Posted By Roger R on 09 Aug 2013 03:43 PM
So I would lose a little "R-Value" correct?
Is it "a little" R-Value only, if we seal the seams and other areas of concern as well as possible? Would this leakage promote mold on the inside walls (between the CMU and sheetrock that is "glued" directly to the CMU)?
I could lose a little R-Value, as right now the unfilled CMU is under R-2, so 4" of EPS will be w-a-y better than we've ever had. But I do not want to promote mold growth.
AT 4" Type-II EPS has a vapor permeance of about 1-perm, and the assembly can still dry. Neither the EPS or CMU is particularly good mold food, and both are quite mositure tolerant, but you'd still have pretty good seasonal drying rates in both directions, particularly with rainscreened siding.
A square inch of air leakage moves FAR more moisture than vapor diffusion through a whole wall's worth 2-3 perm latex paint, let alone 1-perm EPS. Make the wall assembly as air tight as possible both inside and out, and you've fixed the #2 biggest mold hazard issue.
The #1 issue is still bulk-moisture handling- flashing at the windows & doors properly lapped to the drain plane/WRB (=Weather Resistant Barrier) is the most critical aspect. If the pre-existing windows are flashed to the exterior of the CMU, a liquid-applied weather resistant barrier can be applied to the CMU, and the crinkled-type Tyvek applied between the foam & liquid WRB as a capillary break that still allows the bulk water to gravity-drain out the bottom without the a huge convection gap that a 6mm cavity would be. Flashing at the bottom of the CMU needs to be added to direct bulk water to the exterior of the foam too, best installed before the liquid WRB. If you're re-installing the windows or installing new (not replacement) windows you have the option of installing them "outie", where the glass is roughly co-planar with the exterior side of the foam, and a sheet type WRB lapped properly to the window flashing can be on the exterior side of the foam.