Does traditional plaster serve as a sufficient air seal between living space and attic in an older home?
We’re restoring an older balloon-frame structure (1890s) in a northern Gulf Coast climate (extreme humidity, some heating load, large cooling load). Fortunately, we’ve been able to design the house’s first ever HVAC system so that all the ductwork and mechanicals are located in a fairly short run down the long central hall — within the conditioned space, and just beneath the existing ceiling plane (a false drywall ceiling will sit a foot or so below the old ceiling). The existing ceiling over most of the house is a rather thick plaster (with surprisingly few visible cracks). Does it make sense to blow in insulation directly on top of the old lath and plaster, or is there any economic advantage to using house wrap or additional air (or vapor?) barrier before installing attic floor insulation? A small area of the house has beadboard ceiling — I assume it would make sense to use some sort of air or vapor barrier over this in addition to the insulation?
(It may also be worth noting that this house sits on piers more than 3 feet above the ground, and that it will be impractical to add insulation to or house wrap the entire house — so for this and many reasons, it would be wrong to anticipate a completely “tight” house — but we did want to address the roof thermal issues as best we could.)
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User 6970120,
First of all, can you tell us your name?
Q. "Does traditional plaster serve as a sufficient air seal between living space and attic in an older home?"
A. Yes. Uncracked plaster is usually a good air barrier. However, ceiling air leaks can occur at all the usual locations, including at penetrations, chases, and the cracks between the plaster and partition top plates.
Q. "Does it make sense to blow in insulation directly on top of the old lath and plaster, or is there any economic advantage to using housewrap or additional air (or vapor?) barrier before installing attic floor insulation?"
A. What you want is a ceiling air barrier, not a ceiling vapor barrier. (In your climate, where you have more annual hours with air conditioning than with heating, you definitely don't want an interior vapor barrier.) If you seal the penetrations and cracks, your ceiling can be made airtight. If you are unsure of the leak locations, use a blower door to find them. For more information on this issue, see Air Sealing an Attic.
Q. "A small area of the house has beadboard ceiling -- I assume it would make sense to use some sort of air or vapor barrier over this in addition to the insulation?"
A. Again, you want an air barrier, not a vapor barrier. You have three choices: (a) You can install spray foam against the boards from the attic side, or (b) You can install drywall on the interior side of the boards and abandon the beadboard look, or (c) You can temporarily remove the boards, then install drywall, and then re-install the boards.