Sealing the Ceiling

It was pretty easy to make the walls airtight. Zip-Systems sheathing around the exterior, Zip-Tape on the joints, foam to plug every incidental hole. The ceiling is much more difficult because we must think in three dimensions to seal “inside” the house entirely from “outside” the house.

From an HVAC perspective, the attic is “outside” the house. We’ll install a thick layer of blow-in insulation directly above the second-floor ceiling to match the R-value of the house’s 12-inch-thick double walls, and the ceiling drywall will serve as the air barrier. To join the ceiling drywall to the Zip-Systems sheathing, we included an overhanging 16-inch-wide plywood nailer around the perimeter back when we built the exterior walls, as you may recall. Since the nailer fits between the walls and the roof trusses, it leaves a 1/2-inch gap between the ceiling drywall and the truss bottom chords. And if we can’t nail the drywall to the trusses, we need some other way to support the ceiling in the middle of a room.

The colorful parts of this diagram constitute our air-seal system.

The colorful parts of this diagram constitute our air-seal system.

Don’t worry if that last paragraph confuses you… it took a long time for me to get it, too. The important thing for construction is that we need strapping above the second floor so we have a place to nail the ceiling drywall. Slowly but surely, Carson and I are installing 2-1/2-inch-by-1/2-inch straps across each second-floor room, spaced 16 inches on center measuring from the insulating stud walls. Carson made a guide by marking 16-inch intervals on a scrap piece of strapping, and I can hold up the guide to a truss bottom chord to mark where I should align each strap. Then we cut the materials to length and attach them with sheathing nails. Lots of working above our heads, standing on stepstools and ladders, eyes pointed skyward. The strapping fills in that 1/2-inch gap I mentioned, and the regular spacing will inform us where to nail the drywall when we reach that step.

Strapping, spaced 16 inches on center above the master bedroom.

Strapping, spaced 16 inches on center above the master bedroom.

Above the mudroom was a different story. There’ll be blow-in insulation below the shed roof here, but the ceiling to support it is 12 feet above the floor, which is uncomfortably high for a traffic-heavy space. Instead, we’ll build a false ceiling 9 feet above the floor. The true ceiling, 12 feet up, we built from leftover pieces of Zip-Systems, and we sealed it just like we did the exterior sheathing. It took some tricky sawcuts, some awkward lifting, and some even more awkward holding-in-place to get the mudroom ceiling up… so Terry helped me.

The mudroom's true ceiling, sealed with Zip-Tape.

The mudroom’s true ceiling, sealed with Zip-Tape.

Colin closes up above the mudroom ceiling so the insulation won't fall out.

Colin closes up above the mudroom ceiling so the insulation won’t fall out.

We’ve made fantastic progress on the electrical front in these last few days, including Colin’s first attempt to turn on power for the whole house. Did he succeed? I’ll give a full report next time!

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