Vapor barriers or, more correctly, vapor diffusion retarders (VDRs) should mainly be considered as just part of a building’s moisture control. The term “vapor diffusion retarder” is more accurate because vapor barrier implies that that material can completely stop moisture transfer, but no material actually does that much. They reduce the rate at which water vapor can move through a material, but not completely stop it.

A material’s ability to retard water is measured in perms, or permeability. Any material with a perm rating of less than 1.0 is considered a vapor retarder. To prevent the trapping of moisture in a cavity, the cold side of the material should have a perm rating at least five times greater than the perm rating on the warm side.

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So, what does that plastic-y, paper-y stuff on the outside of your house, underneath the siding, really do? If installed correctly, housewraps perform three functions critical to energy costs, comfort, and, over time, structural integrity. Those functions are primarily: act as a second weather barrier (prevents wind-driven rain from getting any further into your home), serve as an air barrier (thus lowering energy costs), and allows interior moisture to escape (vapor-permeable membrane, VERY important!).

Now that we understand what housewraps do, how do we choose from the dozens of varieties on the market? Well, of course there’s no easy answer. Siding sometimes helps determine what wrap to use: vinyl siding comes with built-in drainage holes and wouldn’t need the same type of wrap as brick, stucco, or board siding. If, however, the siding is no help whatsoever in coming to a conclusion the perm rating may help. Permeance ratings reflect the measure of a material’s ability to transfer water vapor. So, the higher the perm rating the faster interior moisture can escape. However, this does not always mean it’s better. Some wraps are perforated to obtain a high perm rating, but the perforation allows exterior moisture in! Not good.

Depending on the type of siding you’re using, it could affect the housewrap’s weather resistance. Cedar and redwood both leech surfactants (surface-active contaminants) which increase water’s ability to pass through the wrap. Not only the siding, but also power-washing chemicals, soaps, and even some types of paint can reduce your home’s efficiency and protection.

Installation is extremely important too. Working from the ground up, vertical and horizontal joints should have sufficient overlap: 12 inches for vertical, 6 inches for horizontal. Also, all joints should be taped with special housewrap tape. Even the nails used are important. The nails most housewrap manufacturers recommend are plastic cap nails.

Once again, much of the information in this article can be found here.