Warm Roof Air Gap

An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it.

Warm roof air gap.

A cold roof is where all of the insulation is placed either between and under or completely under the joists or deck. A warm roof uses solid pir insulation boards that sit above the roof deck topped by moisture tolerant boards. This changes the dew point where warm and cold air streams merge and turn to liquid. With a warm roof moisture is isolated before it can reach the roof deck meaning condensation can t form and weaken the structure.

There should be a ventilation gap created to allow air from the outside to be drawn into this space. A sealed warm roof construction can also be achieved by creating a 50mm ventilated air gap between the surface of the insulation and the undertile lining to allow water vapour to be safely removed from the roof construction. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. The amount of ventilation or air change needed can be calculated by.

The map at right which is based on table r806 4 of the irc lists the minimum r values required to prevent condensation in unvented assemblies in various climate zones. Ventilation is not needed in a warm roof.