Using I-joists for Larsen truss web and building wrap for blown cellulose
I am building a 2 story house on the Kenai Peninsula of Alaska and I am planning on installing a Larsen Truss on the exterior of a 2 story 2×6 framed house sheathed with plywood. As an owner builder who is trying to balance time efficiency vs cost efficiency I am trying to think of ways to speed up and ease construction of the Larsen truss. I recently thought of using I joists for the Web. It seems like this could speed up construction significantly allowing for the building of the truss on the ground prior to installation.
My plan was for 12″ thick exterior walls blown with cellulose. I could use 9 1/2 I joists with a 2 x2 on the outside. This would give me a truss similar to Thorsten Chlupp’s version from his “Arctic Wall.” I am Planning on my siding having a ventilated rain screen. Should I be concerned about the longevity of the OSB stringers in this wall?
I am having an extremely difficult time finding a framer who is willing to install Larsen trusses for me. I have gone over the design with about 8 different guys with no takers. My thoughts are that if I construct them this way I might be able to find an installer. I may also just build them like the original design, dadoing out the 2×2 for web installation.
How much, if any, is the structure of the truss compromised by only having the outside be continuous, with blocks on the structure side?
My last question involves the exterior of the truss. I am planning on covering the trusses with some sort of membrane, Tyvek, Typar or Solitex Mento Plus are the contenders right now. Can Tyvek hold back the cellulose or do I need to get the fancy European membrane to do the job. I’m in Alaska and shipping is quite expensive up here. Mento Plus is over 3x as expensive. Granted I want to build my house right, but I don’t want to blow money where I don’t have to. As a side note I purchased Tescon VANA tape to seal the plywood and I am really impressed with it.
Thank you in advance for any insight or feedback. I have gained an inordinate amount of knowledge from this site, and specifically to the topic the thread, comments and links entitled all about Larsen trusses.
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Replies
Chris,
Typically when I joists are are used as part of a Larson truss system they are fastened to the outside of a sheathed, stick-framed, load-bearing wall, acting essentially as spacers. So your first obstacle is to find a manufacturer or engineer who will stamp them for use vertically as a structural element. You will also need to figure out details for headers and cripples to take loads over openings.
regular Tyvek is fairly flimsy, but Commercial Tyvek is robust enough to contain the cellulose.
Thank you Malcom. My house is framed up with typical 2x6 construction. And the headers have been installed. I'm looking for feedback on using the I joists as spacers. I won't be using the joists for the entire truss, just replacing the ply wood with the I joist/osb, I'll work on a drawing to try and paint a better picture on what I'm envisioning.
Sorry Chris, If I had taken the time to read you post carefully I would have seen that.
I don't see any problems with what you propose. The stringers in the trusses are no more susceptible to damage than OSB sheathing would be. Your main concern will probably be cost. I joists are expensive, but that may be mitigated by the price of alternatives in your local economy.
Chris,
Here are links to two relevant GBA articles:
All About Larsen Trusses
The Klingenberg Wall
Q. "Should I be concerned about the longevity of the OSB stringers in this wall?"
A. Probably not, although there are a few (conservative) builders who wonder whether OSB will really last 100 years.
Q. "How much, if any, is the structure of the truss compromised by only having the outside be continuous, with blocks on the structure side?"
A. This question is confusing. I don't know what "having the outside be continuous" means, and I don't know what you mean by "blocks on the structure side."
Q. "Can Tyvek hold back the cellulose or do I need to get the fancy European membrane to do the job?"
A. Malcolm has suggested that you use the commercial version of Tyvek, and he is probably right that that will work. (I've never tried doing that, so I have no opinion). I know that people who have omitted exterior sheathing on Klingenberg walls have usually ended up going with two layers of furring strips on the exterior side of the vertical trusses -- horizontal plus vertical furring strips -- to make sure that fabric bellying is not a problem.