Air/Moisture Sealing Between Ceiling and Unconditioned Attic
Background: I am about to build a new, two-story house, with a full height basement in the Roanoke, VA area (Bedford County – Climate Zone 4A – Mixed Humid). It will have a vented, unconditioned attic space, R49/R50 blown cellulose insulation, raised heel trusses, asphalt shingles, and composite wood (LP) lap-siding over OSB sheathing using Benjamin Obdyke Hydrogap self-adhered WRB. Walls will be 2×6, 16″ O.C., advanced framing techniques, dense packed cellulose in the wall cavities.
My question: What is a good, cost effective (Pretty Good House approach) way to air and moisture seal the interface between the second floor ceiling and the unconditioned attic space? I do not plan to have any HVAC equipment or ducts in the unconditioned space.
Not a new problem, but I am looking for some recent examples of how folks have solved this issue without going to extremes of time and/or cost.
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Replies
Will the attic be vented?
In zone 4 painted drywall is an adequate vapor barrier.
Caulking all the pipe and wire penetrations in the ceiling plates is generally good enough to stop the air flow.
Advanced framing is generally 24 on center.
Consider bumping up the blown in insulation up to R60.
Consider damp sprayed cellulose for the walls. Lower cost for the same R value.
“I do not plan to have any HVAC equipment or ducts in the unconditioned space.”
Flat out say no to the bad idea of duct work in your attic and draw the duct work in your plans from the beginning. Be strong and ready to say no to all the stupid ideas when they spring them on you at the last minute and there will be many opportunities for stupid.
Walta
Walta,
"It will have a vented, unconditioned attic space"
Mikkee,
Two fairly straightforward choices:
- One is what Walta suggested. If you use the drywall as your air-barrier make sure to run gaskets along the sides of the interior partitions and inside of exterior walls before boarding. Quite a bit of air makes its way into the attic by that route.
- A second approach is to run a variable-perm membrane over the underside of the trusses, and then strap with 2"x4"s. That makes boarding simpler, and means any holes in the drywall don't matter. For continuity either run strips of membrane between the top plates when framing them, or run the membrane 0ver the whole ceiling before standing the interior walls.
This....is the way and what we plan to do on our build next spring.
"A second approach is to run a variable-perm membrane over the underside of the trusses, and then strap with 2"x4"s. That makes boarding simpler, and means any holes in the drywall don't matter. For continuity either run strips of membrane between the top plates when framing them, or run the membrane 0ver the whole ceiling before standing thei9nterior walls."