What is the difference between a warm and cold roof?

In a warm roof, insulation is placed above the rafters (or on top of the roof deck), keeping the entire roof structure warm and within the building envelope. In a cold roof, insulation is placed between the rafters (and sometimes over ceiling joists), leaving the roof void cold and ventilated. Warm roofs have lower condensation risk; cold roofs are cheaper but need careful ventilation detailing.

Which is better — warm or cold roof?

Warm roofs are technically superior: lower condensation risk, fewer thermal bridges, and they make the loft space usable. However, they're more expensive and raise the roof height (planning implications). Cold roofs are cheaper and easier for loft insulation top-ups. For loft conversions, a warm or hybrid approach is usually best. For new builds, warm roofs are increasingly specified.

What is a hybrid roof insulation approach?

A hybrid approach combines insulation between the rafters (mineral wool or rigid board) with additional insulation above the rafters (PIR board). This minimises the depth of insulation above the rafters while still achieving target U-values. It's popular for loft conversions where headroom is limited but thermal performance must meet Part L.

Does a cold roof need ventilation?

Yes — Building Regulations require ventilation above cold roof insulation to prevent condensation. At the eaves, provide a 25mm continuous gap (or equivalent). At the ridge, provide 5mm continuous gap. For mono-pitch roofs, ventilate at both high and low level. Use breathable roofing membrane (Type LR) to reduce ventilation requirements. Without proper ventilation, condensation will damage timbers.

What U-value should a roof achieve?

Part L 2021 requires a maximum U-value of 0.13 W/m²K for new build roofs and 0.16 for roof renovations. Passivhaus targets 0.10-0.12. For a warm roof with PIR (λ = 0.022), you need about 150mm. For a cold roof with mineral wool between 150mm rafters plus 100mm over, you can achieve about 0.15 W/m²K.

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Warm vs Cold Roof Calculator — Insulation Comparison

Compare warm roof (insulation above rafters) vs cold roof (between + over) buildups. Calculates U-values, condensation risk, and insulation volumes.

Roof Area

Rafter/slope area

Roof Type

Rafter Depth

150mm typical (6×2)

Target U-Value

Price Per m² (optional)

Cold roof base price

How We Calculate This

This calculator compares warm and cold roof insulation strategies by calculating the required thickness and U-value for each approach given the same target performance.

The formula

Required R = (1 ÷ Target U) - Existing R - Surface R

Warm: Thickness = R × λ (0.022) × 1000

Cold: Between = Rafter depth, Over = Remaining R × λ (0.044)

For U = 0.13: R needed = 7.69 - 0.2 - 0.14 = 7.35 m²K/W. Warm: 7.35 × 0.022 = 162mm PIR. Cold: 150mm between (R = 4.29) + 135mm over (R = 3.07).

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.