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A reader of mine named Donald from southern Quebec wrote to me about a
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cathedral ceiling issue they have, he has in their home. There's bat insulation
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fiber insulation in the cathedral ceiling and they've had a lot of trouble
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with it over the years. It doesn't work very well, there's lots of heat gain in
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summer and heat loss in winter and the ice and snow is melting on the
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roof and running down and forming ice dams and there's also condensation
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occurring during cold weather within the cathedral ceiling. Now these are all
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very typical issues when it comes to cathedral ceilings in cold climates. I
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hear about them all the time. Donald's plan for solving the problem is to
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completely remove all the bat insulation and replace it with 10 inches of closed
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cell spray foam and he's wondering if this is a good plan. It is actually a
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very good plan. It's going to solve all of the problems that he's been having
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and even some he probably doesn't realize he has, namely air leakage
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Spray foam insulation is great because it insulates well and it acts as its own
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vapor barrier but it also seals out drafts remarkably well so the plan's
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going to work fine. Donald was wondering about ventilating that space and he had
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some suggestions for how he was going to ventilate this cathedral ceiling but no
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venting is required. When you have completely spray foamed the underside of
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a roof structure, what you have is a sealed roof and it's considered a hot
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roof so it doesn't need ventilation and you really couldn't ventilate it anyway
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It's all going to be sealed up with the foam and then the drywall or tongue and
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groove lumber or whatever he's going to put on the inside. So typical cathedral
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ceiling problem and excellent solution on Daniel's part. Oh and I forgot to
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mention one more thing. When you've got a roof that is unventilated, as in
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Daniel's case, it's going to become very hot in the summertime and that can be a
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problem with regular organic asphalt shingles. They don't like all that heat
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and they will fail prematurely. In five, six, seven years they'll be curled and no
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good. So in Daniel's case he's going with a metal roof which will work fine. The
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metal roof is going to be sitting on strapping that he fastens to the roof structure. If you didn't want a metal roof and you want the look of shingles
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then fiberglass shingles are the way to go. They look just like regular organic
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asphalt shingles and in fact they they are a tar based product. You can't really
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tell them apart from from traditional asphalt shingles. But the difference is
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that the substrate, instead of being a kind of a thick tar paper, the substrate
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is actually fiberglass, non-woven fiberglass. And those can really withstand heat very well. I've used them in some of my own projects on roofs that are hot
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and insulated in a similar way that I'm telling you now. And they look as good
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today as as they were when I put them on 10 years ago. They've got I think it's a
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50-year warranty and they've made a good start on that too. So the only issue with
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the hot roof in a cathedral situation is the roofing. Either metal or fiberglass
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shingles are the way to go
