Michael Lobby
12/2/2010
I'm at the insulation/drywall stage of converting half of my 1700sq ft pole barn shop into an apartment for cancer-fighting mother. I want to finish the vaulted ceiling with drywall. Between the trusses are 2x6 purlins 24"OC, above those is vinyl-backed fiberglass batting and over that is the metal roofing. Can I remove the vinyl backing (as I did for the walls) and add fiberglass insulation between the purlins and then drywall over (under) that? My concern is about venting and condensation. Another idea I had was to nail 2x4's 16"OC across the purlins and then the drywall. This would serve 2 purposes - one to better attach the drywall to prevent sagging, and two, to create a channel for air to circulate from the top of the wall to the peak of the ceiling. Anyone out there have some suggestions. You can view a few pics here: http://www.michaellobby.com/pbapt.html Thank you for your help!
Eric Novotny
An informed customer is the Best Customer!
12/3/2010
The air circulation should be right underneath the roof deck and not between the ceiling and the roof deck. The insulation is already there so I would leave what is in place because I think you will have problems removing it and setting the roof back. Your proper insulation layer should be the other side of the finished ceiling. I would fill those 2x6 cavities with the proper insulation depth and then make sure you have a complete and proper vapor barrier that will not allow any humidity to leave the space and collect against the roof line.
Michael Lobby
12/3/2010
I can remove the vinyl backing pretty easily, revealing the thin layer of fiberglass (that's right against the metal roof). That fiberglass stays in place and I would add to it with 6-1/4 in. x 23 in. R19 Batt Fiberglass Insulation with paper backing toward the room. The drywall would go directly over that. When you say "proper vapor barrier" should I cover the insulation (paper side) with plastic before the drywall. Thank you for your reply.
Eric Novotny
An informed customer is the Best Customer!
12/6/2010
The kraft facing on the fiberglass is not a proper vapor barrier because it is not complete and unbroken (i.e. the gaps will allow air/humidity to pass through). Don't bother with removing the plastic on the existing insulation and you really don't need the kraft faced batts. Install unfaced batts and put up a 6 mill poly barrier prior to drywalling. Seal the seams and seal up at the edges.
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