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Why am I supposed to seal insulation baffles?

user-6868842's picture

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?

My assumption wouldl be that (post #215093, reply #1 of 3)

My assumption wouldl be that you are directed to seal the baffles to maximize the "chimney effect", to maximize airflow.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Air tight is not necessarily (post #215093, reply #2 of 3)

Air tight is not necessarily vapor tight.  Wood, cardboard, eps foam are all vapor permiable to some extent, and will allow drying to the outside.  Sealing the baffles is only to control wind washing.  You want the moisture to diffuse into the vented space. 

The ceiling should be a vapor barrier, either with polyethelyne sheet, or airtight drywall with a suitable vapor barrier paint.

Insulation baffles (post #215093, reply #3 of 3)

It seems to me that sealing insulation baffles is a necessity. I'm not an expert in insultaion, but a friend of mine has recently done that on the attic and he also sealed the insulation baffles. I'm going to ask him about that and post a reply here soon.