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Skip to main content. Vapor Barriers. Joseph Lstiburek. September 15, Abstract: Most of us are not aware of just how differently these two barriers work in building assemblies. Air Barriers vs. Vapor Barriers The key to moisture control is the control of air transported moisture. Rights: Building Science documents are intended for professionals.
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Confusion between Vapor barrier and Air barrier The function of a vapor barrier is to retard the migration of water vapor, which is not typically intended to retard the migration of air.
Get more info. Recipient email address:. Personal Message optional :. Copy Me. Please log in to share this item by email. Related: learn more about building envelopes and why they matter. Vapor barriers or vapor retarders are materials used to slow or reduce the movement of water vapor through a material. Vapor barrier materials are installed on the warm side of the insulation in a building assembly, as determined by climatic conditions.
In warm climates, it will be on the exterior and in cold climates, it will be on the interior. A vapor barrier can be a mechanically fastened sheet-material, adhesive membranes depending on composition , fluid-applied materials, insulating board stock or medium density spray polyurethane foam. The thickness of the material will impact whether it is a vapor barrier or not.
Water vapor may be transported by air leakage, but you address this issue by installing a proper air barrier, not by a vapor barrier. Vapor barriers are intended to control the rate of diffusion into a building assembly. Therefore, the vapor barrier does not have to be continuous, does not have to be free of holes, does not have to be lapped, does not have to be sealed, etc. A hole for example in a vapor barrier will simply mean that there will be more vapor diffusion in that area compared to the other areas of the vapor barrier.
To simplify, consider this wool sweater analogy: A wool sweater is insulation. It will keep you warm when there is no air movement, but it still allows the wind to move right through it. A wool sweater with a raincoat will keep you warm, but hold moisture inside and soak your insulation. A wool sweater with a windbreaker will keep you warm, stop the wind from stealing your heat, yet allow moisture to diffuse through it. So think of a windbreaker as an air barrier and a raincoat as a vapor barrier.
In high-performance buildings, air barriers and vapor barriers, as well as water-resistive barriers, can be combined. There are also vapor-permeable air barriers, and there are water resistive barriers which are not air barriers.
As an example, you can have two, three or even four air barrier materials in a wall assembly, but its effectiveness will depend on which material you have chosen and how you have connected the air barrier materials together. Now that you understand the difference between air barriers and vapor barriers, the bigger question is why do they really matter?
Unlike impermeable vapor barriers, the Barricade Wrap can resist moisture while allowing humidity to escape from exterior wall cavities, which is of particular importance in hot and humid climates. Influence of driving rain and vapor diffusion on the hygrothermal performance of A hygroscopic and permeable building envelope. Air Barriers What is an Air Barrier? What is the Purpose of an Air Barrier?
Thermal conduction is the action of hotter molecules moving towards cooler molecules. Thermal convection is the flow of heat energy from a warmer space to a cooler space by the flow of fluids usually liquids and gases. Vapor Barriers A vapor barrier prevents vapor diffusion through building materials. Vapor Retarder vs. Vapor Barrier It is important not to confuse a vapor barrier with vapor retarders. Class I is a vapor barrier: 0.
Class II is a vapor retarder: 0. Class III is a vapor retarder: 1. Problems with Vapor Barriers Not only do vapor barriers not significantly help a wall system stay dry, but they may also damage the integrity of the home.
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