Sorry if this is a dumb question but hey, I’m curious. I know double vapor barrier equals bad idea. How come its ok then to put two layers of tar paper on a wall under the siding. Isn’t the tar paper a vapor barrier? I’m guessing that since some cultured stone manufacturers want 2 layers its not a vapor barrier but I figured I’d ask to truely understand. Thanks.
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The Mfg. is not putting a double vapor barrier, but a drainage plain. Assuming that water will always get behind the siding at some point, the 2nd layer is there to allow the water to drain down, not into the structure.
A double vapor barrier presents a problem when you are trapping moisture between a wall assembly, i.e. a VB inside the building and then outside.
Tar paper is not a vapor barrier.
It acts as a drainage plane, but it is not airtight.
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Gotcha. I figured they wouldn't call for it if it would present problems. Thanks