Poly air barrier on outside vaulted ceiling
I’m in climate zone 5A about to install a tongue and groove pine ceiling on the underside of the vaulted scissor trusses above a covered back deck. The homeowner had us to throw in Kraft faced insulation he had on hand incase he ever wants to close it in. From my understanding if he was going to close it in and blow any heat it would be necessary to create an air barrier behind the T&G. Inside we ran drywall to create an air barrier but this being outside I don’t think that would be the right thing to do. Would I be fine to run poly and carefully air seal to create my air barrier in case he ever decides to close this space in? We don’t run poly on the ceilings inside here but in theory I would be fine?
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It is difficult to install drywall as a long term air control layer, it is somewhat easier to install and detail poly as a long term air and vapour control layer.
For long term air tightness installing a service or installation cavity inside of the air control layer materials is suggested. In your case using a synthetic membrane and compatible tapes (SIGA, Pro-clima or Rothoblass) connected to the underside of the scissor truss then dropping a 2x4 installation cavity for electrical to run in and then install the tongue and groove pine to the 2x4's. This way the owner can remove the pine in the future without affecting the air control layer. You need to connect the new air control layer the existing air control layer of the house.