Climate Zone 5A Wall Assembly
Climate Zone 5A
I’m looking to come up with a wall assembly that neither uses foam or a poly vapor barrier. Ideally, I like to utilize exterior insulation, but it’s not in the budget I want to get your thoughts on the following wall assembly, from exterior to interior.
7/16″ Zip Sheathing with Zip Tape and Liquid Flashing
R-19 Unfaced Fiberglass Batts
I was speaking with the insulation contractor and he says it’s not a good idea in our climate zone. He said he always uses poly and has never had an issue with moisture in the wall. His argument is our climate is so unique. To some extent, I agree. We can go from 70 degrees to below 30 degrees the next day. He said if i was all out against poly, he’d recommend kraft faced fiberglass batts. The other option he offered was 2″ of spray foam on the interior side of the sheathing with R-11 unfaced fiberglass batts and no poly.
Will my proposed assemble be safe in my climate zone or should I make a change?
Thanks for all your help.
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First of all, you need to make sure that your wall assembly meets the minimum code requirements for walls. In Zone 5, many codes require R-20 batts for walls (not R-19) or R-13 batts plus R-5 continuous exterior insulation.
Second, you should realize that even if your wall is code-compliant, it isn't a great wall. It's a wall that barely meets code, at best.
There is no requirement for interior polyethylene. However, most codes require that walls have an interior vapor retarder (not a vapor barrier) in Climate Zone 5. You can meet this code requirement with either (a) vapor barrier paint, (b) kraft facing on the interior side of your fiberglass batts (the easiest compliance path), or (c) a "smart" vapor retarder like MemBrain.
R19s actually perform at R18 when compressed to 5.5" whereas R20s do not. It's still "worth it" to use high density R21 fiberglass or R23 rock wool for the significantly improved air-retardency over R19s.
In NW Ohio there are several rigid foam reclaimers that can take the financial sting out of an exterior foam option. At the same wall thickness and lower cost couple inches of reclaimed roofing polyiso on the exterior of a 2x4 wall is superior in almost every way to 2" of closed cell foam + R11 batts in 2x6 framing.
In zone 5 code does not require an interior side vapor barrier tighter than standard latex on wallboard if the siding is back-vented, or with R5 or more on the exterior of a 2x4 wall. Vapor barrier latex (about 0.5 perms) would offer an order of magnitude better seasonal drying toward the interior than 6 mil polyethylene, but an order of magnitude LESS drying capactity than standard interior latex paint.
Thank you Martin and Dana for your insight.
I know my idea isn't great. I really want to use exterior foam because I know that's the right thing to do. I will dig deeper into the reclaimed polyiso because it makes the numbers look much more appealing.
What are your thoughts on eliminating the Zip Sheathing if I went with exterior foam? Should I go with OSB or plywood and a house wrap for my WRB? Not sure if the Zip is worth the expense with the exterior foam.
Thanks again for all your help.
It has been mentioned here before. When using foam for an exterior sheathing you can use a 4' x 8' sheet of CDX plywood on the corners for bracing and then foam sheathing over it for insulation value. An example would be 1/2" plywood on the corners covered by 1" polyiso. Inbound of the corners would be 1 1/2" polyiso to equal the thickness of the 1/2" plywood and the 1" foam sheathing. I have used this in combination with 2" x 6" walls (R-21) and it makes for an R-30 in a 7" wall.
What ZIP buys you over other OSB is the already-applied weather resistant barrier. It's possible to air seal other OSB using appropriate tapes & polyurethane caulk, and housewrap or #15 felt for the weather resistant barrier.
If going with 2x4 + 2" foam there's a decision to be made about mounting the windows which determines whether the housewrap between the foam & OSB or whether it goes on the exterior side of the foam. With "innie" mounted windows the it goes between foam & OSB, and using a crinkled type housewrap (eg Tyvek Drainwrap) is preferable to flat goods. See:
With 2x4 framing and 1.5-2" foam the window mounting is pretty straightforward, since even with "outie" windows the window bucks can be made with standard dimensioned lumber and would be pretty familiar to most framing carpenters with only minor adjustments. With fatter foam it's a bit more customized, but it doesn't take rocket science to get it right:
There are even pre-made higher performance versions that might be appropriate for your stackup/thickness to consider:
> come up with a wall assembly that neither uses foam or a poly vapor barrier.
If you want to do that, then use a 5:1 or greater ratio of outside perms to inside perms. Also use vented cladding and do a very good job of air sealing (preferably including the interior side). A smart-retarder and/or cellulose are also beneficial.
Thanks to all of you for your guidance. I really appreciate it.