Is there ever condensation within a roof vent cavity?
Based on Martin’s answer here: https://www.greenbuildingadvisor.com/question/vented-attic-vapor-retarder US building codes do not require vented cathedral ceilings in zones 1-5 to have a vapor retarder (just need an air barrier). I also understand that the majority of roof moisture problems come from air leakage, not vapor diffusion.
Assuming the ceiling is air tight, any incidental moisture from diffusion should be whisked outside via the vent cavity. Is there any mild condensation that may develop on the underside of the roof decking from that vapor as it passes through the vent cavity? Or are we really talking about almost unmeasurable amounts when in zones 1-5, assuming at least a 1.5” vent cavity and a 6/12 roof pitch?
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There could be some condensation that forms, but it would be of little concern in the warmer climate zones. There's potential for more in zones 4 and 5. It will be dependent on the interior humidity level and exterior temps. The condensation or frost should clear quickly, as temps moderate or interior RH levels reduce. It's a different story in zones 6-8.
I'm in a colder climate (zone 7), the image shows frost that I found accumulating in my wall cavity (an air leakage issue). The pic was taken in December with low RH, under 20%. This area probably had frost forming for the life of the home, 75 years. It dries completely before the next frost cycle, so no damage accumulates. I just had a small amount of water staining on the bottom plate from the years when it would melt quickly in the spring.
StephenN,
In some climates you can also get some moisture accumulation from night sky radiance. Barring other problems with the roof, I don't think it's something to worry about.
https://www.greenbuildingadvisor.com/article/night-sky-radiation
https://www.greenbuildingadvisor.com/article/moldy-roof-sheathing-in-the-pacific-northwest
I don't know enough about hot/humid climates to comment usefully about them. From what I've read it might be better to forgo venting and use a vapour-diffusion port.
Randy and Malcolm, thank you for the very helpful and interesting observations and references. I did not know about the phenomenon of outside ventilation air condensing.
My winter climate is high-humidity and somewhat cold (average temperature 23 to 41 F), and my house has little sun exposure during the winter (on the north-facing side of a hill), so I’m not sure if I can hope for much drying between condensation/frost cycles, as with you, Randy.
In my case, my roof decking will be boards, not plywood, so I think it will be more resilient to moisture/mold.
However, in this sort of situation where conditions for drying are not good, maybe I should also try to cut down on vapor diffusion (in addition to air sealing, of course) through my roof assembly?
What I’m dealing with is a tongue and groove vented cathedral ceiling with a very vapor open membrane above it as the air barrier. So I’m thinking maybe I should add a smart membrane behind that air barrier before installing rockwool insulation between the rafters.
(Malcolm, I’ve begun building the roof whose design you helped me to evaluate here: https://www.greenbuildingadvisor.com/question/summertime-condensation-with-persist-roof. As you can see, I’ve gotten as far as the air barrier membrane.)