Zip R6 vs. Exterior Foam – A financial equation
Another wall assembly insulation question — I know.
My original plan was to have the following wall assembly: Vertical cedar siding > rainscreen > Zip R6 > 2×6 rockwool cavity > smart vapor barrier > drywall. Easy enough, framing crew can install without concern, all good.
But then I saw that the Zip R6 panels were coming in around $95 a sheet (and I’m needing about 72 sheets) and I started to wonder about using exterior rigid foam and if it would be a good option. I had previously dismissed this because it seemed too complicated.
A big detail — I would personally be the one installing the foam and furring strips, so even though there would be extra laps around the house for these extra steps, we could assume labor to be a cost of zero. I’m considering this a pretty non-technical bit of labor, but if someone has other thoughts on that, please let me know.
I want to also consider the other items that are affected when using extra outsulation. I think the biggest one would be bucking out the windows — which I’m still a little fuzzy on in the first place even after reading several articles. Other than that, longer attachments and extra tape for the foam seams are really all that’s needed for extra materials, no?
If so, and assuming a sheet of 2″ Polyiso at around $40-$45 — it might be the same price for a much beefier thermal assembly.
Interested to hear people’s thoughts. Anything I’m missing? Is the math off? Is it not worth it put myself through all this labor when I could just stick with the simple Zip R6 solution?
Thanks,
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What's your climate zone and/or approximate location?
Sorry - thought I had it. CZ 6
With 2" polyiso you can still use 5" deck screws ($0.20 a piece), keeping fastener costs down. Switching to exterior insulation requires you to make a choice about the air and vapor barriers: either at the sheathing or at the foam's outer surface. I'm going with Zip wall sheathing, taped at the seams, for my build's air and vapor barrier. There will be 2" of Rockwool over the sheathing, fastened with furring strips and screws. I'm leaning toward window bucks, but they need to be flashed so add some costs for extra flashing tape. You need something to protect the exterior insulation from bugs at the top and bottom (stainless steel mesh is about $2-4 per linear foot for that). Regular zip is going to provide more shear for your wall than Zip R as well, if that's a concern for your region.
The reason for ZipR is labor savings. You could come up with something that is cheaper to buy with better performance but will cost more to install.
Your labor might be free but time isn't, always good to try to save on materials but you also have to finish the build.
If ZipR is too spendy, do the cost on 2x8 24"OC with R30 batts instead. About the same assembly R value and you don't have to do any of the work.
Thanks Akos -- makes sense and will explore
Without exterior insulation you get no thermal break though...is that not the holy grail these days?
Thanks for this question. I have the same exact one but for a retrofit and plan for seconds polyiso which is very cheap. I prefer the zip r9 which is 2 inches. Would be over board sheathing. Debating myself.