Warm Roof Air Gap

The amount of ventilation or air change needed can be calculated by.

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. There should be a ventilation gap created to allow air from the outside to be drawn into this space. Ventilation is not needed in a warm roof. With a warm roof moisture is isolated before it can reach the roof deck meaning condensation can t form and weaken the structure.

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. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. A warm roof uses solid pir insulation boards that sit above the roof deck topped by moisture tolerant boards.

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. A cold roof can be cheaper as ridged warm roof insulation tends to be more expensive a warm roof is sometimes the only option when it is difficult to get the required cross ventilation rigid warm roof insulation can allow for smaller timber depths as the 50mm air gap does not have to be maintained.