Closed-cell foam, unvented ceiling, and vapor barrier
This is a follow up to a previous question: https://www.greenbuildingadvisor.com/community/forum/general-questions/54636/metal-shed-roof-ice-water-shield-insulation-over-and-under
Roof is 2:12 slope with metal standing seam over 5/8″ decking. The joists ends have been blocked, sealed and wrapped.
I now have 2″ of poly-iso, above roof decking, which according to this chart (http://www.weatherbondroofing.com/?page=ecomm&p=view&cid=1002&cn=XPPolyiso) gives us averaged R value of 11.4.
The overall target R value for the roof assembly is R49.
We are using closed cell foam for some other areas in the house and am considering using it to address the underside insulation needs of the roof. I also just a bit worried about any water build up in cellulose.
At a target total of R49 and with 11.4R on top – that leaves approx. 37.6R to fill. This would create 23% of total top side R. I could add another 6R below and still be above the 20% ratio.
If I target 37.6R below and use CC foam (6R per “), I’d need 6.3″ of foam. My joists are 12” deep.
– Should I add the additional bottom side foam to get closer to the 20% ratio? I’d rather stay closer to 22-23% to be safe.
– If I use CC foam do I still need a vapor barrier between the sheet rock and the joist bays?
– Is having a 5.5″ gap between the foam and sheet rock an issue? Should I look into alternate methods – to fill the entire bay – like Dana describes in the previous thread – to reduce convection? We are not using any cans in the ceiling.
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