What U-value does a loft conversion roof need?

Building Regulations Part L (2021) requires a U-value of 0.15 W/m²K for a converted loft roof (sloping ceiling). This is the element U-value target. With PIR insulation (λ=0.022), this requires approximately 150mm total thickness.

Should insulation go between or over the rafters?

Ideally both. Fill the rafter depth with insulation between rafters, then add a layer over the rafters (or under, using counter-battens) to reach the required U-value and reduce thermal bridging through the timber. A continuous layer over the rafters is more thermally efficient than between alone.

Do I need a vapour control layer (VCL)?

Yes. A vapour control layer must be installed on the warm side (room side) of the insulation to prevent moisture from the room reaching cold surfaces where it could condense. Use a minimum 500-gauge polythene sheet or proprietary VCL, lapped 150mm and sealed at all edges with tape.

What is the best insulation for a loft conversion?

PIR rigid board (Celotex, Kingspan, Recticel) with λ=0.022 is the most space-efficient — achieving U=0.15 in about 150mm. Mineral wool is cheaper but needs about 230mm. Multifoil systems are thin but may need supplementary insulation to meet Part L. Always check Building Control approval.

Do I need a ventilation gap above the insulation?

If the roof has a breathable membrane (modern felt), no ventilation gap is needed above the insulation. If the roof has a non-breathable traditional bitumen felt, a 50mm ventilation gap must be maintained between the insulation and felt, with ventilation at ridge and eaves.

TradeCalculator

Loft Conversion Insulation Calculator — U-Value & PIR Thickness

Calculate insulation thickness between and over rafters to achieve Part L U-value of 0.15 W/m²K using PIR, mineral wool or multifoil.

Roof Slope Area

Total sloping ceiling area to insulate

Rafter Depth

Depth of existing rafters

Insulation Type

Target U-value

Part L 2021: 0.15 W/m²K

Insulation Price Per m² (optional)

Material cost per m²

How We Calculate This

This calculator determines insulation thickness needed to achieve the target U-value based on the chosen insulation type.

The formula

Thickness (mm) = (Lambda / Target U-value) × 1000

Between rafters = min(Rafter Depth, Total Thickness)

Over rafters = Total Thickness - Between Rafters

Insulation types

PIR board: λ 0.022 — ~150mm for U=0.15
Mineral wool: λ 0.035 — ~233mm for U=0.15
Multifoil hybrid: λ 0.030 — ~200mm for U=0.15

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.