Good enough wall assembly question
Hello,
My name is Jim. I am a builder in the Upper Peninsula of Michigan (climate zone 6b but close to 7). I am currently building my own “Dream Home”. It is common around here to build to code minimum which would consist of 2×6 walls with R-21 in the cavity. Which is the way the home was drawn. I’ll admit I was not real well versed in real high efficiency building when I started the project but after reading as much as I could over the last couple of months (mostly on this web site) I decided I wanted to make a more efficient building envelope. Here is what I’m thinking for my exterior wall assembly from the inside – out.
1. 1/2″ Sheetrock
2. 2×6 studs 16″ oc
3. Stud cavity filled with mineral fiber bats.
4. 1/2″ osb sheathing
5. WRB –
6. 2″ Rockwoll comfortboard 80
7. 3/4″ furring strips
8. LP Smart side
Here are my questions.
1. Do I need anything between the studs and Sheetrock? If so what brand do you suggest?
2. What wrb would you suggest? Keep in mind that I will be installing it this year and it is pretty much winter here, temps 15-30 degrees Fahrenheit.
3. Can I get a little sympathy? There was an article in the paper today saying October had 28 days with precipitation and November had 29. Both shattering records. Its been one heck of a fall to build my house…..
Thanks in advance for your help,
Jim
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Replies
Jim, that's a good start. Somewhere in your assembly you should have a designated airtight layer; I usually find it's easiest to do at the outside face of the sheathing. Regular OSB is usually airtight but not always. If Huber's Zip sheathing is available in your area, you could use it for sheathing and tape the seams as the airtight layer, and then you wouldn't need another WRB--especially with the Zip located safely behind the Comfortboard.
LP's installation instructions for lap siding require 2x4 furring 16" on center for installation over foam more than 1" thick. Presumably you can substitute mineral wool but it would be smart to contact their technical services department to make sure.
Because you'll have ventilated cladding and Comfortboard is vapor-open, you don't have to install a vapor retarder behind your drywall. If you want extra insurance against moisture accumulation, though, you could install a variable permeance membrane such as Pro Clima's Intello, Siga's Majrex, or Certainteed's Membrain.
Jim,
As to #3. That's brutal. You've got my sympathy, even though I'm in the PNW.
You should design better than code min, so assume zone 7. This means a minimum of R-15 external to wood sheathing for 2×6 walls. 2" Comfortboard, while highly beneficial, doesn't come close. So you need to do additional things to reduce OSB rot risk.
Air barrier location matters, so tape the OSB *and* create an interior side air barrier. Blower door test to verify.
Vapor diffusion caused moisture problems relate to interior/exterior perm ratio, so create an interior side vapor retarder (I'd use Membrain or Intello).
Consider damp spray cellulose - which has several beneficial properties over mineral fiber.
Zip is low perms and would make things worse. Plywood would make them better.
Thank you Michael, Malcolm, and Jon.
Michael:
I am planning on making my designated airtight layer the outside face of my osb. Is taping the osb seams, caulking the osb to the foundation, and air sealing penetrations enough. Im thinking of using 3m all weather flashing tape for this. The regular osb sheathing is already installed.
I did check into LP's instructions and they do allow for 1x4 furring strips if southern pine is used.
Jon R,
Regarding OSB rot risk. I understand the concept of minimum thickness of foam to keep the osb dry and warm. In one of the posts I read on this site Martin stated that if using mineral fiber there would be no minimum thickness as the osb would be able to dry. (I have tried to look back and find that article again but could not find it) Can anyone confirm this?
I do like a lot of the properties of damp spray cellulose. The only problem I have is that one side of my house is within 5 feet of the property line. The code states that i need a 1 hour fire rated assembly on both sides of that wall. I have chosen to use LP Flameblock osb sheathing. In their assembly instructions is specifies mineral fiber insulation in the cavity. I could do that wall with mineral fiber and the rest with damp spray cellulose but it seems easier to stick with one product throughout.
I still have not chosen what product to use for my wrb. Any suggestions?
Thanks again for all the great advise on this site,
Jim
The articles about foam thickness are probably these:
//m.etiketa4.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
//m.etiketa4.com/article/rethinking-the-rules-on-minimum-foam-thickness
You should have lots of drying ability with mineral wool on both sides, and a rain screen. You probably don’t need an interior vapor barrier, but I, not positive on that since I’m more familiar with using vapor impermeable exterior foam.
You have my sympathy too. There must be a lot of wet yoopers up there! It’s been warmer and wetter than usual down here in troll land (I’m in Davisburg), but nothing like you describe. Last year was brutal record-setting cold though.
Bill