Certainteed Membrain Permeance
Was looking at the permeance chart for Certainteed Membrain. Looks like at 40% RH or so, permeance is greater then 1, probably closer to 2. I’m looking at this for a basement, Zone 5 Toronto. 40% RH in a basement is not that high, so curious as to GBA member’s thoughts on this product. It seems like it potentially would allow to much diffusion through the product during the winter months, if RH is greater then 35%.
Also, does anyone know if Membrain requires drywall to be installed and painted to function correctly? I came across a few references to that but have not heard definitively. Was hoping to leave it unpainted as the basement will remain unfinished, and I have no desire to drywall a ceiling.
Lastly-I have seen that many on this forum mention that to use rigid foam as a vapor barrier (forget the Membrain in this case, forgive me for jumping around), drywall must be painted with two coats of latex paint to function as an interior vapor retarder. Why is this the case? If the rigid foam provides a perm rating of <1, why is this additional step needed, assuming it is taped/canned foamed as appropriate and appropriate thicknesses for dew point control were used?
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