VAPOR PERMEABLE OR IMPERMEABLE BUILDING ENVELOPE MATERIALS, DOES IT MATTER?
Durability of the building enclosure is part of a sustainable building design. The expected service life for the building enclosure is longer than for other building systems, which makes the building envelope durability even more critical. A designer can significantly affect the service life of multi-component assemblies through materials selection, placement of materials within the building assemblies, or design detailing.
Moisture is one of the major factors affecting durability. Building assemblies may periodically get wet, but if allowed to dry they can have an acceptable performance. Moisture problems only occur when buildings get wet and stay wet long enough under adverse conditions for materials to deteriorate. Good enclosure design must consider both sides of the moisture balance: minimize the risk of wetting and maximize drying potential. If a wall assembly is able to dry, it may experience some wetting without long-term durability risks. Vapor diffusion as a wetting source is highly overestimated while vapor diffusion as a drying mechanism is poorly understood and often neglected.
One of the most often asked questions is about how to address diffusion wetting: do I need a vapor barrier, what type/class of vapor retarder is best (I, II or III), and where should it be located in the wall assembly? Unfortunately it is less recognized that every time a vapor impermeable material is used, an unintentional vapor barrier is introduced in the assembly, which eliminates a very critical diffusion drying pathway. While vapor diffusion cannot handle large amounts of moisture and cannot compensate for repeated and extended wetting events, it can provide a very effective drying pathway for incidental moisture intrusion. Still have questions about why vapor permeability of building envelope materials is relevant to moisture management and durability?
Still have questions if you should use a vapor permeable or vapor impermeable air and water barrier? The simple things to remember are: if a material is vapor permeable it can be located anywhere in the building envelope without interfering with diffusion drying pathway; if a material is vapor impermeable, it should only be located on the side with higher water vapor concentration (where the diffusion wetting comes from) and not on the side with lower water vapor concentration (where the diffusion drying wants to transport the moisture to). Another thing to keep in mind -- watch for unintended vapor barriers located in the path of diffusion drying, these will close the drying pathway!
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