Roof Purlin Metal Building Air Gap

Warm wet air rises and meets the cold roof and cools.

Roof purlin metal building air gap.

Metal roof purlins are installed horizontally under metal roofs. The purpose of the purlins is to provide a solid foundation for the metal roofing panels and trim to be screwed to. They give added support to the roof and also provide a nailing surface for the end panels and drip edge. If you do not completely fill purlin cavities code requires airflow from eave to ridge over top of the insulation.

Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. On a sloped metal roof there is a vent at the bottom to allow air in and natural convection takes air at the top out notes wingfield. But probably the best benefit that comes from using purlins is the air gap it creates between the metal roof and the existing shingle roof. You would have to seal the ridge vent it isn t working anyhow because your building does not have an air intake from enclosed vented soffits.

Do all roofing projects require purlins. Purlins are 2 by 4 feet and are installed much like metal roofing. The metal will be attached to at least 3 4 purlin and 1 2 plywood instead of just 1 2 plywood which should make the overall roofing system stronger. This lets any condensation on the underside of the metal roof evaporate.

They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top. Warm air is actually lighter than cold air known as avogadro s law essentially condensation occurs at night when the temperature inside a building is warmer than outside. Purlins are non pressure treated 1 x 4 x 12 lumber that is installed on to the roof in a specific layout pattern and screw pattern. The air space allows natural ventilation when there is a vent at the low part of the roof structure and another at the high part of it.

You don t even want to go there. Research has confirmed that creating an air space under a metal roofing system will increase energy savings during both summer and winter months. 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. You have no way to achieve this without a major remodel.

Warm air holds more moisture than cold air. In the case of sips it s essential in my view to have an air channel between the roofing underlayment and the metal roofing. Ventilation air follows exactly the opposite path. Getting a perfectly sealed vapor barrier under purlins would be nearly impossible to achieve.

The colder the climate the more essential the air gap.