Replies

1. Expert Member
   Dana Dorsett|Jun 27, 2013 05:18pm|#1

   You'll still need to caulk the bottom plate of the framed wall to the subfloor, and air-seal the subfloor to the band joist, the band joist to the foundation sill, and the sill to the foundation, eh?

   Acoustic sealants are not big VOC emitters when compared to construction adhesives or some other types of caulks:

   http://ca.brockwhite.com/files/Resource/Other/LEEDAcousticalSealant.pdf

   The notion that the primary air-barrier will be inviolate & air tight forever, therefore redundancy would be a waste is a well demonstrated fallacy. If you're not insulating the studs with air-impermeable insulation (foam), you're well advised to caulk the framing to the sheathing, and to foam-seal lateral electrical & plumbing to eliminate lateral thermal bypassing, and if you're not putting more than the IRC prescriptive minimum of sheathing insulation on the exterior you're well advised to use air-tight methods on the gypsum too, for moisture transfer reasons. (Mold spores, anyone?)

   The interior side air barrier isolates any VOCs in the wall cavities from the conditioned space, limiting the rate of transfer into the air you're breathing, and increasing the dilution factor with ventilation air by orders of magnitude. So what if it takes some caulk to get there? It's improving rather than harming the indoor air quality, despite containing VOCs. (It's not as if the OSB in the ZIP has zero VOC emissions, eh?)

2. 
   UBuildIt_Indy|Jul 01, 2013 09:53pm|#2

   What about placing 1" of rigid polyiso foam on the outside of a wall assembly? If doing this over the OSB sheathing, would it then be overkill to use ZIP?

3. GBA Editor
   Martin Holladay|Jul 02, 2013 05:21am|#3

   Allen,
   No, it would not be overkill to use Zip sheathing. But your proposed wall assembly (using thin foam) will only work in a warm climate. In a cold climate, you need thicker rigid foam. More information here:Calculating the Minimum Thickness of Rigid Foam Sheathing.

4. 
   designthis|Jul 02, 2013 07:30am|#4

   Hi Dana,
   Thanks for the thorough response. I should have explained the cottage better:
   It's a modern,1000 sq/ft single story with a shed roof (in climate zone 6) that I'm building as a vacation rental on my property. And since it's not a full-time residence, I'm doing some fun details for the interior while trying to make it as energy efficient (as I can afford) to keep the monthly operating costs low.
   I am using a full-fill open cell spray foam in the wall cavities. I'm finding the costs of spray foam to be on par with cellulose around here and since I have had very positive results with spray foam in the past, it seems to be a no-brainer to use it. I am spraying 3" of closed-cell foam under the roof-deck and filling the rest of the 12" iJoist cavity with R-21 batts to give me a total of R-40 in the un-vented cathedral ceiling. This method will save a lot of money plus gives me the vapor-barrier I need in the ceiling. I'm not putting foam on the exterior walls primarily for budget reasons. I'm figuring that the small footprint, with limited glass surface, and careful attention to details will yield a very tight envelope.
   I do have 2 more questions involving the use of plastic vapor barriers:
   I'm doing a conditioned crawl space and bought the 6mil (under the rat slab) 12" up the poured walls. I plan on putting blue foam on the interior walls down to the slab. I also waterproofed the exterior with impermeable Grace Bituthane. Can I terminate the 6mil poly at 12" or do I really need to bring it all the way up the wall to just underneath the band-joist? This is another seemingly redundant detail.
   Also, I am thinking of using AC plywood as a finished interior wall surface in some areas including one exterior-facing wall. Since I have open-cell foam on the walls, would you recommend putting a poly liner on the wall behind the plywood?
   Details, details...
   Thanks!

5. 
   LucyF|Jul 02, 2013 01:05pm|#5

   David,
   Would you tell us what zone you are in?

6. GBA Editor
   Martin Holladay|Jul 02, 2013 01:11pm|#6

   Lucy,
   Look at Comment #4. The cottage is in climate zone 6.

7. 
   LucyF|Jul 02, 2013 03:23pm|#7

   Sorry,
   I missed that.