Rigid Foam vent channels
Martin Holladay, if you are out there, or etal:
Martin some time ago you posted a reply to Gary Steinfeld where you stated “you can use rigid foam to create a ventilation channel.” I like this idea and would like to plan on using a foam ventilation channel in a cathedral ceiling application, followed by FG between the rafters and another layer of foam on the underside of the rafters. Some have suggested firmly that the vent channels be WELL air sealed.
My concern is that this method might trap any mositur which might get into the FG. Of course the goal would be to air seal the underside foam and the upper side so as to not have a moisture issue, BUT…..!
Martin since suggested that this method was ok and you are a GBA Advisor could you comment.
Thank you.
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Replies
Bob,
As you wrote, "Martin since suggested that this method was OK."
Martin still suggests that this method is OK.
If you use EPS or XPS foam to create the ventilation channel, and the foam is 1 inch thick (or less), it will be somewhat vapor-permeable and will allow some drying to the exterior in a pinch. I think everything will be fine.
A follow-up to Martin
I had been thinking of using 1 1/2 inches of foam for the shoots, but 1" would be fine. 1. Do you agree with air sealing the shoots?
2. What type of foam would you then recommend for the underside of the rafters and would you still add a "vapor barrier" (ie plastic) to the underside of that foam? Naturally tape seems on the foam.
Thank you again.
Bob,
Q. "Do you agree with air sealing the shoots?"
A. Yes. More information here: How to Build an Insulated Cathedral Ceiling.
Q. "What type of foam would you then recommend for the underside of the rafters?"
A. Any type of rigid foam will work, but foil-faced polyiso is the most environmentally friendly foam, and is also the easiest to tape.
Q. "Would you still add a vapor barrier (i.e. plastic) to the underside of that foam?"
A. No. Foil-faced polyiso is already a vapor barrier. More information here: Do I Need a Vapor Retarder?