Spray Foam In Select Locations
Hey everyone, I’m currently in the process of building in Vermont, zone 5B and am about to begin insulating. My plan is to only spray foam select locations of the envelope to achieve an improved air seal (truss heels, top/bottom exterior wall plates, rim joist and perimeter chords of gable trusses). All other areas will be a mix of rockwool in wall cavities and a batts with loose fill combo in the attic. I just want to make sure I’m not setting myself up for failure with moisture related issues if I proceed with this approach? Also wanted to get any input on whether or not using spray foam at these locations would be beneficial since both wall and roof sheathing is ZIP. With the ZIP system already in place would this just be throwing away money using spray foam strictly to air seal? Thanks, everyone.
– Garrett
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Replies
Garrett,
Of the locations you mentioned only the rim-joist makes sense to me. If you are using Zip, and have thought through the sealing of it as the primary air-barrier, I'm not sure what the spray foam adds.
At the rim-joist it acts as a vapor barrier to keep moist interior air from getting to the sheathing. If you end up not using spray foam at the other locations, I'd use foam board on the rim-joists instead.
The other tricky spot is where the air-barrier transitions from the Zip on the walls, to underside of the trusses. That is best handled by laying a strip of membrane over the top plates of the walls to connect the two.
Spray foam is always the most expensive option, needing just a little make it even more expensive as the set up and clean up cost become a bigger percentage of the job.
If the house is still in the design stage with some thoughtful planning there is almost always a lower cost solution giving you better performance using greener materials.
At the truss wall connection getting a raised heel on the truss allows you to get R60 of cheap fluffy insulation over the wall at a very low price.
All too often is seems the trusses have been ordered or installed before anyone thinks about how to insulate.
Walta