Truss Uplift and Airtight Attic Ceiling
My project is a small cabin (somewhat delayed) that will have a vented attic space built from raised heel trusses. There will be about 16″ of insulation up there but I’d like to completely air seal the attic from the main floor. The problem is when you use trusses they can uplift, causing the ceiling plane to pull away from the tops of the interior walls and leave cracks between the ceiling drywall and the wallboard.
In the main room there are some 12″ tall ceiling beams criss-crossing the space at a height of 8′ and the bedroom, bathroom and entry have dropped ceilings.
What if you filled in between the main room beams with 2×4’s (there would still be 8″ of beam showing) and then put ZIP sheathing on top of the 2×4’s and beams (the underside of the 2×4’s would have drywall)? You would then tape the ZIP ceiling sheathing to the ZIP exterior wall sheathing and then put the trusses on top of the ZIP ceiling.
Since the trusses are what carries the roof load down to the exterior walls they can rise and fall whenever they want and the drywall ceiling plane never moves. Plus, your main floor is now completely air sealed from the attic.
Thoughts?
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Here's a picture of the idea.
Matt Risinger built his own house using the "Monopoly" method (basically taping the pitched ZIP roof sheathing to the ZIP wall sheathing and then adding the overhangs later) but this idea is to tape the ZIP attic floor sheathing to the wall top and bottom plates and then add the ZIP wall sheathing later (a sheet of ZIP wall sheathing covers the gable form and comes down past the wall top and bottom plates to help lock the gable to the wall).
Should there also be Membrane between the ZIP attic floor sheathing and the 6x12 beams or between the ZIP attic floor sheathing and the bottom cord of the truss?
Look at the truss calculations for the anticipated deflections. In flat ceiling (bottom Chord) trusses the deflections are usually 1/8" or less when loaded. That should not cause a problem. To keep the air barrier complete if you don't like the 1/8" then install the dry wall first then the interior walls.