Has anybody sprayed a 1″ layer of 2 LB (R7) foam in a 2×6 wall for air sealing and then put 5-1/2″ R21 Fiberglass unfaced batts in the wall? I am thinking of doing this because of the cost of Foam. 3″ of Foam would give me R21, but cost about $3.50 a board foot. The combination of both would give R27 at a lesser cost.
Any help would be appreciated, Thanks.
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That is version of the "standard" approach to wall cavity insulation, as done by many of the large insulation contractors in Indiana.
A "veneer" of sprayed urethane foam is put in each stud cavity, against the sheathing, lapping onto sole plates, top plates, and studs. The formula and application result in a cured foam thickness of 3/8" to 1/2".
Cavities are then stuffed with faced fiberglass batts.
I thought is was a great method.
I would aim for at least 3/4" thickness of foam, to make sure the dew point is in the foam.
Yeah, as Taylor suggests the problem is the dew point. If you spray a very thin layer of foam and then put in FG the dew point (in any part of the country that actually needs R27) will occur somewhere in the FG, and condensation (badness) in the FG will result. If the foam is thick enough then the dew point will be in the foam, which is to say on the "outside" of the vapor barrier, and the gods are happy.
I'm not entirely confident that an R7 layer is thick enough, though. Keep in mind that most commercial buildings maintain a lower inside humidity than homes usually do, so commercial building practices won't necessarily be sufficient for home use.
Another disadvantage to this approach is the air movement within the FG itself. This simply cannot happen in the closed cell foam product.
The price seems really high. 3 inches should be less than $3 per sq ft. The labor for 1" vs 3" is virtually the same. Only material costs are additional. Of course this material is expensive but also the best you can buy.
Stu
The answer to that depends on the part of the country you are building in.
My cold climate requires that the VB be on the inside of the wall. Your plan puts it on the outside. That would be fine for a moderate or a cooling climate, but not for any heating climate with more than about 4500 degree days of heat required. Your FG could end up sopping wet.
The price you mention is a mis-statement though. installed is nominally a buck per board foot, so that insatllation might be three fifty a SQUARE FOOT
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Thans guys i will contact Stu and go all foam