Hybrid Roof Habitable Roof Space

his type of roof is most commonly used in loft conversions as any type of membrane can be used and because either there is no original roof membrane or it is a non-permeable one. This system should incorporate effective use of a vapour control barrier such as Permavent-DRY installed directly underneath the insulation.

The ventilation at eaves level must be connected to the ventilation at the ridge via a min 25mm gap (BS5250:2000) between the membrane and the insulation although good building practice has established a 50mm gap as the norm.

Non-permeable membranes can be prone to problems because any problems with the vapour barrier could result in interstitial condensation (especially on long rafters). The resulting condensation would then run down the insulation boards to the join and leak through to damage the finished ceiling.

Some specifies have, in recent years, designed a warm roof on new build with a 25mm vented space (in essence a hybrid) between the insulation and the breather membrane but there have been reports of interstitial moisture forming and leaking through the joins in the insulation, the BSi now recommends permanent sealing of all insulation joins in this system. This means that the use of a warm roof with Permavent laid directly on, and then ventilated, is now more preferable. Permavent used in conjunction with the Permavent-DRY vapour control would be a permanent and effective use of the hybrid system.