i found a business near me that sells factory second foil faced polyiso foam in 4' X 8' X 3.5 inch sheets for a really good price and was thinking of using something like this to insulate my 2 x 4 walls.  has anyone ever dealt with factory seconds in foam?  what would be some of the problems with the foam.  now even if it was new foam would it be okay to use foil faced poly iso in a wall.   the r value for a 2 x 4 wall would probably be around 24.  all thoughts would be appreciated.  thanks

If the business is near you, go take a look!  I suspect much of it is dinged corners & dents, maybe the occasional foil-peel, etc.

Are you talking about cutting it & fitting it into the cavities, or installing it on one side of the stud or the other?

Cavity installation isn't recommended- the thermal short-circuits of the studs,footers & headers  is huge- 1.5" wide stripes of R3-R4 really destroys the overall clear wall R-value.  The total framing area ususally runs about 15% of the total surface area in 2x4 construction, which will lower the actual clear-wall R value to about R13 (!)  (If you wet-sprayed the cavities with cellulose or filled 'em with 1/2lb foam like icynene the clear-wall R value is about R10)

If applied over the studs you'll get the full R value of the isoboard + whatever the rest of the construction assembly  provides, and sealing/taping the seams of 4x8 sheets is FAR less labor intensive and easier to get perfect than if filling the cavities.  Since the foil is a vapor barrier it's important to be mindful of that and not create a vapor trap by what you put on the other side of the studs.  Housewrap, felt etc are air barriers- not to worry, but avoid polythylene sheeting.  If the ISO board is on the outside of the studs (under the siding), take care that the interior paints are at least somewhat water-vapor permeable (most latexes are fine.)

Applied to the inside of the studs, be sure to foam-seal all electrical & plumbing penetrations/boxes, since air-transported water vapor is large compared to any permeable material, and foil-face isoboard with mastic or FSK taped seams is about as perfect a vapor-barrier as practicable- use that to your advantage!  Gypsum board can be either glued directly to the isoboard or run on furring strips that are held in place with long screws to the studs. (Furring strips are better if you ever plan to hang pictures etc on the walls.)

A bit of background:  I insulated my poured-concrete basement with recycled 3" un-faced recycled isoboard (that had previously been installed on a factory building roof). I studied the vapor & construction issues quite a bit before proceeding, but the performance has been OUTSTANDING.  (They're literally the best-insulated walls in the house, by far.)  I glued it to the concrete with plastic-compatible construction adhesive in vertical striping (to allow any condensation to run to the floor rather than soak into the iso), and held it in place with furring strips, over which I applied 1/2" mold-resistant sheet rock (required for fire code with unfaced iso).  At the top I foamed-in the gap at the same time that I insulated & sealed the band joist & sill. The result was a slightly better than 20% drop in the whole-house fuel consumption (and a much warmer, still unheated basement.)  It's good stuff- only sprayed on 2lb polyurethane foam would outperform it.

dana,
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.

You can use it, but you won't be gaining that much considering all the thermal bridging. If you are worried about the foil facing, just make a board with lots of nails in it to poke a bunch of holes in the foil. However, once you consider all the labor, it is just so much easier to fill the cavity with fiberglass. You might consider adding this foam around your xeps if that is possible.

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:

http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/2007-04-12.6587338447/download

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.