Ideal perfect wall wood sheet goods location – critical?
user-4885540
| Posted in Green Building Techniques on
In going through the double-stud layout as proposed by many on this board as well as BSC, I’m wondering if there is a “building science related” or other reason for placing the wood-sheet-goods air barrier on the outside of the inside wall vs putting it on the inside of the inside wall? We are in region 6 in SW Montana (high desert) and are looking to hit an R40-ish wall.
Also from my closest supplier Zip, OSB, and CDX are within a dollar or two per sheet…is there any reason to not use taped ZIP for both sheet goods locations?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
You want the sheet good in a location where it is easy to have a continuous air barrier. Usually on the outside of the wall is easiest.
Overall in heating climates, you want your air barrier on the warm side. For many years this was tried with air tight drywall approach but there are just too many intersections and bends to get it tight. So for your middle air barrier make sure you have this detail sorted (you know the trace the air barrier along the house cross section without lifting the pencil).
If you do have an outer-ish air barrier, you still need a decent (but not perfect) warm side air barrier. Usually standard drywall is good enough.
I still have issues with these middle air barriers. It seems to be a solution for a problem that doesn't exist. Standard double stud walls work and work well in all but the coldest climates. This details just seems to add a lot of complexity to insulation and air sealing for little benefit.
Akos,
I think middle air-barriers came from Thorsten Chlupp, who used them a bit differently. He framed and sheathed the house conventionally, then built an outer stud wall, which he didn't sheath.
I understand the logic behind moving the combined air/vapour barrier into the wall where it is protected and has very few penetrations, but agree it seems to be going a bit far to have a second redundant complete layer of sheathing. Especially as we now have a much better idea of the risks of damp sheathing and how to mitigate them.
Everyone seems to agree that Zip is a better grade of OSB than regular OSB, so I'd say your decision should be "do I use Zip, or do I use CDX"? Don't use regular OSB if the cost for all three are about the same.
I personally prefer CDX, but that's for various reasons, all my own preferences. CDX is less likely to have issues if it ever does get wet, it's a bit stronger, it's readily available everywhere at any time, I can put up whatever WRB I want.
Zip has the advantage of having a pre-applied fluid WRB, and is part of a system with a tape to seal gaps. You'll probably have an easier time getting a good air seal with Zip since they designed it to help with sealing.
My ultimate would be CDX with a field applied fluid WRB on the exterior, but that's not cheap.
Bill
Peter,
If those are the two locations you are considering (as opposed to the outside of the double-wall), moving it into the wall does two things:
It reduces the number of holes in the air-barrier that need sealing, and also significantly reduces the labour involved. If you have even hung drywall, you know a fair amount of the time is spent on marking and cutting out penetrations for plumbing and electrical. Now imagine doing the same task again using plywood, which can't just be roto-zipped or cut by a utility knife.