Double layering XPS to obtain required thickness?
I'm planning to use XPS rigid foam against my basement foundation wall. The existing wall is a 8" thick concrete foundation with only the spray on 'damproofing' on the exterior. I am in Zone 4A (St. Louis, MO), where the climate is mixed-humid. I have read elsewhere that a 1-1/2" XPS Layer is needed if I desire the XPS to act as a moisture barrier in addition to an air barrier.
See the 3rd Section titled, "Better Basement Insulation Detail"
With a bit of research I tend to agree by definition that the 1.5" XPS is a Class II Vapor Retarder (although I cannot find a perm rating for 1.5"). I did quick plot of 1" XPS (1.1 perm), 2" XPS (0.70 perm) and 3" XPS (0.60 perm) and then applied a trend line to fit (2nd order polynomial trend line). Using this trend line I find 1.5" has a perm rating of about 0.86 perm.
I plan to do a wall section of XPS foam (1.5" thick) sealed and taped, then studded with Roxul batt insulation, then drywall. I'll be foregoing any sort of vapor barrier poly or 'smart' vapor membrane. Per several relatively recent discussions I've come across on this site. Needless to say the Roxul will be providing the R-Value, so I'm not concerned in using the XPS as an insulator, only as a moisture barrier.
My question lies in the method I take to get to 1.5" thickness. I am having a difficult time getting 1.5" XPS (Foamular Brand) with the Tongue & Groove (T&G) edge; everyone (the big box stores) sells square edge. Special order requires I buy a pallet of 64 (I need 12 sheets). I can, however, put my hands on 3/4" T&G XPS rigid foam. Is there any pitfall to applying a double layer of 3/4" XPS to obtain the 1.5" total thickness. Part of me thinks it may be better in that I can stagger the seams, but another part imagines condensation in-between the panels. For what it's worth, I want the T&G edge to create a better seal between sheets, and I also plan to apply a small bead of sealant in the grooves to supplement the tape. Any thoughts or lessons learned would be appreciated.
Posted Thu, 01/02/2014 - 12:23
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