Insulation for an unvented cathedral ceiling in Zone 4
We are building a a “pretty good” house in southern Virginia. We are using local contractors and trying to keep things within their comfort zone. For this reason, we avoided exterior foam insulation and laid out the house so as to make the details of the air barrier simple.
The house has a 32 X 48 footprint with a cathedral ceiling running the whole length. The roof will built with parallel-chord trusses ($7k worth!), as shown in the last page of the dwgs attached.
Our insulation contractor suggests insulation with open-cell foam – about 10″. This would work as our air barrier, but according to Martin Holladay’s article on cathedral ceilings it would not achieve a vapor seal. Our contractor, on the other hand, argues that it is wise to let the roof decking drain to the inside so that you detect a roof leak before it ruins the decking.
Our questions are:
1) Should we insist on a layer of closed-cell foam against the roof decking as a humidity barrier?
2) If we make sure that we have enough foam under the decking (i.e., 31% of the total R-value of the roof, per another Martin article), would it be wise to fill the rest of the truss with fluffy insulation? (It seems a shame to waste the big void; we could have, say, 8″ of closed and open foam plus 16″ of fluffy insulation below it.)
3) Would choosing Zip roof decking (instead of OSB) help to achieve a better air barrier or humidity barrier?
4) As always, suggestions are welcome – please do answer the questions we should have asked but didn’t think of. 🙂
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