Warm Roof

A warm roof is a roof in which the insulation is placed at rafter level (in pitched roofs) or above the structural deck (in flat roofs), rather than at the ceiling. The timber or steel structure is therefore on the warm side of the insulation, and any roof space is within the conditioned or semi-conditioned volume of the building. The term is common in the UK and Europe; in North America related designs are called unvented or conditioned attics, or compact and insulated roof assemblies.

Pitched warm roofs

In UK practice the insulation runs along the line of the rafters, between and/or over them, and an air and vapour control layer (AVCL) is placed on the warm side. Where a high-resistance underlay is used, ventilated voids are provided between underlay and insulation; with a suitable low-resistance (vapour-permeable) underlay, the void may be omitted according to the product's certification. Counter-battens over rigid insulation boards can carry the tiling battens.

Warm pitched roofs are popular for loft conversions and rooms in the roof, because the whole space under the slope can be used.

Flat warm roofs

In a warm flat roof, rigid insulation boards are laid above the deck and a vapour control layer, with the waterproof membrane on top. In an inverted (or "upside-down") warm roof, the insulation goes above the membrane, protected by ballast or paving, so the membrane is shielded from temperature extremes.

North American unvented attics

The US IRC allows unvented attic assemblies under specific conditions in section R806.5, typically requiring air-impermeable insulation, or a combination of insulation types, directly against the underside of the roof deck so the deck stays above the dew point.

Considerations

Warm roofs generally need careful air sealing, correct vapour control, and adequate insulation thickness, which can raise the roof line when insulation is placed over rafters.