Posted By aksmith42 on 01/15/2009 12:56 AM
i was thinking about using the isoboard for cavity filling, it will be quite a bit of labor. now i forgot to mention that i put up 1.5 inch owens corning xeps (r-7.5) around the entire outside of the house over 7/16 osb sheathing and foamed all the seams. if i used the foil backed isoboard in the stud cavities would that create a vapor trap? if so how about beadboard insulation. i'm building the house myself and would like to be able to install the insulation. i'm not real big on fiberglass and i worry about dry cellulose settling. the isoboard sounds like a good idea to me if it wont cause moisture issues. thanks for the input.
XEPS will indeed be a vapor barrier, but if you're using the isoboard
as cavity filler it isn't much of a vapor barrier as long as you don't foam-seal the
edges along the studs, and glue it in an array of blobs to the OSB leaving a very tiny air gap. You'll have condensation potential on the
inside of the OSB during the coldest weather (IIRC, in MA it's a
code-requirement that if the outer insulation layer is a
vapor-impermeable plastic it needs to be 40% or more of the total
insulation value when fiberous materials are used on the interior. This is presumably to keep the dew-point & frost temperature layers inside the less susceptible plastic, not the wood or fiber insulation. In warmer climates that can be less, but with 3.5"
of iso you're adding another R22-24, so you're at about 25%. You'd still have to be a heluva perfectionist to cut the
isoboard to fit so tightly against the studs that it forms a vapor
tight seal, eh? ;-) But the fact that you can't/shouldn't seal the isoboard in order to not moisture-trap the OSB makes it somewhat more akin using fiberglass or cellulose from an insulation stackup point of view. Even at ~25% of total R-value it may still be the right thing to do, depending on your actual climate.
The worst-case amount of condensation you'd get is far lower if you AIR
seal the interior wall perfectly. (Air transported moisture is
generally orders of magnitude than what can permeate through wall
matierials.) Most of the time it's possible to do this by glueing the
wallboard to the studs at the seams, and caulking/foaming around
plumbing and electrical penetrations/boxes. But even assuming interor air can find it's way between the outer foil of the iso and the OSB, as long as it has some gap it can dry toward the interior when temps rise again.
The amount of convection you'd get around the isoboard chunks in that type of assembly is
non-zero, but orders of magnitude lower than what you'd get with
fiberglass batting (the reason fiberglass loses insulating value with
increasing delta-T), and the net R-value gain is good, even when
bridging is considered. With isoboard you'd get a clear-wall R value (including the R7.5 sheathing) of about R19.5-R20.5, whereas with cellulose or half-pound foam you'd be at ~R17 assuming the standard ball-park of 15% framing area for 2x4 walls.
Worries about dry-blow cellulose settling in wall-cavities are really
overwrought. If installed at a proper density (3lbs. cubic foot or
higher) it simply CAN'T settle- it's under permanent spring-tension of the fibers themselves. Cellulose settling over time is a much bigger issue in low density open-blowing like attic floors, etc.
But if you have the walls open, wet-sprayed cellulose can result in a
tighter air seal (but it sounds like you're already well on the way
with seam-sealed OSB for sheathing.)
There is a host of info on wall structures' insulation stackups & vapor profiles for different climates zones here:
Ideally, if you could get factory-seconds on fiber-faced isoboard (like the recycled stuff I installed in my basement) you could then caulk & seal it to the studs with abandon (or not). It's vapor permeability is high- much higher than gypsum board or latex paint, but still forms a very good air-barrier.