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Community and Q&A

Rigid foam with metal roof and cellulose?

cjaigner | Posted in Green Building Techniques on

Hi have an old carriage barn that was converted to liveable space in the upper catskill mountains, New York (climate zone 6a). It gets very cold (-4 degrees Fahrenheit baseline for heat loss calc) but almost never have to run AC (a few days a year). I am recapturing the attic space in this building as conditioned liveable space but the 7/12 pitched roof is a pretty low ceiling with 5.5″ rafters and I don’t want to lose much more head room. Sheathing is currently 12″ rough cut 1x open sheathing with wood and asphalt shingles above. I am replacing the roof with metal standing seam. Originally I was going to add 5″ rigid foam above roof sheathing (according to recommendations read on this site) to avoid condensation but then I thought, if it’s never going to have AC, then won’t all the condensation happen in winter on the inside of the sheathing? Does that mean a bit of condensation is ok as long as rafters are vapor open and vented? Then would I be able to do only 2.5″ of rigid foam (or a vapor open foam board?) on the outside of the sheathing (avoiding complicated trim work), put in baffles and fill the 5.5″ rafters with cellulose (lower environmental impact), maybe put one more inch of rigid foam on the inside before sheetrock, and put a vented ridge cap? I’m trying to get as close to R50 as possible. 
My goals are – be as environmentally friendly as possible, keep ceiling height, minimize work.
I know I could just cc spray foam btw the rafters but I’m not crazy about never being able to access the roof assembly again, low chance of seeing problems until they are too late, and about the poor environmental impact. Then again, the roof has a saddleback that was heavily reinforced, maybe better off doing spray foam and 60 years down the road, if there are hidden structural issues, just tear the whole roof off and replace.
Thanks for your help!

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  1. Jon_R | | #1

    Some code. Follow it conservatively plus air seal well and condensation won't be a problem.

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