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Why am I supposed to seal ventilation baffles?

Bob Dobalina | Posted in Energy Efficiency and Durability on

One reason for venting a roof (in addition to keeping the sheathing cold) is to release moisture from the attic. Given that (and assuming it is a given) I’m a bit unclear on the benefit of air-sealing ventilation baffles, particularly in a cathedral ceiling application where the baffles are continuous from soffit to ridge.

If I’m trying to ventilate the insulation space, why would I intentionally create an air-tight barrier between it and the ventilation channel? Haven’t I just built an unvented roof? I don’t think I’ve so much eliminated potential moisture problems in the attic as I’ve just moved them from the bottom surface of the sheathing to the bottom surface of the baffle.

I understand the stated reasons for sealing baffles (keeping warm, moist indoor air from reaching the sheathing and reducing wind-washing) but I would think the intent would be to control, rather than eliminate, ventilation. I also understand that much of this is moot if the ceiling plane is properly air-sealed — but if I’m going to go through the trouble of venting, I’d like to make sure I’ve got it right.

Can anyone help me out?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Insulated cathedral ceilings have a high rate of failure. The main reason is the stack effect, which brings moist interior air to the roof assembly during the winter. Every attempt should be made to limit air flow through your roof assembly.

    If your ventilation baffles leak air, they will allow any warm, moist, air between your rafters to escape. This escaping warm air pulls more interior air into the rafter bays through ceiling cracks. You don't want this to happen.

    Paying attention to airtightness when putting together the components of your cathedral ceiling assembly is the most important thing you can do to limit the chance of moisture accumulation and rot. This applies to the ventilation baffles as well as the drywall ceiling.

    The ventilation channel keeps your roof sheathing dry. It's a good insurance policy, because you don't want damp roof sheathing. But just because you have a ventilation channel, doesn't mean that you should encourage air leaks. Airtight details are still important.

  2. Bob Dobalina | | #2

    Thanks, Martin. I hadn’t considered the extent to which venting warm, moist air from the insulation space could draw warm, moist air in.

    This all leads to my next question, though: what is the appropriate material for a continuous insulation baffle like this? If I have a vapor retarder at the ceiling, and I use a sealed foam or plastic baffle, aren’t I sandwiching my fluffy insulation between two low-perm layers? Should the baffle be vapor-permeable?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    I suggest that you read this article, which answers all your questions: Site-Built Ventilation Baffles for Roofs.

  4. Bob Dobalina | | #4

    I should have started there. Thanks, Martin!

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