Zip System R-Sheathing
mculik5
| Posted in Green Building Techniques on
Quick question about Zip System R-Sheathing.
The concept of a REMOTE wall, where exterior insulation is used to ensure that the structural sheathing stays warm (and thus dry), makes perfect sense to me.
The Zip System R-Sheathing is a compelling product because it saves several steps by installing all the necessary wall components in one go.
HOWEVER, from the pictures on their website, it looks like the Zip System R-Sheathing has the WRB on the outside, followed by the sheathing, followed by the insulation. So, doesn’t this mean that the sheathing is cold?
How does that work?
Thanks.
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Replies
Exterior rigid insulation keeps the OSB warm so interior water vapor won't condense on the interior facing of the OSB sheathing.
ZIP-R OSB doesn't have to worry about interior condensation because there's a layer of vapor impermeable foam between it and the interior. Consequently the ZIP-R OSB dries to the exterior (ideally behind a rain screen). In addition cold air = dry air so there really isn't a risk of condensation on the ZIP-R OSB in winter.
My two cents.
Matt,
Zip-R sheathing has many good qualities, including ease of installation. But if you are looking for warm sheathing, choose another product.
For more information on this category of sandwich products -- some of which are designed to be installed with the rigid foam part of the sandwich facing out, and some of which are designed to be installed with the rigid foam part of the sandwich facing in -- see Nailbase Panels for Walls.
-- Martin Holladay
For less money you can install taped 1/2" Plwd/OSB and 1" rigid foam board taped with an approved tape to be a WRB. With that assembly, plus dense packed cellulose in cavity insulation, we get 1ACH50 EVERYDAY!!! You should have the rigid foam on the OUTSIDE of the sheathing... Did I mentioned less money?
In my opinion Zip Rs foam is in the wrong place. Putting the foam between the sheeting and the frame weaken the wall even if you install the 100 plus nails per 4x8 sheet called for in the nailing schedule and you still have a wall that is not good enough for hurricanes and earthquake zones.
Walta
It looks like you've got some good responses on the zip sheathing specifically, but here's something for you to think about.
With sheathing, keeping it "warm (and thus dry)" is not so much the goal, but the means of achieving the goal, if that makes sense.
What you don't want is for the rate of wetting to exceed the rate of drying for a long enough period of time to cause damage.
Keeping the sheathing warm and dry (on the inside of the foam) is a great way to achieve this goal, but it's not the only way.
Zip-R can also achieve this balance, because the foam on the inside controls the rate of wetting from the inside, the membrane on the outside controls the rate of wetting on the outside, and the ventilated rainscreen gap allows the sheathing to dry to the exterior.
Whether or not Zip-R is a better solution than the more standard approach described by Armando is a different question, but hopefully this clarifies why the sheathing being warm or cold is not the critical issue, but just one factor in the flow of moisture.
Brendan,
Well put.
-- Martin Holladay
Brendan,
Your explanation makes sense. I was considering using the Zip-R on a project (zone 4) with 5.5" of open-cell insulation in the wall but I was thinking about thermal-bridging, not vapor. Do you think there could be enough vapor migrating through the open-cell insulation to cause issues with the exterior sheathing? I've heard that cathedralized attics with open-cell spray foam have had issues with decking but I haven't heard of this being an issue in wall assemblies. Thoughts?
Michael,
Do you end up using the ZIP R-Sheathing on your project? If so, how did it work out? I'm in the same climate zone and am considering R-Sheathing on an exterior renovation. Thanks.