Air sealing baffles
Matt_Michaud
| Posted in Green Building Techniques on
Just re-reading Joe’s roof venting article in FHB and wondering about a common warning – that baffles (even for a vented attic) MUST be air sealed. If we assume that the ceiling plane is truly airtight (we are using a taped AB paper), and the wind-wash effect of a vented attic on loose fill is negligible, then why is the airtightness of the baffle important? Wouldn’t a friction-fit foamboard baffle already be better than the open cellulose in the rest of the attic? Caulking around each baffle is a lot of extra work if not truly necessary and the angle and space makes a foam gun nearly impossible.
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Replies
Matthew,
Most ventilation baffles are installed without proper air sealing. If your access is limited, a "friction fit" approach may, indeed, be an acceptable compromise under the circumstances. Do your best, and move on, without regrets.
The need for meticulous air sealing of ventilation baffles is more important in a cathedral ceiling assembly than a vented attic (that is, an attic that only requires 12 inches or 24 inches of baffle length near the eaves).
In all homes, having an airtight ceiling goes a long way to avoiding attic moisture problems, excessive heat loss, and ice dams. If your ceiling is leaky, you're in trouble. If your ceiling is airtight, you're ahead of the game.
Martin,
Thanks for the reassurance; friction fit it is!
Venting the top or bottom side of roof sheathing relies upon a driving force--either wind or stack effect--moving outside air to increase drying potential of the roof assembly.
We are hoping that most of the time, that outside air being driven into the vent space is drier than the air currently in the vent space in contact with the roof sheathing.
If that outside air is mixed with leaking inside air, could be that mixing reduces the drying effect of the moving air in the roof venting.
It's all about managing air movement rather than being uncertain about it.
Peter