Double stud walls & wood shrinkage
My evolving plan for my new house in northern Michigan uses double stud walls. The outer wall sits on the concrete basement walls while the inner wall sits on the floor that sits on an inner stud wall in the basement. The inner stud wall of the basement is separated from the concrete by 3″ of XPS. The outer stud wall supports the roof trusses. The inner main floor stud wall is sheathed on it’s exterior side and this sheathing is the vertical portion of the primary air barrier. The outer stud wall supports the roof trusses. If I tried to use the lower chord of the trusses as the ceiling air barrier I see a problem. The inner wall is nearly twice as tall and has much more wood in cross grain compression compared to the outer wall, so even if both walls dried equally the inner would shrink much more than the outer leading to failure of any attempt at air sealing from wall to ceiling. Is this a valid concern? My best solution is to NOT use the lower chords of the trusses to form the ceiling but add ceiling joists supported by the inner wall and interior partitions. I would put OSB on top of the ceiling joist and detail that as the ceiling air barrier (caulked to the inner wall sheathing) and have the lower chord of the trusses above that, of course this means my outer walls will need to be taller & the OSB above the ceiling joist may as well continue outward to tie the walls together. Before addressing the shrinkage issue I had planned on sheathing the bottom chords of the trusses with OSB to form the top of a service cavity then ‘hanging’ 2×4’s below that to support drywall. Going to 2×8 ceiling joist sheathed above before the trusses is more conventional, probably less labor but more lumber but it does allow can lights inside the air barrier. Am I right that it eliminates any concern over differential shrinkage of inner & outer walls?
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Hi Jerry,
I am more visual than verbal
It is difficult for me to follow your description.
can you post a drawing?
Since you are concerned about differential shrinkage....
check out figure 3 in this BSC Insight
http://www.buildingscience.com/documents/insights/bsi-023-wood-is-good-but-strange/files/bsi-023_wood_is_good.pdf
John,
Thank you for an excellent reference. The problem is much smaller than I thought because I'm using an ISL rim board and 'Trimjoist' which are more dimensionally stable than cross grain wood. But the article mentioned truss bottom chord uplift which can cause a similar problem. I still favor the redundant ceiling joist as it is much easier to construct and allows the can lights the wife wants. After reading the article I'm much less concerned about the ability to achieve a lasting air seal around windows and doors as gaskets should easily accommodate the movements. BTW my scanned wall cross section sketch is to big to upload.
Jerry,
Are you still in the planning stages of your project?
If so you might be interested in checking out some of the details I have posted at my blog.
I also wrestled with some of the issues you mentioned...
I found it very convenient to keep the all the load bearing structure to the inside (inner frame) and then to "build out" the wall afterwards.
This way the sheathing is very easy to "connect" from wall to ceiling.
A drawback to this approach is that it requires an interior service cavity...
Although John Brooks has suggested that maybe drywall could be applied directly to the plywood sheathing (sans service cavity)...
I haven't explored that idea much yet, but I may experiment with down the road...
If your scan is too big
and....you have a PC.....
Start Menu
All Programs
Accessories Folder
Snipping Tool
Pretty easy to "snip" an image from your screen
It will be saved as a not-so-big JPG
Jerry: How are you taking care of any wood sitting on concrete? Gaskets between? Treated wood? You also may want to search for the Sunrise House (Home?) that Thorsten Chlupp has built and posted pics of here. It sounds like you have similarities.
John,
Over concrete I'll have a closed cell foam (blend of EDPM,Neoprene and SBR) gasket and use a pressure treated plate. The gasket is a capillary break, the PT is a code requirement.
I was unfamiliar with ; 'Sunrise House' and am frankly flattered that you see similarities, I see some too, especially now that I've added the ceiling joist. Interestingly Thorsten strongly advocates locating windows near the center of thick walls which I'm planning for a very different reason. I plan on using 'thin brick' as exterior cladding and wanted recessed windows to mimic the "look" of conventional masonry veneer so my windows are essentially in the middle of my walls. Other than my much more modest insulation, r80 ceiling, r40+ walls, r25 basement, r10 slab, the biggest difference is that I use the outer exterior wall to bear the roof and set it on the concrete basement wall and I build basement stud walls to support everything but the outside wall and roof.. Also I gave up on solar thermal as an economical heating source and opted for PV and air source heat pump instead. But I will use a condensing vent less clothes dryer.