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 clear how he would go about sealing it, and then build the rest of the roof structure on top of it.
Yes, adds Akos, use some kind of sheet material above the timber beams and tape the seams. If Miller tries to seal each seam between a truss and the drywall, failure is likely — even if he’s meticulous in his attention to detail.
“Sealing drywall along long seams is a pain (speaking from personal experience),” Akos says. “If you want it to last, it has to be a very clean, even joint, and you need to put a backer rod into the gap, and then caulk. I did most of this work myself and even then had to redo a section where the joint failed after a couple of years. (No matter how careful, human nature is to take shortcuts and will bite you in the $%^). The probability of this being done properly by a contractor is near zero.”
Adding a layer of foam above
With those suggestions in mind, Miller considers adding a 1-inch-thick layer of extruded polystyrene (XPS) insulation above the drywall strapping, leaving a few small openings to blow in cellulose. Those openings could be sealed later.
“This would make the XPS the primary air barrier rather than relying on the drywall layer,” Miller says.
“Or,” he adds, “instead of XPS, just put down a 4- or 6-mil sheet of poly, with a few openings to blow cellulose and then seal them shut. I would think poly or XPS is OK if the [assembly] can dry to the exterior, and in this design there is a 2-inch ventilation space under the roof sheathing.”
Zephyr7 suggests that Miller use polyisocyanurate insulation rather than the XPS (since the blowing agents in XPS have a high global warming potential). Reducing the thickness to 1/2 inch would produce the same air-sealing result.
“Polyethylene sheeting would work, too,” Zephyr7 adds. “You’d still have to detail everything at the edges, which might be tricky if you have timber framing. You could caulk the poly to the timber pieces, that might be better than trying to tape it.”
It’s the detailing that will count, points out GBA moderator Brian Pontolilo.
“Airtight drywall is a legitimate approach to air-sealing,” he writes, “but it is much more involved than hanging and taping drywall panels.”
Pontolilo said he initially wondered whether ZIP System sheathing might be an option for the continuous layer of material above the timbers. Whether Miller opted for that, or rigid foam, carefully taping the joints and detailing other potential trouble spots would be essential, and to that end Pontolilo points Miller toward an article and video series on the topic.
Or, move the air barrier
Trevor Chadwick has another idea: move the air barrier so it’s above the rafters, rather than in the area immediately above the trusses, as the drawing below illustrates.
“Seems to me that the extra materials to do it this way would be less than all the labor trying to get a quality sealing job another way,” he says.
The idea puzzles Miller, who has been planning on an interior air barrier that prevents moist interior air from getting into the wall and reaching an area where it might condense.
“I assumed I would also have an exterior air barrier, basically what is shown in Trevor Chadwick’s drawing, to prevent external air from blowing into the insulation and undermining its R-value,” Miller adds.
He also had imagined only drywall on the ceiling, rather than adding a layer of polyiso or XPS, so that the assembly could dry to the interior.
“If you were doing the ceiling without the timbers breaking things up, wouldn’t you just make a giant drywall plane in an airtight manner, have an exterior air barrier (per the Chadwick drawing) and be done with it?” Miller asks. “But I’ve complicated things by allowing the timbers to be penetrations? And that’s the issue I’m trying to resolve.”
Chadwick points to the ventilation channel between the insulation and the roof sheathing. He contends that given where Miller is building, and that he’s going to be using dense-packed cellulose, an air barrier on both sides of the insulation is probably unnecessary.
“You’re right that I might not need so much air barrier,” Miller replies. “… So maybe the ventilation channel, uncaulked, is just fine. But one thing I was hoping to prevent was the moisture from humid summer air condensing in the insulation as it gets near the air conditioned ceiling. But again, maybe I’m over-thinking this — if it’s OK that the insulation in the attic doesn’t have air and vapor barriers over it, then why should my assembly need it?”
Our expert’s opinion
Here’s what GBA Technical Director Peter Yost has to add:
The way in which Dennis Miller is thinking this through — and using the skilled advice of other GBA folks — is a testament to exactly how GBA does great work: collectively.
Here are some additional recommendations to consider:
- Connecting exterior wall and roof air-control layers: So often with timber-frame structures, rafters and ridges extend from conditioned space to the outside. In my experience, these are never continuously sealed to start, nor maintained over time. There’s just too much wood movement for sealants to manage. So, healthy overlapping at the eaves (because rafter tails don’t extend to the exterior) eliminates a tough and losing battle.
- Mechanically supporting adhered overlaps: The other great performance feature of overlapping the roof and wall barriers happens at the eave, where you can reinforce your airtight seal with sticky stuff by mechanically supporting subsequent rigid materials. Relying on both chemistry and physics is a great “belt-and-suspenders” approach.
- Do you need two air control layers in this roof assembly? If your cavity insulation is air-permeable, then it must be confined on all six sides for the full insulating effect. Cellulose insulation is air-permeable — not nearly as air-permeable as, say, fiberglass batts, but cellulose insulation is 50 times too porous to qualify as an air barrier material. And if there is an air leak into the roof assembly, there will be moisture carried with that air, and moisture that will certainly reach dew point as it moves towards the first condensing surface.
- Using Huber Zip sheathing system on your roof: The thing about this is that the material compatibility of the sheathing surface and the pressure-sensitive adhesive (PSA) tape has been worked out by the manufacturer. Zip tape sticks to the Zip sheathing very well (see my series of blogs). It’s at least worth considering.
- Polyethylene sheeting: I know that there are many Canadians who have successfully detailed polyethylene as an air control layer, but this remains a mystery to me personally. And I will take a rigid air control layer over a flexible one every time. The latter is simply more durable and easier to detail for sticky (chemistry) and mechanical trapping (physics).
- Test your air barrier: Since Miller will be doing most of the work himself, and also for the first time, I think it is essential that he test the continuous control layers with a blower door test supported by infrared imaging. This sort of performance testing is incredibly revealing, particularly at your first rodeo.
- Your ceiling interior vapor retarder: From what I can gather from reviewing your Pennsylvania code and the 2009 International Residential Code, there is no explicit guidance or requirement for an interior vapor retarder for your roof assembly. But if we apply the language of the code for exterior walls — and use the logic of vapor movement by diffusion during the winter in cold climates — I think you have three options: (a) Warm your air-permeable roof cavity insulation with topside rigid insulation according to R-value ratios that keep your first condensing surface temperature above the dew point for the three coldest months of the year; or, (b) replace enough of your cellulose with air-impermeable insulation, such as spray foam, to avoid first condensing surface temperature issues using the same R-value ratios as above; or (c) install a Class I or II interior vapor retarder in your roof assembly. For more on how much rigid foam to install, see “How to Install Rigid Foam On Top of Roof Sheathing.”