Vapor barrier under metal roofing

Mark Empric
I am completing construction of a second floor addition to a two car detached garage. I have designed and completed a gambrel (barn style roof) in which all roof segments are identical. The roof trusses are 5 1/2' wide, laminated 7/8"(1 3/4" total thickness)exterior grade plywood, glued and screwed. They are placed 12" on center. The purlins are 3 1/2" wide, 7/8" exterior grade plywood fastened with 3" deck screws and run purpendicular to the trusses, 11" on center. I am planning to use a standing ridge roofing system, to avoid as many holes from through screwing as possible. Soffit and ridge venting will be installed on the entire roof. My question is regarding a vapor barrier. Does a vapor barrier have to be installed over the purlins? I have done all the work myself so far and would like to also install the roofing. I do not have access to a bucket truck and am forced to climb the purlins to install the roof. If a vapor barrier is installed correctly,over the purlins, no holes or seams can be present which covers all possibilities of climbing the roof via the purlin footholds. Will Kraft faced 4" fiberglass insulation installed from the inside between the 5 1/2" wide trusses 12" on center be sufficient to inhibit condensation on the back of the metal roofing? If my calculations are correct I should still have 1 1/2" of open space from the fiberglass insulation to the bottom surface of the purlins for soffit to ridge vent air flow. Any help would be greatly appreciated.
Guest User
Mark, I would use a 3" fiberglass insulation with a vinyl face. The roll will be 6 feet wide and will leave room for you to get up and down beside it. Some standing seam roofs will "rumble" in high wind. The insulation helps prevent it and to quiten it when it does happen. The vinyl face is you vapor barrier. If you tear a hole in the vinyl face, patch it with a piece of vinly tape. Good Luck Wade
Todd Miller
Classic Products, Inc.
Wade's advice is sound.
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