Unvented Roof for an existing Barnodiminium
FishGuy2
| Posted in Energy Efficiency and Durability on
I have attached a drawing of the proposed roof remodel. At present there is sheetrock to the underside of the purlins, the MBI blanket over the top of the purlins, then the metal roof screwed down.
Will the proposed detail work to insulate, and prevent condensation?
And is there a specific suggestion for the layers of underlayment paper ( as to specific type ) If it matters ?
Thanks
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Replies
Some of this depends on which climate zone you are in. For unvented assemblies with exterior rigid insulation you have to maintain a certain ratio between the rigid and interior fluffy. The colder climate you are the more exterior rigid you need. This is a good read on what is required:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
In colder climate like mine (zone 5), that 2x1.5" of polyiso so ~R17 or so, you can have up to R25 of fluffy in the inside. That is an R43 roof so pretty good overall.
Any vapor barrier directly under the rigid or under the 3/4 roof deck is fine. You don't want a vapor barrier between the fluffy and the drywall as this cavity can only dry to the inside.