I’d like to learn the pros and cons of this insulation technique. I’m leaning towards requesting it for a new home for a couple of reasons; R-value versus cost and possible post-installation low-voltage wiring changes.
BTW, I’m building in SE Mass.
Thanks in advance.
In this FHB Podcast Segment, the crew chats about how and where to place XPS insulation in a basement.
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A couple of the fiberglass companies are now promoting a sprayable caulk to do all air sealing followed by bats. Do a quick search and you'll articles on it. Might be hard to find installers though as it is new.
Why use flash and batt?
From your mention of cost and ability to fish wires through the finished wall, it sounds like you are considering the pros and cons of foam versus fiberglass batts. But if you use fiberglass, why use the flash and batt system? Why not just use fiberglass batts and wrap the house with air infiltration barrier?
Maybe I am missing something, but I don’t get the point of flash and batt. You end up with an air barrier, which if fine. But the air barrier is also a vapor barrier, which is completely wrong for a house in the northern climate. If you use flash and batt, and also use a warm-side vapor barrier, you would have the dreadful “Double vapor barrier.” With that, any vapor that gets past the interior barrier might condense and be trapped in the wall by the outer barrier. This is one of the worst possible outcomes.
What you want is the most perfect vapor barrier possible on the room-side of the wall, and a barrier on the outside that lets water vapor pass, but prevents air infiltration. The theory is that the interior vapor barrier will leak to some minor extent, even if installed as perfectly as possible. So any moisture that gets past it should be able to pass easily through the rest of the wall and get out as easily as possible. That is why the outer barrier has to be able to “breathe” i.e. be able to allow moisture to pass through it.
Read FHB issue #217 - march
The Fine Home Building article pretty much covers it all. You can order a back issue or sign up to read it online.
The short story is that flash and batt makes sense, but in a cold climate you may need thicker foam than you think to avoid condensation inside the wall cavity. In Mass you are looking at about 2" of the fancy stuff.
In my non-professional opinion it seems like a good option. I am planning this on my new house in New Brunswick, but I will use insulated sheething instead of OSB if the architect/engineer can get the details to work (for 'racking' strenght we'll need obs on corners only plus some additional supports, etc).
Be sure and cost the flash/batt option vs exterior foam (think thicker...) plus batts or cellulose fill.
I'd recommend conduit for future changes
I'd highly recommend conduit to accomodate future changes to wiring, and then insulating with either foam or blown in fiberglass.
The blue smurf tube for the low voltage stuff is inexpensive, and easy to run.