How best to prevent truss lift air leakage?
I’m building a new home, and trying to make sure that my drywall ceiling is airtight. I’ve read about how truss lift can sometimes form cracks at the tops of interior walls, which would defeat my air sealing efforts.
I’ll be installing truss clips to allow the trusses to float above the walls, but I’m trying to decide how best to hold the ceiling drywall in place at the top of the interior walls while still allowing the trusses to move as needed. Any suggestions on what to use?
Also, it’s recommended to not secure the ceiling drywall to the trusses within about 16″ of the interior walls, to allow the drywall to flex and not crack. But how does that work in a 42″ wide hallway running down the center of the house, in the middle of the trusses? That would only allow screws to hold the drywall in a 10″ width running down the center of the hallway. Doesn’t sound right.
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Replies
Debra,
In such a small area you'll need to reduce the screw-free zone a bit. I'd keep them back 12", not 18". The edges are supported by the wall boards, so there won't be any sagging.
If you do get the odd crack at the intersection of wall and ceiling, it's easy to fix with paintable caulk.
Three options to eliminate problems with truss lift, from least to most practical:
1) Ideally you would keep the entire truss above and outside of the attic insulation in some manner.
2) Frame the attic as a floor platform, in plywood/osb atop floor joists, before putting on the trusses.
Tape the sheet goods and let them be the air barrier. Then you don't have to worry about the drywall, and you get a service cavity as a bonus that you can use for recessed can lights to your heart's content.
3) Apparently strapping helps for some reason. I'm not sure why a single layer of strapping would work (perpendicular double layers with an offset should certainly act like a shock absorber), but these guys say it does:
https://www.finehomebuilding.com/1998/09/01/dealing-with-truss-uplift
One more idea: install the truss as normal, install an air barrier and vapor retarder at the bottom of the trusses, then frame a service cavity below the trusses--2x4s on edge provide plenty of space for wiring and fixtures.
We followed the technique Mike is suggesting. We have details available here:
https://kimchiandkraut.net/2017/08/16/ceiling-details-air-sealing-4/
We're in our second winter and, so far, no cracks from truss uplift.
To answer your question in my opinion , poly if you are using it. Wrap it around the wall ceiling intersection. How big are your trusses ? 30 ft is a lot bigger than 24 ft. 36 ft are big. The longer the bottom chord the more you should worry. You could design your building to have shorter trusses. Trusses are a triangle and if the bottom chord gets shorter than the top chord they start rise. The 1970 is the first time I personally saw this. We quickly learned not to design trusses with a center post. This helped a lot. Maybe only because the bottom chord could bend a bit more in the middle. I think lots of ventilation is the cheapest answer. Keep the top and bottom chord the same moisture content. Lumber does change in length with different dryness. Not much but in a truss to much.
Yeah a center post seems worse case scenario since those connecting chords are really what's preventing things from happening uniformly along a plane. It's kind of like a bow and arrow set-up. In addition to the top chords lengthening and the bottom chord shortening, what's needed is the restrictive connection nearer the center. (perhaps obvious and is why its not stated usually; otherwise it's basically just rafters)
(edit): I've no observation that the type makes any difference, I was just riffing on Mike's observation... sources seem to say truss type doesn't much matter. Since a triangle will provide similar restraint as a single vertical member it may not matter much; perhaps marginally. Either way, probably irrelevant in the grand scheme as it'll happen to a degree regardless).
As a woodworker I've always found truss uplift confusing since we are taught wood doesn't shrink/expands hardly at all along its length. I think this becomes an issue with long spans (certainly much longer than furniture!) and with construction grade wood more likely to have abnormal (compression and juvenile) wood that exhibits increased longitudinal shrinkage.
I like Michael's suggestion for a robust but simple solution.
If relying only on drywall, in addition to leaving fasteners off close to the walls, it may be helpful to use some sort of fin to catch the ceiling drywall and hold it tight to the wall drywall (something like osb/ply protruding from and connected to the top plates (but not ceiling)— or they make clips and vinyl corners too I believe which may serve similar function).
Michael, do you know if its generally OK to run romex in 3/4" or 1.5" ceiling chases, so long as they're not too close to the framing 'likely to receive nail and screw penetrations'?
Thank you for the suggestions. There's a limit to what I can do since the house is mostly framed already. Our trusses are 28 feet long. No center post, as we had them frame the trusses to allow for an elevated walkway running down the center of the trusses. We're finishing the interior wall framing next week.
We may end up having to strap one of the ceilings with shimmed furring strips, as at least one truss in the main room is over 1/2" lower than the rest (don't ask me how the subcontractor managed to do that!). But in the rest of the house I'll probably use drywall clips along the interior wall top plates to secure the ceiling drywall in place.
I have a rental property that had BAD truss uplift issues. I did remove nails in some areas of the ceiling (last 18" or so) and installed blocks in the attic attached to top plate of partition walls, the underside of which I screwed the ceiling drywall into.
HOWEVER---what really made the difference is something Mike mentioned in reply #4: ventilation. Right at the time I did the work I mentioned, I had a new roof installed. Crucially, the old roof had no ridge vent, just gable vents to exhaust the intake from the soffit vents. I had a ridge vent installed with the new roof. Voila, no more truss uplift, as evidenced by the disappearance of the seasonal cracks in all areas, even in the areas I had not removed nails from.
Apparently the top chords, in the old scenario, were getting too moist, expanding, and pulling the drier bottom chords up along with them via the webbing or struts of the scissor trusses.
So yes, good ventilation is vital. And even more so: air sealing the ceiling up against the attic. The devil is in the details.
Thanks for sharing your experience. Fortunately, my attic will have excellent ventilation. So, hopefully that will help.
Hi Debra---
You are quite welcome. And the air sealing bit is key: much more moisture comes via air leaks as opposed to diffusion through materials. Thorough airselaling plus good ventilation, and you likely won't have a problem. Good luck!
Exactly. The small things you need to do to mitigate truss uplift - clips, keeping fasteners back from wall/ceiling intersections - assume it will be a minor problem. If they are not effective, something much bigger is in play and needs to be addressed.
I'd have no problem leaving 18" to first screw in 42" wide hall. The ceiling board is installed first, hopefully within 1/4" or so of the walls. Then wallboard is pushed up tight to the ceiling (top course first). Once that joint is taped and mudded, it's not going anywhere.