Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16

Working backwards from the target U-value to a PIR thickness.

To reach the common 0.15 W/m²K target, a warm flat roof needs roughly 150mm of PIR; the 0.16 limiting value needs about 140mm, and the 0.11 new-build notional pushes past 200mm. U-Monkey’s "Hit a target U-value" solver does this backwards calculation for you against your exact build-up.

Open this build-up in U-Monkey — pre-filled, free

Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report

The build-up (outside → inside)

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Target 0.16 (limiting value)~140mm PIRMinimum for a new thermal element
Target 0.15~150mm PIRThe usual extension spec
Target 0.11 (notional)~210mm PIRNew-build performance

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

Lambda sets the exchange rate: phenolic (0.020) saves ~10% thickness over PIR (0.022); mineral wool (0.038) nearly doubles it. Upstands, rooflights and falls all eat into the build-up height — solve for the real target before the joists are ordered.

Does the thickness change the condensation risk?

Hardly — in a warm deck the layer order does the work. Run this build-up through U-Monkey’s free BS EN ISO 13788 check at 100mm, 140mm and 210mm of PIR and the result barely moves: condensation under the waterproofing peaks at about 7 g/m² in spring and 3–4 g/m² is left at the end of the year, so with the waterproofing at BS 5250’s figure for bitumen felt, sd 90 m, over a 500-gauge polythene VCL, it fails at every thickness. What moves it is the vapour control, not the insulation: a high-performance VCL clears it, and the check’s How to fix it gives the rating it needs, lower once you enter the membrane’s declared value.

Where thickness does matter is when it is split to save height. Put 60mm of PIR above the deck and 150mm of mineral wool between the joists below it, leave the vapour control layer on the deck, and the check puts heavy condensation on the underside of the deck every winter, more than dries out by summer: a fail, with its own timber check warning about the deck. Move the VCL down to the ceiling and the amount falls to a few grams, but it still does not clear each year, so that fails too. Solve the thickness first, then check the build-up you will actually build.

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

Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16?

To reach the common 0.15 W/m²K target, a warm flat roof needs roughly 150mm of PIR; the 0.16 limiting value needs about 140mm, and the 0.11 new-build notional pushes past 200mm. U-Monkey’s "Hit a target U-value" solver does this backwards calculation for you against your exact build-up.

Can U-Monkey tell me the thickness directly?

Yes — build the roof, press "Hit a target U-value…", pick the target, and it solves the insulation thickness for you. Free, in the browser.

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.

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