A builder I am doing energy consulting with has used tech shield roofing material on the exterior walls. The foil is manufacturer perforated for moisture. Is this a good or durable application for this sheathing? Will it effect the wall system?
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It all depends on the climate where it is used and the fraction of the surface area that is not foil. I assume the product is as described here: http://www.lpcorp.com/techshield/
That site doesn't give enough information to give you the answers you need. It is basically a foil covered OSB product, with holes of unspecified size for "vents." It does say: "This patented, post-lamination incising process allows LP TechShield panels to dry more quickly from construction moisture than other radiant barrier panels." It doesn't say that it lets the panel have a high enough overall permeability to make it suitable for wall sheathing in areas and wall designs where the wall assembly ought to have some ability to dry to the exterior.
OSB by itself is a low-perm sheathing, and its permeability does increase a little with moisture content, although not as much as does plywood. However, molecular diffusion is an area-based thing. If you cover 99% of the surface area with an impermeable material such as aluminum foil, the overall permeability of the sheet is only about 1% of what the uncovered sheet would be. In the case of OSB, 1% or even 5% of a very low permeability is essentially impermeable.
As used on the roof of a vented attic, the material ought to work just fine, as long as it can dry to the exterior often enough between rain events. It certainly won't dry quickly to the interior, if that foil covers most of the surface. If the sheet is used as exterior wall sheathing in a situation where an exterior side vapor barrier should not be used, I'd say it has been misapplied. That's my view. I'm sure Martin and others will have something to say.
Dick is right on. More info needed. Location of project and more info about exactly how the material is used in what assembly.
Low-perm materials usually add risk to a wall assembly or a roof assembly -- unless the low-perm product has a high R-value. This is a low-perm product that doesn't add R-value -- so it's risky. If you want a wall that performs better, choose rigid foam, not aluminum foil.