I have read many posts and articles and believe I know the answer to this question but would just like some confirmation. I am building a cathedral ceiling, vented under the sheathing, and using 24 ga galvalume R panel fastened directly to the sheathing. Since I live in a warm climate, I don’t think I should use asphalt felt due to the potential for it to adhere to the metal roof. Does it matter if the synthetic underlayment is breathable since I am venting under the sheathing and if so, what are some recommended products for this use?
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One way to address the concern about metal roofing sticking to asphalt felt is to install furring strips (purlins), parallel to the ridge. These are usually 1x4s or 2x4s, installed 24 inches on center, installed on top of the roofing underlayment. Not all types of metal roofing can be installed over purlins, but for those that can, it's a good approach -- because it allows condensation drops to evaporate faster than when the metal roofing is installed over plywood or OSB.
You can definitely use synthetic roofing underlayment if you want -- even the types that are vapor barriers -- as long as you have a ventilation channel under the roof sheathing.