Void space behind drywall
deangio
| Posted in General Questions on
Is it OK to leave a void or empty space between the back of the drywall and spray foam insulation if the required R value is reached without filling the cavity? The insulation contractor has suggested only spraying 3″ of closed cell foam against the inside of the sheathing of a 2 x 6 framed wall. This would leave approximately 2.5″ of void. Northern NJ, 1000′ elevation.
thanks in advance.
Dean G
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Replies
Hi Dean.
It is common for closed-cell spray foam to be installed not filling the stud bays. Unlike open-cell foam, it is not easily trimmed. And unlike air-permeable insulation, convective currents are not a concern. Finally, because of its high R-value, it can often meet R-value goals without filling the cavity.
All that said, closed-cell spray foam is not necessarily the best option for stud bay insulation. First of all, it's both expensive and environmentally costly. Secondly, it does nothing to mitigate thermal bridging for all those costs. I suggest you read this and perhaps consider some other options: Installing Closed-Cell Spray Foam Between Studs is a Waste
If you need the SPF for air sealing, I would go with 2" of closed cell and fill the rest of the cavity with 2x4 batts. This would be much cheaper for the foam install and if you can DIY the batts, you would end up with a higher R value assembly.
Your 2x6 with 3" SPF is around R17 whole assembly. Going with 2" SPF and 3.5" mineral wool batts you'll end up with an R20 wall for a bit less money.
thanks for the replies. I apreciate both suggestions. I am not comfortable with leaving large void spaces in walls. I think they become playgrounds for critters. Maybe open cell foam is a good way to go, but its very costly. What do you guys think of just an air seal closed cell foam application around the inside perimeter of each stud bay, followed by 6" batts? Not a full flash and bat, and the moisture migration mitigation would be accomplished by the kraft paper on the batt?
Dean,
Is this a new house or a reno? I'm curious as to why you need to air-seal the stud bays from inside , rather than taping the sheathing from the exterior.
Malcolm,
Its a renovation with a small extension onto a room. I extended a second floor bedroom 2', so it has partial new construction attaching to existing construction. The sheething is done, with housewrap over the sheething , All housewrap seams taped.
Dean,
It may be a bit late then. What you now need the spray foam for is air-sealing, but much simpler and more effective would be to seal the sheathing from the outside.
Working from the inside you can still use a combination of tape, caulking, reserving canned foam just for penetrations, forgo the spray foam altogether and fill the cavities with batts.