Air Barriers | Green Construction Contractor

Understanding the Vapor Barrier

November 17th, 2011 — Air/Water, Green Construction

vapor The first thing to understand is that there is a difference between vapor barriers and what air barriers are. When properly defined, a vapor barrier alone is not capable of controlling the movement of air, but rather it is responsible for controlling the movement of moisture. Vapor barriers are not actually barriers, but rather they are retarders of vapor diffusion. They regulate the flow of moisture from outside in or inside out on a molecular level. When the vapor diffusion retarder is used in the structure, the moisture control function happens. Unlike with an air filtration barrier then, the VDR does not have to be sealed, continuous or free of holes in order for it to do its job. These vapor barriers are rated based on the level of vapor diffusion control that they are capable of providing.

The original reason behind using vapor barriers was to keep ceiling and wall assemblies from becoming wet. Now when VDRs are installed onto an assembly interior, they can prevent inward drying as well which is actually capable of leading to serious problems such as mold and rot. Another variable to consider is climate. Another complication to the vapor flow between exterior and interior is cladding choices as some claddings are capable of retaining significant qualities of water when they become wet. In weather that is hot and humid, the moisture can be drawn inward, increasing the pressure of the vapor on the assembly.

The industry surrounding residential building has plenty of controversial construction techniques, and vapor barriers tend to rank high when it comes to these kinds of things. Few builders really understand how vapor barriers work and as a result, they do not know why or how to use them. Builders are not the only people that are confused, as building code officials also often misinterpret and misunderstand their requirements. Just because things were done a certain way in the past, that does not mean that today's modern day wall systems will be the same way.

The current proposals are to define:

Vapor barriers of 0.1 perm or less as Vapor Impermeable rather than Class I Vapor Retarder,
Vapor barriers greater than 0.1 perm and less than 1.0 as Vapor Semi-Impermeable rather than Class II Vapor Retarder,
Vapor barriers greater than 1.0 perm and less than 10 perms as Vapor Semi-Permeable rather than Class III Vapor Retarder,
Vapor barriers greater than 10 perms as Vapor Permeable

When in doubt, Class I and Class II should only be used in the coldest zones, zones 7 and 8. In these zones you can use MemBrain, which installs like poly but behaves similar to a vapor diode which allows the perm rate to be increased when the vapor pressure is increased. Reducing the amount of vapor that enters a wall during the winter months will allow Class II requirements to be met. For other zones you can use a good quality wall primer in lieu of a traditional VDR. Before you prime the drywall, it has a perm rate of 50 but after a few coats of a latex based paint you will have a perm rate of only 2 or 3.

Photo Credits: RobW_

Originally posted 2009-07-22 05:17:11. Republished by Blog Post Promoter

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