Framing Roof Vapor Barrier

To know if you need a vapor retarder ask yourself a few basic questions.

Framing roof vapor barrier.

Post frame buildings with metal roofs and especially buildings with minimal insulation hold warm moist air. The two main types of substrates used with metal roofing systems are open framing and solid substrates composite or plywood. It is frightening indeed that construction practices can be so dramatically. We ll add a vapor barrier so the moist air does not flow up through the insulation and condense on the roof.

The history of cold climate vapor barriers itself is a story based more on personalities than physics. Whether or not you need a vapor retarder hinges on three main factors your climate your home and the location of the wall you re insulating. It serves as a structural member transferring the weight of both live ex. Rose 1997 regales readers of this history.

That air condenses on the cooler concrete floor. Wind snow rain and dead ex. A vapor barrier under your concrete slab is essential for moisture prevention. A vapor barrier against the concrete surface is generally recommended before wood framing or flooring materials are installed.

Crawl spaces benefit from a polyethylene moisture barrier placed directly over the exposed earth. Add a preinstalled membrane to absorb and release the condensation as it forms with the natural weather cycle traditionally we ve insulated the roof panel to try and keep the temperature on the panel from reaching the dew point. 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.

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. That reduction of air and moisture infiltration can then help improve roof longevity. Understand these are climate specific recommendations. 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.

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. 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. Class i vapor retarders i e.