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Using Both Fiberglass and Polyiso for Vented Cathedral Ceiling

kb57442 | Posted in General Questions on

I know this has been hashed to death but I have a couple of questions because we are in a bit of a bind.

I had originally planned on doing a unvented cathedral ceiling on the home we are building by getting 3″ of closed cell foam sprayed against the roof deck and then filing the remainder of the 2×12 rafters with R30C insulation.

We can’t get spray foam now so I’m scrambling for a new solution.  The ceiling is a simple sloped ceiling (with the 2×12 rafters) and was made so it could be vented.  But I can’t get to R49 if I use the 1″ to vent the roof.

The only way to get to R49 (or close to it) would be to use 3.5″ of polyisocyanurate.  The only way I can figure out how to do that and still use my R30C fiberglass I already have is to put the fiberglass 1″ below the roof decking to allow for a vent channel, and then put a 2″ piece of poly iso between the rafters and a 1.5″ piece across all of the rafters. (although all I can get right now is 1″ and 2″ pieces. )

If I do this is there any issue with putting the poly iso on the bottom instead of on the top of the fiberglass?  Is there a better way to do this?  If I put the 2″ poly iso on the top and then put in the fiber glass, and then a layer of 1″ poly iso across the bottom of the rafters I figured that would create a vapor sandwich from what I’m reading so I assume that method wouldn’t be good?

Any feedback would be greatly appreciated.


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  1. brian_wiley | | #1

    Hi Kyle,

    This article might be old news to you, but I thought I’d mention it on the off chance that you hadn’t seen it yet:

    1. kb57442 | | #2

      That's a good article. Unfortunately the cathedral ceiling we have is about 1000 sqft. so it wouldn't qualify for the reduced insulation requirements.

      1. brian_wiley | | #3

        I’m sure I could be mistaken, but my reading of the code is that the 500 sq ft requirement only applies to the second option in the article. Option 3 seems to apply regardless of square footage.

  2. the74impala | | #4

    Have you considered furring out into the room for mor depth. More awkward with 2/12 pitch, but if you go vented on that slope I thought more vent, like 2" would be better than 1" if you get down an extra 2x4 on edge, you could get close to what you are attempting, and still have plenty of vent. Just a thought.

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