Framing Roof Vapor Barrier

We ll add a vapor barrier so the moist air does not flow up through the insulation and condense on the roof.

Framing roof vapor barrier.

That air condenses on the cooler concrete floor. Class i vapor retarders i e. Hvac units roof materials etc loads to the supporting joists or purlins. 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.

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. Post frame buildings with metal roofs and especially buildings with minimal insulation hold warm moist air. A vapor barrier under your slab will also protect you from harmful ground pollutants like radon gas. 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.

That reduction of air and moisture infiltration can then help improve roof longevity. Understand these are climate specific recommendations. The history of cold climate vapor barriers itself is a story based more on personalities than physics. Wind snow rain and dead ex.

A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. Crawl spaces benefit from a polyethylene moisture barrier placed directly over the exposed earth. Rose 1997 regales readers of this history. It serves as a structural member transferring the weight of both live ex.

A vapor barrier against the concrete surface is generally recommended before wood framing or flooring materials are installed. 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. The two main types of substrates used with metal roofing systems are open framing and solid substrates composite or plywood. 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.

It is frightening indeed that construction practices can be so dramatically. 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 also a cold climate artifact that have diffused into other climates more from ignorance than need.