Advice on my wall assembly
Hello, I’m getting ready to build a new home in western North Carolina. I would appreciate advice on my proposed wall assembly. 2×4 walls with huber’s R-Zip 9.6 (2″ thick overall) and coravent studi battens for a rain screen assembly. The idea is to end up with a wall assembly thickness so I can order windows for a 2×6 wall and have them work. So I end up with drywall(1/2″) + 2×4 wall (3 1/2″)+ Rzip board (2″) + sturdi batten coravent (7/16″) = 6 7/16″. I was planning on closed cell foam in the 2×4 wall cavity.
Does this seem like a reasonable assembly?
My other thought was to use huber’s R zip 12 ( 2 1/2″ overall) and skip the rain screen and end up with the 6 1/2″ thickness I was shooting for. any thoughts?
Thanks, Jeff
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Forgive my potential ignorance about residential windows... but isn't it quite common for walls to vary dramatically in thickness depending on insulation levels, interior and exterior finishes, etc., so if you're using flanged windows attached to the outside of your Zip sheathing, you just use trim or a drywall return (depending on aesthetic) on the inside to make up the difference in depth?
Regardless, closed-cell foam on the inside is probably a waste of money and is unnecessarily environmentally harmful. It doesn't seem like there's any reason not to just use cellulose in your wall cavities. If you must use foam for some reason, open-cell foam would be a better choice.
Don't skip the rainscreen. Rainscreens are great!
Agree, Closed cell foam is overkill. R20 is suggested for the wall mockup in your climate zone. With R9 exterior, you could use dense back fiberglass, cellulose, Rockwool and exceed your target.
WNC receives a lot of rain (especial Cherokee area) so rainscreen is key. I'm trying, unsuccessfully, to get my brother to install rainscreen on his home in Asheville.
Just an FYI. You'll want to review for the additional bracing requirements which may be required when using that thick of a ZIP panel.
Closed cell foam in a 2x4 cavity only adds about R1 to the "whole wall" performance compared to using open cell foam or R20 batts, despite the much higher center-cavity R. It's the thermal bridging of the ~R4 framing fraction that constitutes 25% of a typical wall's area. That's a pretty expensive R1, and it comes with a hefty environmental hit to boot.
Also 3"+ of closed cell foam dramatically inhibits the drying rate of the stud edges next to the sheathing. The R9 ZIP-R offers significant dew point margin to avoid wintertime moisture build-up in 2x4 wall cavites, even if it were air-permeable batts.
Thanks for your input. I'm no expert by any means but I was hoping the closed cell inside the 2x4 wall would provide a air seal. Do you think that the R zip taped up up properly would provide that? Or how about a flash and fill in the 2x4 cavity to provide additional sealing? Also on my wall assembly build up, I just figured that if I could come up with a proper thickness to avoid window modification or extension jambs would be wise. I'm trying to educate myself on the proper way so the builder doesn't just throw stuff together without me knowing any better. Thanks, Jeff
Open cell foam at 3" or more qualifies as an air-barrier all on it's own, and seals as-well or better than closed cell foam. It's 1/4 the amount of polymer as closed cell, and uses water as a blowing agent rather than HFCs. A flash-inch of closed cell foam won't air seal nearly as well as a full cavity fill of open cell foam.
No cavity fill will air seal between the bottom plate and the subfloor, or between the doubled-up top plates or other doubled-up framing, so get used to laying a bead of polyurethane sealants/caulks to ensure those are air tight, even though you are also taping the ZIP.
Jeff,
I agree with the other readers who have responded.
1. Don't worry about the total wall thickness. Choose the wall details that perform well, and build your own interior jamb extensions out of 1x pine lumber, or use drywall returns.
2. When it comes time to choose an insulation material to install between your studs, you need to choose something that is vapor-permeable (something like cellulose, mineral wool, fiberglass, or open-cell spray foam). The principles are explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
3. Adding more R-value on the exterior side of your wall sheathing (by choosing thicker exterior rigid foam) always makes more sense than trying to add more R-value between the studs.
4. You don't need spray foam to reduce air leakage. For more information on this issue, see Questions and Answers About Air Barriers.
5. For an overview of these issues, see How to Design a Wall.
-- Martin Holladay
Thank you for your info and guidance. So it sounds like my proposed assembly would be fine if I were to use open cell foam in the 2x4 cavity.
please advise....on my proposed wall assembly. Changing the cor-a-vent strips with 1/2" plywood furring strips still gives me the overall thickness of 6 1/2" and provides me the rain screen, but does it create a issue with attaching fiber cement siding? nailing thru the furring strips into the exterior of the R zip will be only 15/16" of combined wood thickness, will I need to use 4" fasteners and go all the way through into the 2x4 studs to have enough support for the siding? bear with me, I'm a novice trying to learn. Thanks for your patience, Jeff
Jeffrey,
Q. "Do the furring strips create a issue with attaching fiber cement siding?"
A. The answer depends on the type of fiber-cement siding that you install. You have to follow the manufacturer's installation instructions. Most fiber-cement siding manufacturers specify the type of fastener and the depth of the fastener into the substrate (sheathing or studs) that receives the fastener.
Fortunately, most manufacturers post their installation instructions online. If you know what brand of siding you will be using, it's time for you to use Google.
-- Martin Holladay
Hi Jeff
I am joining you in building in western NC so I am very interested in the responses here. I have the same concerns as well. Here is the Hardiboard bulletin on foam, which I think would apply to Zip-R as well.
http://www.jameshardie.com/JamesHardieMainSite/media/Site-Documents/TechnicalBulletins/19-jh-over-advanced-framing-or-continuous-insulation.pdf
Thank you for your input Martin and Kevin! as well as the previous posters. :)