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Could plastic attic ventilation baffles on a 5/12 roof create a moisture problem?

charlie_sullivan | Posted in Green Building Techniques on

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.

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  1. Expert Member

    5/12 is a fairly steep roof. I've always found something in that range to have good air movement from soffit to ridge.

  2. GBA Editor
    Martin Holladay | | #2

    I addressed your question in the article you linked to. I wrote:

    "One of the reasons that builders install ventilation channels is to help damp roof sheathing dry out. Researchers now realize that ventilation channels can help a little bit at this task, but not as much as some people think. ...

    "What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?

    "The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.

    "That said, there really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles. The main reasons:
    Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
    The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
    Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting."

  3. charlie_sullivan | | #3

    Thanks, Martin. My only follow-up question is rhetorical. How did I miss that?

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