Orienting Foil-Faced Foam-Board Insulation
I have an old house that was prefab (1950’s). The walls are plywood inside and out with what appear to be furring strip material used as studs. Every 37″ there is a metal strip about 4″ wide that runs the entire height of the wall inside and out with more fur strips under the metal for strength. These metal enclosures also were areas to run electric throughout the house. I believe all of the furring strips were nailed and glued and the plywood as well. They really didn’t insulate the walls other than some kind of spray that looks like possible sprayed tar on the plywood between the wall. I would like to insulate a small portion of the external wall about 10′ long. I would like to use foam board on the interior side of the wall glued to the interior plywood and then fur on top of the foam insulation and finally drywall over that. The foam board I purchased has a foil face on one side. I am not sure if the foil facing should face inside or to outside. What I am concerned with is the foil facing going to create moisture in the wall? I would appreciate any advice!! Thanks so much! (Location St. Louis, Missouri)
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You are probably on the edge of Zone 5, correct? (See here to determine your zone: https://www.energy.gov/sites/prod/files/2015/10/f27/ba_climate_region_guide_7.3.pdf).
Would you be open to having a contractor blow cellulose or rockwool into the walls? That might be preferable to giving up (maybe) 2 inches (R-7.5 foam plus new drywall and all that entails.)
Usually, the experts advise improving air sealing and adding attic insulation as the steps that offer the best value in terms of energy savings and comfort.
Regarding the orientation of the foil face, Martin Holladay answered the same question from another reader this way: “If your brand of polyiso has foil on only one side—many brands have foil on both sides—then the foil should face the air space. The foil is a radiant barrier. If the radiant barrier faces an air space, then the air space will add between R-1 and R-2 to the R-value of your wall assembly. If the foil doesn't face an air space, it won't add any R-value. Of course, the polyiso has an R-value independent of the R-value of the air space.”
The foil facing, unless it's perforated, is a vapor barrier regardless of which side it goes on. I would consider the entire sheet of foam a "vapor barrier" if you have such a facer, and all the usual rules about where to put it in an assembly would apply.
The one case I can think of where the "which side" issue really matters a lot is if you have a fire-rated product such as Dow Thermax (or similar). These products typically have one facer that is much thicker than the other, and ONLY that facer has the fire rating, so it MUST be installed facing towards the FIRE RISK side of the assembly. In the case of a basement, that would mean towards the interior. In an attic, it would mean towards the interior of the attic.
Regular foil faced polyiso only matters in terms of orientation if you're trying to get that extra R1ish radiant barrier benefit. It's worth trying for that if you can easily do it, such as if you're putting furring strips over one side of the polyiso anyway.
It sounds like in your case you'll have an assembly of studs/plywood/polyiso/furring/drywall. I would face the foil towards the air gap created by the furring strips between the polyiso and the drywall in this case.
That sprayed tar sounds like it might be some kind of water resistant barrier (WRB)? I've not seen something like that sprayed on an above-grade wall. Asphaltic coatings are common on below-grade masonry to protect against water intrusion. You might want to post a pic on here so that you can get some commons as to what that material might be, and if it creates a moisture trap risk with your wall assembly or not.
The sheathing may be fiberboard. (See here for photos: https://inspectapedia.com/structure/Fiberboard_Sheathing.php.)