR and a polythene vapour control layer on the old 18mm deck, the joist void below unventilated. The pre-filled link opens as an upgraded element.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
As it is: felt and deck on empty joists, plasterboard ceiling≈ 1.83Far worse than the 0.35 threshold
150mm PIR between 150mm joists only, 50mm vented gap above (a common spec)≈ 0.22The joists bridge it: misses 0.16
100mm PIR between + 50mm PIR under the joists≈ 0.17Celotex: 0.17
100mm between + 60mm under≈ 0.16As pre-filled (0.159; 0.1605 at BR 443’s 12.75%); Celotex: 0.16
100mm between + 65mm under≈ 0.15Celotex: 0.15
175mm joists: 120mm between + 50mm under≈ 0.15Celotex: 0.15
Warm deck instead: 140mm PIR over the deck, joists left empty≈ 0.15Meets 0.16 (0.152)
Warm deck: 150mm PIR over the deck≈ 0.14Meets the 0.15 for a new roof too

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 joists decide how much the between-joist insulation is worth: they run through its full depth (11.7% at 400mm centres on Celotex’s sheet), so 150mm of PIR between them only reaches 0.22, while 100mm between plus a continuous 60mm under them reaches 0.16 — the layer under the joists covers the timber. The gap above the insulation is the other rule: a cold flat roof needs at least 50mm of ventilated space under the deck, cross-ventilated (Celotex asks for “a 50mm minimum ventilation gap above any original insulation”), which is why only 100mm of PIR fits between 150mm joists. Everything above that gap — deck and felt — is disregarded, as BS EN ISO 6946 does for layers outside a well-ventilated air space. Celotex’s garage-conversion data sheet gives, for 150mm joists with 100mm between, 0.19, 0.17, 0.16 and 0.15 with 40, 50, 60 and 65mm of PIR in the insulated plasterboard below, which U-Monkey reproduces. The regulations make the target: a garage roof kept in a conversion is a retained element (Approved Document L paragraph 4.12(b)), to be upgraded to 0.16 if it is worse than 0.35 (Table 4.3), with a lesser standard allowed where the frame’s load-bearing capacity or the upstand heights prevent it (note 4); a new or rebuilt roof is a new element at 0.15 (Table 4.2). Building control guides such as CNC’s say that “a warm roof construction is the best option” where it can be done — insulation over the deck, the void below left unventilated — and BS 6229, as IKO summarises it, no longer recommends cold decks.

Condensation risk in a converted garage roof

A ventilated cold flat roof is one of the constructions BS 5250:2021 covers with prescriptive guidance rather than a calculation, so U-Monkey’s free BS EN ISO 13788 check reports the pre-filled roof as outside a calculation’s scope, with the Glaser result (no interstitial condensation in any month) shown for information. The guidance is the ventilated gap above the insulation — open at both ends, which is hard where the joists run across a garage — and a continuous, taped vapour control layer at the ceiling.

The warm-deck rows are calculated, and they show the vapour control layer has to suit the membrane: with polythene under a bitumen felt, the check finds condensate building up (3.5 g/m² still there at the end of the year). Under a felt or bitumen membrane, use a high-performance vapour control layer, as the warm flat roof page explains.

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 garage conversion roof (flat roof insulated from below)?

Insulating a flat garage roof from inside as part of a garage conversion — 100mm of PIR between 150mm joists, leaving a 50mm ventilated gap under the deck, and 72.5mm of PIR insulated plasterboard (60mm PIR) under the joists with its joints taped — gives a U-value of about 0.16 W/m²K: the 0.16 Approved Document L asks of a roof kept in a garage conversion (paragraph 4.12(b), Table 4.3), and the figure Celotex’s data sheet gives for it. 150mm of PIR between the joists alone, a common specification, reaches only 0.22, because the joists bridge it through its full depth. Building control guides usually prefer a warm deck where the roof is being re-covered anyway: 150mm of PIR over the deck gives 0.14.

What U-value does a garage conversion roof need?

In England, 0.16 W/m²K for a flat garage roof that is kept and upgraded: Approved Document L treats elements retained in a garage conversion as upgrades (paragraph 4.12(b)) and asks for 0.16 where the roof is worse than 0.35 (Table 4.3). If the roof is rebuilt, the new roof needs 0.15 (Table 4.2). From inside, 100mm of PIR between 150mm joists with 72.5mm insulated plasterboard below meets 0.16 on Celotex’s figures, and 77.5mm (65mm of PIR) leaves a margin; a warm deck with 150mm of PIR over the existing deck meets 0.15.

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

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? 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.

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