Could plastic attic ventilation baffles on a 5/12 roof create a moisture problem?
The article on attic ventilation baffles notes two pretty good commercial products, Accuvent and Smart Baffle. Both are made of plastic, with essentially zero vapor permeability. With a steep pitch roof, that’s fine. Any vapor that come up through the ceiling, through a leak or through vapor diffusion through the drywall, can keep going up through the insulation and reach the vent space above the insulation, so there’s no moisture trap. But there must come a point where the roof pitch is low enough that the moisture is as likely to condense on the underside of the baffle as to make its way at a diagonal to the safe exit into the attic air space. Has anyone seen a problem with that? Do you think it’s a concern, or perhaps more precisely, how low could the roof pitch get before it became a concern?
Case in point: A 5/12 roof in zone 6, where I’ve started putting in site-built ventilation baffles, making them out of Coroplast, inspired by Smart Baffle’s use of that material. I’ve convinced myself that it would have been safer to use fiberboard or something else vapor permeable, but I’m not sure it’s worth worrying about and changing materials. This is a retrofit, not new construction, so there’s no raised heel, and the cellulose insulation depth under the baffle will only be about 4 inches at the outer edge of the top plate. No vapor barrier at the ceiling plane other than paint on the drywall, but the air sealing is pretty good, and anything near the eaves that looks dubious is getting more one-part foam on it as the baffles go in.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part