R-14 batt, Vaproshield, and 3-inch EPS in Alberta, Canada
I’m doing two retrofits for clien’s and my plan on both homes was to fit inside and out, then insulate with Roxul in the existing 2×4 walls, add Vaprosheild IT to the exterior which is an air barrier and WRB, then insulate with 3″ of Neopor EPS, then 3/4″ strapping and then siding.
But now I’m worried that my air barrier is in the middle of the thermal barriers (basically R14 on each side) as opposed to 2/3rds in. Do I need to worry about condensation in this assembly if I’m insuring an air tight building? I have my own blower door so I can test for air leakage during the construction phase.
I want to avoid poly as it’s so hard to get it 100% and it seems to cause problems when it isn’t done perfectly, and since these are both retrofits, I know I probably can’t get it 100%.
So as far as I can see is rely on a vapor retarding primer, airtight drywall or add more r value to exterior to bump the ratio more towards the 2/3rd mark.
I’m trying to hit that mark for a long lasting building assembly, that performs well and doesn’t drain the client’s budget. (I know, aren’t we all?…)
Thanks for your help.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Josh,
With 3 inches of exterior EPS, your wall will be fine. (To learn more about this type of wall assembly, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
To verify whether my assessment is correct, you can start by consulting this climate zone map. As the map shows, most of the population in Alberta is in Climate Zone 6, although the northern part of Alberta is in Zone 7.
The table in my article on minimum R-value requirements for exterior rigid foam shows that in Zone 6, your rigid foam layer (for a 2x4 wall) needs a minimum R-value of R-7.5. In Zone 7, you would need at least R-10 of rigid foam. Since your 3 inches of EPS probably has an R-value of about R-12, your wall will be fine.
More information:
1. Your air barrier can be anywhere in your wall -- at the exterior, at the interior, or in the middle -- without causing problems.
2. You seem to be confusing air barriers with vapor barriers.
3. Walls with exterior rigid foam are designed to dry to the interior.
4. When a wall has exterior rigid foam, it's best to omit the interior polyethylene. That said, if you think that the wall already has interior polyethylene, and you don't want to open up the wall to remove the poly, you'll probably be OK. For more on this issue, see The Exterior Rigid Foam is Too Thin!