Repurposed polyiso location
I’m in NE Colorado in climate zone 5. I’ve got a 7″ deep wall cavity in a post frame building w/ 8′ post spacing. I want to maximize the insulation value in these walls, but I’m prioritizing air sealing over pure r-value. I have access to good quality 3″ reclaimed polyiso, so my plan is to utilize those 4×8 sheets of polyiso to go into my 8′ wide bays between structural posts. I intend to use foam sealant around all seams to improve air tightness. After that, I would put a slightly compressed R-19/21 batt in the wall cavity. The goal is to get above R-30 whole wall assembly.
The question I have is this: Is putting the polyiso at the exterior of my wall the best location, or would the polyiso be better off on the interior side of my wall? From what I’ve read on this site, it seems the 3″ foam would be fine on the inside of my wall given my climate zone and probably allow the polyiso to perform better as it would be warmer. But I’ve also read things about wind washing of fiberglass and the negative impacts on fiberglass performance, thus my original idea of putting polyiso outward of the fiberglass to better air seal it. Any advice is much appreciated.
Thanks,
Brandon
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Replies
Hi Brandon,
It might be helpful for GBA members to know more details about the sheathing, wrb, siding details, etc. to share their thoughts on this one.
If I read your post right, you are working from the inside, so those details are likely in place already. If that's not the case, you should install the rigid polyiso on the exterior side of the sheathing and then fill the cavities with other insulation. If you cannot install the rigid foam outside of the sheathing and your shell is leaky, you should probably install the rigid polysio in the wall cavities first, cut-and-cobble-style to create your air barrier, then fill the cavity with other insulation.
Builders are relying less and less on canned spray foam to air seal these days as it has proven to be unreliable, particularly smaller beads of it. You might consider caulk or an air sealing tape instead. I am sure you'll get some advice on what to use.
Thanks Brian. To clarify, my project is a barndominium (aka post frame w/ living quarters and shop combined). With steel roof and siding. The original plan was to spray foam the building and be done with it, but the more I've researched and read this site the more I've realized that's probably not the way to go. I have no exterior sheathing, but do have tyvek between wall girts and steel siding. So, it's looking like the polyiso will need to serve as the primary exterior air barrier in this case?