The house that Dennis Miller plans on building next spring will include a cathedral ceiling with timber trusses exposed on the interior. The issue, as Miller explains in this Q&A post, is making sure the ceiling gets an effective air barrier that will prevent moisture problems in the roof.
“In a regular ceiling I’d think a continuous plane of drywall would do the job,” Miller writes. “However, this is a cathedral ceiling built on timber kingpost trusses that are exposed to the interior. In this case, the ceiling drywall is not continuous but broken into sections by the exposed timbers.”
As the drawing at the top of this column shows, the ceiling will be insulated with cellulose. Rafters in this Climate Zone 5 house will be supported by a ridge beam and the outer wall in this double-stud wall design.
Miller wonders whether his only option is to do a “tedious caulking-gluing-sealing job” where the drywall meets the trusses. Or is there another way?
“I think my question really applies to any situation where someone has a ceiling with exposed beams,” he says. “What are some good methods to ensure a good air barrier when the ceiling plane is segmented?”
That’s the topic for this Q&A Spotlight.
A continuous layer above would be best
In general, Malcolm Taylor replies, the best approach is to include a continuous layer of material above the exposed trusses. “That’s in part why many timber-frame houses have tongue-and-groove or plywood roof decking, which can be fastened to the tops of the trusses,” he says.
Taylor thinks that sheathing and taping the tops of the 2×4 strapping would be the safest and most effective approach. As an alternative, Miller could use rigid foam insulation for that layer, although it isn’t…