How to attach smart vapor between collar ties
Mill_house
| Posted in General Questions on
Climate zone 7; cape cod style house; second floor
I’m wanting to install smart vapor on the flat attic ceiling of the second floor. Are you able to provide me with some options of how I can install smart vapor so it makes contact with the spray foam between the collar ties, please?
Thanks.
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Normally you would have some framing at the point where the drywall transitions from the slope to the ceiling. Seeing that is all spray foamed in already, adding that in would be hard. Easier option would be to install a splayed sheet metal angle for supporting the drywall (unsupported corners plus drywall ends up with cracked corners) and you can also attach your vapour barrier to the angle.
Another options is to bring the vapour barrier down part way down over the spray foam section and seal it to the spry foam with acoustic caulking.
If the roof section is well vented, you might might be able to skip the vapour barrier, go with air tight drywall and barrier paint, which would be simplest.
If your smart vapor retarder is simply a vapor retarder, you don't need to have this sealed at its intersection with the sloped ceiling spray foam.
If your smart vapor retarder is also acting as an air control layer for the flat ceiling, then it does need to be continuously sealed at this intersection.
Membrain by CertainTeed has different installation instructions for its installation as a vapor retarder (https://www.certainteed.com/resources/30-28-137.pdf) or air control layer (https://www.certainteed.com/resources/30-28-137.pdf).
I am thinking of running acoustical sealant between the collar ties and stick the membrain to that. (The person above mentions this as an option.) Do you see any issues with this? Will acoustical sealant damage the closed cell spray foam in any way?
I assume you are using this as an air control layer. It looks like your SPF ends at the collar ties and you are going to use fluffy insulation above with an airtight control layer.
I think you could hang the membrain so it extends down the sloped wall a bit. Then use can foam from above to seal the membrain to the SPF sloped wall after the drywall ceiling is hung if you have access. You could do it from below, but it would be trickier.
Check what your worst gaps would be, but it looks doable.