Insulating a wall with loose and blown-in foam
JohnBoy28
| Posted in Energy Efficiency and Durability on
We are in Zone 6 . Most builders here recommend the code which is 2×6 walls with R20 loose insulation plus optionally 1″ of rigid foam outside of the plywood . One builder recommended R-20 loose insulation and 1″ of blown in foam between the loose insulation and the vapor barrier. He said it creates a better seal.
I haven’t seen this method, is it a sensible alternative to what the other builders are recommending?
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Replies
John,
I don't know what you mean by the "vapor barrier" -- most codes don't require an interior vapor barrier, only a vapor retarder, which is a less stringent layer, and in most U.S. locations you don't want an interior vapor barrier like polyethylene.
It sounds like you are trying to describe the "flash-and-batt" or "flash-and-fill" technique. Contractors who use that approach put a layer of closed-cell spray foam on the exterior side of the stud cavity (against the exterior sheathing) before filling the rest of the stud bay with fluffy insulation.
For more information on this technique, see these articles:
Flash and Batt Insulation
Why Flash and Batt Makes Sense
Thanks Martin:
"flash and fill" is what he suggested but he wants to fill with fluffy insulation against the exterior sheathing and then put a one inch layer of closed -cell spray on the house side. He said it would give an extra seal around plug receptacles etc.
John
John.
I have never heard of that approach. Does your contractor intend to work from the exterior of the house? I'm not sure how this could be done otherwise -- and you can't really install the drywall before you install the wall sheathing, so I'm confused (unless this is a double-stud wall or a Mooney wall).
I am surprised that Martin didnt bring where the condensation point would be. In zone 6 1 inch of exterior foam is not enough.
Flash and fill doesnt make sense to me because of thermal bridging.
I assume the builder wants to use the foam for an air barrier. Is spray foam really a good long term air barrier in a flash and batt. With stories of foam shrinkage and other factors will a thin coat of spray foam be a durable air barrier
I'm surprised that builders are able to get away with 2x6/R20 + 1" of foam on the exterior without running into moisture issues. R20+5 would be 1" of XPS, which is somewhat vapor permeable at 0.8-1.5 perms so it CAN dry toward the exterior (slowly) but as Robert points out, it's insufficient exterior-R for dew point control at the sheathing without an interior side vapor retarder.
If they're installing air-tight 4 or 6 mil polyethylene on the interior it solves the wintertime loading issues, but it reduces the moisture resilience of the assembly- any moisture that get's in takes a long time to leave through 1" XPS. A better solution would be air-tight wallboard and half-perm paint ("vapor barrier latex"), or a "smart" variable permeance vapor retarder sheet. But none of those solutions are as resilient as simply installing sufficient exterior R that the average temp at the sheathing in winter is above the presumptive ~40F dew point of the conditioned space air, and using only standard latex paint (3-5 perms) as the interior vapor retarder. In zone 6 that means a bit more than 1/3 of the total-R has to be on the exterior of the wall sheathing:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
I instead of 1" foam they installed R5 rigid rock wool, the much higher vapor permeance of the rock wool would be sufficient to skip the interior side vapor retarder despite the colder sheathing, since the drying path to the exterior would be essentially unimpeded.
A flash-foam of ccSPF on the wallboard puts a ~0.8-1.2 perm vapor retarder on the interior side, which would be sufficient protection for the sheathing, but is not a substitute for the R5 continuous sheathing on the exterior. With the thermal bridging of the framing through the 1" foam the "whole-wall" R is only about R0.25 more than 2x6 /R20, which does NOT meet IRC 2012 code for zone 6. But R13 + R10 continuous sheathing does, and it would have HUGE dew point margin at the sheathing. The better solution would be to drop back to 2x4 framing and install 2-3" of foam on the exterior to hit that mark.
publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
Response to Robert Hronek (Comment #4),
You wrote, "I am surprised that Martin didn't bring up where the condensation point would be. In zone 6, 1 inch of exterior foam is not enough."
My response to John Ball didn't address that issue because it didn't sound like John was considering the use of 1 inch of exterior rigid foam on a 2x6 wall. He seemed to be describing some type of flash-and-batt system with the spray foam on the interior side of the wall.
GBA readers are probably sick of hearing me rant about builders who install exterior rigid foam that is too thin. But if any GBA readers are feeling the need for a rant, they can read this article: The 2012 Code Encourages Risky Wall Strategies.