We visited a builder yesterday (Streetman homes in Austin) and they gave us this talk about how great their insulation was with exterior foam sheathing. We asked for more details and that’s what I could gather.
So from what we knew, foam is usually associated with sprayed foam insulation in the form of Icynene (open cell) or another type of foam. This provide most of the insulation of the house but at a more expensive cost (both the material and work needed increase the cost).
The sheathing is usually something that helps with the structure of the house, when plywood or OSB (oriented strand boards) is applied but has usually a poor insulation value. Which isn’t a problem because the insulation stay inside between the plywood and the dry wall.
So the advantage of using foam is that it adds an additional R-value to the house while a less expensive insulation is used on the inside (typically glass fiber batts). The version they used was a 1/2 inch styrofoam with an added R-3 value. This isn’t much but is in addition to the rest of the insulation typically a R13 from batts. So total it would easily reach R16. An addition to the insulated sheathing is a problem most stick built house will have is that the wood inside the wall is touching the exterior and interior side at the same time and doesn’t have the insulation value of the other materials put there. If say 20% of the exterior wall is in contact with wood, there’s a consequent “thermal bridge” or leakage of heat through those. This is the case even with sprayed foam, as the sprayed foam typically goes in between the wood of the wall. So the foam itself has an high R-value (say R20), but doesn’t cover the wood parts of the wall itself, causing some thermal bridge with the outside. At least that’s the theory.
This type of foam (styrofoam) also provides an air and vapor barrier so that the inside of the insulation is prevented from becoming wet, wetness according to the people we talked cause a decrease in R-value of batts or foam based insulation (with open cell like icynene) and in the case of glass-fiber batts will cause them to reduce and sag over time.. The foam itself is really durable and may not suffer through the lifetime of the house.
Possible disadvantage is probably the extra cost vs the usual sheathing used in a house, as well as the fact that it’s petroleum based.
If you’ve built a house, own a house, or had some experience with this type of insulation please let me know in the comments !