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.

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. With a warm roof moisture is isolated before it can reach the roof deck meaning condensation can t form and weaken the structure. Rising along the underside of the angled purlin until it reaches a gap then rising up to the next purlin and so on to exit at the ridge or gable. This changes the dew point where warm and cold air streams merge and turn to liquid.

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. A warm roof uses solid pir insulation boards that sit above the roof deck topped by moisture tolerant boards. Ventilation air follows exactly the opposite path. The amount of ventilation or air change needed can be calculated by.

Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. Ventilation is not needed in a warm roof.