ventilation / drainage plane

Charles Morris
I am building a creek side cabin in a North Georgia mountain valley. The climate is humid with an annual rainfall of 56 inches. The annual heating degree days exceed the cooling degree days. We have 200 plus sunny days. The cabin will be shaded. I plan to use corrugated metal to best fit the cabin design. The minimum roof pitch is 3/12. The sheathing will be 5/8ths OSB with a premium underlayment or ZIP roof sheathing. Rafter bays will be insulated with 5-1/2” of open cell foam. Closure strips will be installed to keep out insects. If the roof is vented a screened opening will be installed at the eave and ridge. If ZIP is installed and properly taped will I benefit by also installing an underlayment? I am concerned about ventilation and a drainage plane between the sheathing and metal. Is venting necessary in my climate since I am using the open cell foam for insulation? Should vertical battens be installed so moisture cannot collect behind horizontal purlins? Thank you
Eric Novotny
An informed customer is the Best Customer!
I know they say the ZIP tape will not come apart but I would love to see some lap seamed synthetic underlayment as well. Cheap insurance if you as me. Is the roof going on battens? If so and you are only using horizontal, make sure the are notched to allow water or use the plastic ones...or run verticals over them. Venting above the deck is never a bad idea as it will allow any water that makes it past as well as some convective cooling.

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