Warm vs Cold Roof Calculator — Insulation Comparison

In a warm roof, insulation is placed above the rafters (or on top of the roof deck), keeping the entire roof structure inside the thermal envelope with no ventilated void. In a true cold roof (NHBC Standards 7.2.15 for pitched roofs / BS 5250:2021), insulation sits between the rafters only, with a ventilated void above it (50mm minimum air gap, eaves + ridge ventilation). If insulation is added both between and above rafters, that's a hybrid buildup — not a cold roof. Warm roofs have the lowest condensation risk; cold roofs are cheaper but need correct ventilation detailing.

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.

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.

Yes — BS 5250:2021 requires a ventilated void above cold-roof insulation to prevent condensation. Two distinct figures apply and should not be confused: (1) a 50mm minimum clear ventilated air gap, which is the void DEPTH maintained between the top of the insulation and the underlay; and (2) the equivalent continuous free ventilation OPENING area at the eaves, which is pitch-dependent — 10mm continuous for a standard cold pitched roof (pitch over 15°) and 25mm continuous for a low-pitch roof (10–15°). A further 5mm continuous high-level (ridge) ventilation is needed where the pitch exceeds 35° or the span exceeds 10m, and for mono-pitch roofs ventilate at both high and low level. A vapour-permeable (LR-type) underlay can reduce ventilation requirements, but always follow the membrane manufacturer's BBA-certified detail. Without correct ventilation, condensation will rot the timbers.

Part L 2021 limiting (backstop) U-value for new-build pitched roofs at ceiling level is 0.16 W/m²K; the Part L 2021 notional dwelling roof (the compliance-modelling target) is 0.11 W/m²K. The default 0.13 used here is a typical design target that betters the limiting value. For roof renovations the limiting value is also 0.16. Passivhaus targets 0.10–0.12. For a warm roof with PIR (λ = 0.022) you need about 165mm to reach 0.13 W/m²K — 150mm PIR only reaches about 0.14. For a cold/hybrid build with mineral wool between 150mm rafters plus 100mm quilt over, you achieve about 0.15 W/m²K.