Zip R sheathing condensing surface
Looking to build a new construction home in climate zone 5a. House will be a ranch with approximately 3600 square feet of conditioned space. Shooting for an air tightness of 1.5 ACH50. 2×6 walls and the builder typically does blown in cellulose in the cavity. I let the builder know I’d like to upgrade the insulation but haven’t given specifics yet. I don’t think there’s a lot of high performance insulation happening in my market so I would like to be informed.
I’m thinking of requesting zip r9 for continuous exterior insulation for simplicity. Looks like that will cost an extra $4500 to upgrade from regular zip. That’s just the materials. I’m concerned a little about condensation on the interior of the wall. I’ve listened to a video from fine home building on this and it was a little confusing. Is it fair to say that if I get the ratios right, I shouldn’t have to worry about condensation? 2×6 walls in climate zone 5 requires 7.5 r or greater exterior insulation. Correct? Should I be concerned about cavity insulation choice?
Thanks,
Matt
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The whole point of those interior to exterior R value ratios is to keep moisture under control in the walls. If you use AT LEAST the recommended exterior R to interior R, you will be OK. If you want to go over the ratio, you want more EXTERIOR insulation compared to the amount of interior you have. Basically for a given interior R value, you want AT LEAST the recommended exterior R value, but you can put as much more exterior R value on as you want and you just make the wall safer and safer from a moisture standpoint.
Condensation would be on the interior side of the exterior sheathing, not on the finished wall surface on the interior of the home. In the case of Zip-R, that "interior side of the exterior sheathing" is actually the interior facing side of the polyiso that is bonded to the Zip-R panel, since Zip-R puts the "exterior" insulation on the interior face of the exterior sheathing, which is opposite of the way it gets done if you put up seperate exterior sheathing panels and exterior rigid foam panels.
The short of all of that is that if the recommended ratio for your climate zone and wall assembly is "R7.5" of exterior insulation, using R9 instead on the exterior is in the safER direction, so you'll be fine. If you want a little extra insurance, put a smart vapor retarder on the inside under the drywall -- something relatively inexpensive like MemBrain is all you need if you want to do that.
Bill
Thanks Bill, I really appreciate the answer. I was thinking that was the end result. I saw Steve Baczek likes to use 2 inches of closed cell, putting a vapor barrier there. More expensive but it has other benefits. My budget prefers your suggestion. Again, much appreciated!
Matt
Bill,
One thing I forgot to ask. Should I be selective about the insulation in the stud cavity based on a small chance of moisture in the wall? What would you recommend?
Thanks,
Matt
Just for the record, 2021 IRC R1102.1.3 (R402.1.3) requires in CZ5 - R30 or R20+R5ci or R13+R10ci or R0+R20ci, so you are good with your plan.
Thanks Armando, good to know it's exceeding code.
Matt99: Blown in cellulose is perfect for your wall assembly with exterior insulation. Be sure to do an intermediate Blower Door Test PRIOR to installing the blown-in or attic insulation, so if it's over 1.5 ach50, you can fix it! If you get 3 ach50 after insulating, there isn't much you can do to fix it; before, you can. You might need to find (rent, possibly) a fog machine, they're fantastic for this - I've used them several times with great results. You basically set the fan to blow in, not out, fill the house with fog and see where it comes out - and there's your problem! I've built a number of NZR homes under 1ACH 50; it's really not that hard. (Oh, and lots of good high quality peel and stick tape)
Thanks for the information, I really need to talk to the builder about the iterative steps of blower door tests and air sealing. They use the energy star certification which requires 2.4 ACH50, I think. I've already talked to them about aerobarrier but they'd never heard of it. Maybe we can get to 1.5 or better without it.