Adding rigid insulation on top of an existing cathedral roof
I’m dealing with an existing, 60-year old building with a 12:12 cathedral ceiling. It’s actually a small church. It has a simple gable roof with no odd geometries or significant penetrations. Just a couple of plumbing vents and a masonry chimney.
The original drawings show 2×6 rafters with 4″ of “balsam wool” insulation and a 1-1/2″ air gap between the insulation and the 3/4″ decking. There were – and are still – asbestos shingles installed over the decking, and continuous soffit vents at the eaves. NO ridge venting. 20 years ago, someone talked the church into installing foam roofing over the top of the existing asbestos shingles. It looks pretty bad, as you might imagine. We’re going to tear off the existing foam/asbestos mess and build back up from the original 3/4″ roof decking.
I’m proposing 3″ of EPS and a 1/2″ plywood coverboard, then stone-coated steel shingles with underlayment. We’re planning to install new vented soffits to replace the tired-looking ones that are there now. But again, there’s never been any ridge venting, and it doesn’t seem to have caused any issues. The roof looks terrible because of the sagging, nasty foam application, but there have never been any leaks or problems inside, and so my thinking is that what we’re putting on in place of the foam & asbestos shingles could only improve things, not create any new problems. Am I thinking about this right? Will the added insulation in fact cause some new problem? While we have everything stripped down to the decking, should we install some ridge venting? Some other type of roof vents? Would really appreciate your assistance!
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