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.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- Waterproof membrane
- PIR (thickness = the question)
- VCL
- 18mm deck
- Ceiling
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| Target 0.16 (limiting value) | ~140mm PIR | Minimum for a new thermal element |
| Target 0.15 | ~150mm PIR | The usual extension spec |
| Target 0.11 (notional) | ~210mm PIR | New-build performance |
Figures computed with the BS EN ISO 6946 combined method (bridging included) using BR 443 conventional lambda values — indicative for the generic build-up shown. Your products, thicknesses and (for floors) footprint will move the number: 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.
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):
Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.
Frequently asked questions
How much insulation does a flat roof need??
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) and the condensation-risk screening check 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 and air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- U-value of a cavity wall with full-fill insulation
- U-value of a cavity wall with PIR partial-fill insulation
- U-value of a timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of 270mm loft insulation
- U-value of insulation between rafters (loft conversion)
- U-value of a warm flat roof
- U-value of an insulated concrete ground floor
- U-value of an insulated suspended timber floor
- U-value of a beam and block floor with PIR
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.