Does sealed rigid foam prevent air infiltration like sprayed in?
For insulating between rafters in an unvented attic ceiling, does cut-to-fit and foam sealed rigid panels achieve the same air infiltration benefit as spray foam? I’d prefer to not have to come up with some kind of roof deck venting strategy.
Replies
You're not making sense.
You're not making sense. There's very little air infiltration through a standard asphalt shingle roof, so no real need for an air infiltration barrier.
Sorry - I'm referring to the
Sorry - I'm referring to the vented vs. unvented issue. As I understand it, the roof doesn't need to be vented if you use spray foam insulation, as it completely prevents any air movement. I'm wondering if the same effect is achieved using rigid foam panels sealed with canned foam.
The roof doesn't need to be
The roof doesn't need to be vented with sprayed foam because the foam is (if you pick the right type) an effective vapor barrier.
Right - exactly. So, then it would make sense that rigid foam board cut to fit, and sealed on all edges with canned foam or caulk would achieve the same thing. I was just asking if there was something I was missing???
Assuming your sealing is good, it will act as an effective vapor retarder/barrier as your post intended.
Good question. Ive'
Good question. Ive' considered this in the past as well, which is one of the reasons I like cellulose instead of foam.
At initial install I would think both spray foam and foamed-in ridgid foam would seal well. After several years of framing movement with temp and humidity swings, I would expect gaps to develop between the foam and the framing in both instances, and the resultant leakage would be different in the case of ridgid, I would think, because once you've broken the seal at the perimeter, moist air could migrate behind the ridged foam. Spray foam may have developed gaps at the framing members but would probably still be mostly adhered to the decking preventing widespread moisture migration.
The above is all speculation of course. My cellulose contractor says he sees gaps in spray foam at the framing members frequently.
Steve
Thanks for the reply. I'm
Thanks for the reply. I'm trying to determine the best method for insulating a cathedral ceiling over an attic space I want to convert. I'm trying to preserve as much headroom as possible. The roof is currently framed with 2x4's, 24" on center. I think I'll need to sister on 2x6's for both structural integrity and insulation depth. However, I don't have soffits or roof vents for ventilation.
Will you be needing new roofing shingles anytime soon?
If so, then re-roof now but put foam panels on top of your roof decking. Then more sheathing on top for the new shingles.
The most recent JLC mag has an article that shows the details.
Thanks, but my roof is in pretty good shape. I have exposed rafter tails and no soffits, which is the standard in my area. The attics were intended to be ventilated through opening of windows or gable louvers. So, I think I need an insulation option that is also a vapor barrier.
Can I install a plastic sheet vapor barrier and then blow in cells for the same effect?
I'm wary of cells behind
I'm wary of cells behind poly. I think it manages moisture best when it can dry to th interior when conditions are favorable for that.
Steve
I think the problem with this is that the rigid foam will not fill every gap on the underside of the roof like spray foam will. There will be space for moisture to collect but no space for ventilation. Not good for the roof. I don't think it would pass inspection.