Piece of mind with two layers of CDX
Ian_homebuild
| Posted in GBA Pro Help on
I am building a home in climate zone 7. Engineering dictated due to earthquake risk that I have cdx on both sides of the wall assembly. Assembly also includes 1.5” furred out area inward of inner CDX, with mineral wool in between furring strips. So whole assembly, from outside in is housewrap, CDX, stud bays with 6-8” of mineral wool, CDX, 1.5” furring cavity with mineral wool, 6 mil poly, drywall.
I am concerned about any sandwich effect but am reassured since inner layer of CDX is semi permeable and should dry outward right?
Here is hoping for positive feedback since I just hung drywall. Thanks.
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Replies
That sounds like a safe assembly to me. Personally I would change from poly to a responsive vapor retarder, because the future is not going to be the same as today, and the only downside to the increased resilience is a relatively minor increase in cost.
Thanks, Michael. I’ll switch to a responsive retarder going forward but sounds like you think okay to leave existing poly on dry walled sections (as opposed to pulling a weeks worth of drywall hanging). Sound right?
Poly has worked in CZ7 for decades and likely will for some time to come. The more you find you need to run an air conditioner, the more risky the poly; that's the only time when vapor drive is strongly toward the interior to be a problem. Running an a/c for a few days or maybe even weeks should be ok. When you're running it on high for weeks on end, you could end up with microbial growth because moisture will build up behind the poly.
Thanks, Michael. For the quick piece of mind. Much appreciated.
Correct, CDX is not a vapor barrier, so you should be fine. The only point at which you could potentially have an issue is if you put a vapor barrier material on the exterior. An example would be foil faced polyiso, if you added continuous insulation. The easiest way to address that would be to use a smart vapor retarder on the interior instead of the 6 mil poly, which I'd recommend you do regardless since the assembly will be safer with the smart vapor retarder, and you'll have no restrictions on exterior cladding materials that way either.
Bill
Thanks, Bill. Similar to Michael above, I’ll do the smart retarder going forward, but sounds like both of you agree the assembly as is now is okay to leave for completed sections (I,e. No need to rip of hung drywall, just go smart retarder moving forward)?
Great minds think alike I suppose :D
The physics of this stuff is the same for everyone, so while you may get slightly different methodologies from different posters, ultimately we're all going to be recommending that you do basically the same thing for similar end results.
Bill