Ceiling air barrier detail with plenum
We are going to build a Pretty Good House in the PNW (zone 4c), using 2×6 24” oc af, with zip sheathing and 2” of CI. We’ve spent a lot of time trying to figure out our continuous air barrier for the attic, and find ourselves with a silly question despite all our research.
Three things:
1. We are going to install a fire sprinkler system
2. Our HVAC ducting will be between the 1st and 2nd story
3. Attic is vented, with ~R60 fluffy stuff resting on top of our air barrier
We have tentatively settled on a plenum truss design to accommodate the fire sprinklers and electrical inside our conditioned space, as opposed to building a continuous service cavity everywhere (we’ve floated using i-joists, open web joists, or 10” dimensional lumber). This means we shouldn’t need a service cavity anywhere else in our ceiling.
We are undecided on whether to create our air barrier using a membrane like Intello, or taped plywood—either option would be on the underside of the trusses. Even though my partner is a builder, he’s never done a continuous air barrier like this, so has no experience with the labor or material cost differences between the two (any advice or thoughts on this is welcome). Regardless of which method we use, our silly question is this: can we fasten the drywall directly to the taped plywood (or the underside of the trusses if we use the membrane) in the non-plenum part of the trusses, or is this a bad idea? It seems wasteful to use strapping just to hang the drywall, but we are wondering if there are good reasons for going to that trouble?
Thanks, Sterling
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Replies
Sterling,
If the majority of your electrical will be in the plenum with just a few boxes on the ceilings elsewhere I think you are fine not strapping the ceiling, and attaching the drywall directly to the trusses or plywood.
Air-sealing aside, the preference breaks down on an east/west divide. Builders on the east coast can't see how you can build any house without strapping the ceilings, those in the west somehow manage to do just that successfully.
A Membrane works fine as an air-barrier on the ceiling. Make sure you add strips of it over interior partitions while framing. Also think through how it connects to whatever is your primary air-barrier on the walls.
If you decide to up-grade to plywood, you can sheathing the ceilings before the interior partitions go in. It also makes dry-walling a lot easier - but plywood does come at a cost premium.
Thanks, Malcolm. I'm a bit surprised to hear that in your experience Intello is cheaper than plywood. Are you pretty confident in that? We are in the Portland OR metro area, and a crude search for price per sq foot of each material seems to indicate that plywood is cheaper. Is it possible that pricing varies a lot regionally?
We have no preference for one material over another other than cost, as both have a reputation for performing well if taped properly (and connected properly to wall assembly).
Sterling,
Sorry, I meant generically - whether poly or some other type - membranes are usually cheaper than plywood. If Intello is similar in cost, and you feel those are the two choices you are comfortable with, then I'd use the plywood.