Metal roofing and condensation
We are building a home in Redmond, WA. We are going to use NU-Ray 24 gauge metal roofing. Our roof will get a lot of sun so we’ll go with a light color to maximize heat reflection. Somewhere in FHB(I can’t find the article) over the last year I read that metal roofs will condense on the underside in the evening following hot days and an accommodation needs to be made to allow that condensation to escape. We were planning on installing the metal roofing directly on Ultra Ice and Water Shield over a very well insulated roof. I would like to avoid the cost and complexity of building a double roof as we have zero risk of ice dams in this location. I also don’t understand how moisture would get to the underside of the sheathing if it is installed right up against the membrane.
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Replies
Culbertson,
I agree. For condensation to occur you need moist air to condense. You are much more likely to have that with an air-gap below than if the channels are tight to the roof underlay and sheathing. There is always going to be some small gaps under the panels due to their profiles, but that also allows a drying path for any incidental condensation to be removed.
The two reasons I can think of to add a vent channel over the sheathing would be to avoid ice damming, or to provide a drying path if the sheathing had impermeable insulation on the underside. Beyond those I don't see any advantage.