Vapor retarder help!
I built a new house last year in mn (climate zone 6) using nexcem icf walls. I Monopoly framed the roof, using zip sheething, and taping the seems to the wall wrb (siga majvest). I ended up with 0.43 blower door test, and have an erv to move air. The house was pretty wet over the winter at 40-45% rh for most of the winter (I think the concrete is still drying, and I did all plaster walls). I chose to do a split insulation detail on the roof. I have 3 layers of 1.5″ poly iso above the zip sheething, and put a r30 rockwool in the truss cavity. I was under the understanding that a vapor barrier is not needed in this setup, but may have been wrong as I am getting condensation and mold on the underside of the zip sheething.
Question I have:
1.Should I install a smart vapor retarder under the top chord of the truss?
2. I know cut and cobble is generally talked down on, but wouldn’t be difficult to do it this setup as it is a simple gable in a walk up attic. Would this be a good or terrible idea?
3. Would it be a good idea to spray foam the underside of the zip with a couple of inches, and then batt the rest?
4. Did I screw something up, and am completely screwed?
No matter what I do I will have to pull all the rockwool off and mitigate the mold.
Thanks for your help!
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Replies
Dodn,
Your ratios are fine, but you need an interior air-barrier to stop warm moist air making its way through the permeable insulation to the roof sheathing. A variable-perm (smart)membrane will work well, although it will be difficult to effectively seal around the truss members. Here is how Michael Maines described the problem with these hybrid roof or wall assemblies:
" I got many calls and emails from people who were convinced to use that system and found condensation on the interior face of the foam, sometimes running down the wall in quantity. That's when I realized how important it is to keep indoor air from easily circulating through the fluffy insulation.
An air barrier (such as drywall) is much more important than a vapor retarder, but because many things can go wrong, I prefer to make sure there is a vapor retarder as well. The IRC calls for a class 3 vapor retarder when you have a proper ratio, i.e. painted drywall, so that's a minimum."
Would the spray foam be any better, or does the risk or traping moisture out weight the "difficult to effectively seal" issue?
Dodn,
With an interior air-barrier the assembly should be pretty safe. I don't think it warrants going to spray foam.
Why don't you enclose several bays underneath and on the sides as a test to see how effective the warm-side air barrier is? That might dispel any misgivings.
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