Zip or not in double stud wall
user-4885540
| Posted in Green Building Techniques on
Based on my previous questions and responses, i’ve seen the light and am planning on using double stud walls (climate zone 6) and trying to achieve ~R40. Insulation will be Rockwool Comfort batts or blow cellulose. I’m drawn to the idea of using some sort of sheet good (wood) in between as a smart vapor barrier. My question is whether or not Zip is advantageous in a double-stud setup? Would it go on the outside wall? In between the walls? Skip the Zip altogether?
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IMO, double walls are usually best built with smart-retarder membranes, cellulose, plywood, rain-screen, a high ratio of exterior perms (wet) to interior perms (dry) and excellent air sealing on both sides. Admittedly, "interior" can be part-way into the wall and wood sheathing can act as a fairly-smart-retarder.
I didn't include it below, but agree with Jon on the ventilated rainscreen detail as very important as well.
Hi Peter.
We have an article coming (probably in December) from a well-respected high-performance builder on his approach to double stud walls in Maine. The assembly is quite simple and includes a fully-adhered, vapor-open WRB on plywood sheathing, dense packed cellulose insulation, and a class III interior vapor retarder (painted drywall).
Many GBA users will claim this is a risky wall, but the author has installed sensors and monitored these walls and has remodeled them and has found no evidence of wet sheathing. I believe the folks at 475 building supply would advocate a similar wall, with the addition of a smart vapor retarder on the interior.
Both the plywood sheathing and the cellulose insulation are important materials in these assemblies, beyond their structural and insulating roles. Plywood allows more outward dying and some believe holds up to wetting better than OSB, and cellulose buffers moisture. That is, it can hold and release moisture unlike some other materials.
You have a lot of options, including ZIP sheathing, a mid-wall vapor retarder, etc. The most important things you can do is a fastidious job installing your WRB, flashings, siding, etc. to keep water out of your walls, and great air sealing to keep moist air out. If you do those things well, vapor drive is much less of a concern.
I agree with Jon on this one--although in the past I've used Zip for exterior sheathing in double-stud walls, data logging shows that moisture accumulates at the sheathing in late winter and spring. While occasional wetting may not be a problem, even high-quality OSB does not stand up well to repeated wetting. I prefer plywood's increased perm rating at high moisture content. Even better would be solid wood sheathing with a self-adhered WRB, or a fully vapor-open design.
Although I've opened up double-stud walls insulated with fiberglass, built in the 1980s and without much moisture damage, I would not do a double stud wall today without borate-treated cellulose, or borate-treated wood fiber insulation once it's available (coming soon, made in Maine)--the hygroscopic nature of cellulose and the borate treatment work together to protect the framing and sheathing from mold and fungus damage. Mineral wool can do some things better than cellulose, but not this.
You could improve the performance of the wall by moving the vapor-retarding sheathing (or membrane) to the interior of the wall, but it greatly increases complexity, and one of the advantages of a double stud system is its simplicity.
So let me see if I can take a stab at the layers here, outside to in:
1. Metal / reclaimed wood siding
2. Raingap - Either horizontal firring strips with relief cuts in the back or horizontal firring strips over vertical firring strips
3. WRB - housewrap, Polywall? Prosoco Cat5? Nothing?
4. Plywood
5. Outer wall studs
6. Gap
7. OSB? Plywood? Zip?
8. Inner wall studs
9. Drywall
10. Paint and/or finishing
Assuming blown in cellulose is between the studs and the gap between the walls, have I got things right or setting myself up for moisture issues? My big question marks are what specific WRB as well as what specifically (product or technology) functions as the “smart” vapor barrier in the wall sandwich. I keep leaning towards Zip or liquid applied as it appears to simplify the complexity around air sealing and flashing.
Peter,
There are dozens of variations on double-walls that can work well. So deciding on one has to do with build-ability, goals - but also to some extent straight personal preference.
Brian, Jon and Mike are all worried about keeping the exterior permeable. My own variants on your proposed stack-up would do that by moving the sheathing to the outside of your inner-wall and not using it at all on the exterior - instead substituting a robust, fairly hi-perm WRB like Tyvek Commercial. That would allow you to use batt insulation without worrying about moisture build-up on the exterior sheathing, as there would be none.
One comment on the stack-up as you have proposed it: Omitting the WRB isn't an option, either from a practical or code standpoint.
Peter,
I think Jon, Mike , and I are more or less in agreement on this. Your WRB choice should be something that is vapor open. If you go with a fully adhered product, like Henry Blueskin VP100, it can also provide your exterior air barrier. Some fluid-applied products are vapor open and air seal too. If you went with a mechanically fastened housewrap like Tyvek, you'd need to tape or use a fluid-applied sealant on the plywood seams for air sealing.
You can use OSB inside the walls for a vapor retarder, however as Mike points out, that makes the build more complex. The variable-perm or smart vapor retarders, like CertainTeed's Membrain or Intello (available at 475 Building Supply) can be installed on the interior face of the interior wall, and can also be detailed to do some interior air sealing.
Focus on air sealing. And verify the air sealing work with a blower door test. It's worth doing.
Sorry to belabor this, hoping you all at least find this interesting. This is the last house I’ll live in and I have dozens of projects I need to check the box on rather than dealing with the issues brought on by moisture related issues...in other words I’m not trying for a “100 year” house or anything, I just want to build that incorporates the best of what we know about how homes work, etc today. In looking at the Blueskin it appears to be roughly equivalent to either Zip or fluid applied Cat-5, Polywall, etc...am I mis-reading their info? Assuming that time and the ability to get my framers to bend to my will is no object, Plywood on the outside with Cat-5 (assuming the windows go here) and Zip or plain OSB in the middle? I just want to make sure I get this “right enough” in order to not be dealing with issues down the road. I’m also willing to spend whatever time and materials are necessary on air sealing the details on whatever system we end up with.