A loft-conversion roof with 120mm PIR between the rafters and 25mm continuous PIR beneath them achieves a U-value of about 0.19 W/m²K; moving to 100mm between plus 50mm under reaches about 0.17. The continuous under-layer matters more than its thickness suggests because it breaks the rafter bridge.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- Tiles + ventilated batten space
- Breather membrane
- 100–120mm PIR between rafters (~12% timber)
- 25–50mm continuous PIR under rafters
- Plasterboard
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 120mm between + 25mm under | ≈ 0.19 | Common conversion spec |
| 100mm between + 50mm under | ≈ 0.17 | Better bridge-breaking for the same total |
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
Rafter depth limits what fits between, and a 50mm ventilated air path (or a suitable membrane strategy) must be kept above the insulation. The under-rafter layer is continuous, so shifting thickness from "between" to "under" usually wins.
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
What is the U-value of insulation between rafters (loft conversion)?
A loft-conversion roof with 120mm PIR between the rafters and 25mm continuous PIR beneath them achieves a U-value of about 0.19 W/m²K; moving to 100mm between plus 50mm under reaches about 0.17. The continuous under-layer matters more than its thickness suggests because it breaks the rafter bridge.
How much headroom will rafter insulation cost me?
Typically 40–65mm below the rafters (continuous board plus plasterboard). That is exactly the trade the between/under split controls — model both splits in U-Monkey and check the headroom against the U-value you need.
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 a warm flat roof
- How much insulation does a flat roof need?
- 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 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.