Framing Roof Vapor Barrier

A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter.

Framing roof vapor barrier.

Hvac units roof materials etc loads to the supporting joists or purlins. Vapor barriers are also a cold climate artifact that have diffused into other climates more from ignorance than need. Wind snow rain and dead ex. A roof design that includes an adhered roof membrane with multiple layers of insulation with board joints offset and staggered over a vapor retarder air barrier helps lower the risk that air and the moisture it carries will infiltrate the roof system.

The history of cold climate vapor barriers itself is a story based more on personalities than physics. Some moisture load in conditioned post frame buildings can be relieved via diffusion hence a vapor retarder in a ceiling rather than a vapor barrier through a roof assembly able to handle it example a vented attic in a moderately cold climate or mixed climate. In theory if a building houses a moisture producing activity such as a kitchen or locker rooms then a vapor barrier is essential to slow moisture from escaping into the roofing system. A vapor barrier under your slab will also protect you from harmful ground pollutants like radon gas.

It is frightening indeed that construction practices can be so dramatically. It serves as a structural member transferring the weight of both live ex. The two main types of substrates used with metal roofing systems are open framing and solid substrates composite or plywood. That air condenses on the cooler concrete floor.

To know if you need a vapor retarder ask yourself a few basic questions. Crawl spaces benefit from a polyethylene moisture barrier placed directly over the exposed earth. Understand these are climate specific recommendations. A vapor barrier under your concrete slab is essential for moisture prevention.

Rose 1997 regales readers of this history. That reduction of air and moisture infiltration can then help improve roof longevity. Post frame buildings with metal roofs and especially buildings with minimal insulation hold warm moist air. A vapor barrier against the concrete surface is generally recommended before wood framing or flooring materials are installed.

Class i vapor retarders i e. Vapor barriers can be installed in vented roof assemblies in climate zone 6 and climate zone 7 but should be avoided in climate zone 5 as top side condensation may occur in the summer months during air conditioning periods. Vapor barriers are a recommendation but not a requirement by a number of roofing panel system manufacturers.