Matt McDonough
2/8/2012
I'm buiding a small cabin (12x16) and am considering the following somewhat unconventional insulation under a metal roof. Here's the order of materials... 1. rafter 2. 1/2" ply cdx sheathing 3. ice and water shield (lower 1/3); tar or felt paper (upper 2/3) 4. 2x4 purlins running horizontal 24OC with 1.5" foamboard/polystyrene lying in between the purlins 5. 29 gauge "pole barn" style/raised rib metal roofing with ridge vent I'm a little concerned about condensation. will this plan work? SHould I lay an additional barrier fabric (tar/felt paper) over top the insulation? thanks Matt
Todd Miller
Isaiah Industries, Inc.
2/9/2012
The extra layer of underlayment would not be a bad thing. But let me ask you a couple of questions. What do you have as far as a vapor barrier between the living area and the attic, and also what sort of attic ventilation do you have? Have there been any problems, issues, or concerns with the current roof system that you are attempting to remedy?
Matt McDonough
2/9/2012
it will be an open loft. no attic. its a new build, not fixing a current system.
Todd Miller
Isaiah Industries, Inc.
2/10/2012
Especially considering where this is located, I strongly encourage getting a vapor barrier and additional insulation into this roof system, as well as ventilation. I believe that what you're proposing will result in a high potential for condensation issues as well as winter ice damming if the living space is heated.
Eric Novotny
An informed customer is the Best Customer!
2/10/2012
Where you are located, inside a heated space, you will still have condensation across the rafters. 1.5" of foam is not enough to ward off condensation in this case and the lack of thermal break between outside to inside is not enough in this case. You are going to need at least 4" (probably more like 6") of rigid foam on that roof deck. Make sure the seams in the CDX are sealed as well so that you don't have any air loss via those pathways.

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