Open-web trusses for walls and roofs
GBA Editor
| Posted in Green Building Techniques on
Anyone out there have experience of using open-web joist trusses as studs, rafters or both? I know Katrin Klingenberg has used TJI joists as studs but I was intersted in something with a more open structure and no, or little, OSB. It’s really to save the time in making my own Larsen trusses on-site.
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Dear Interested,
Open-web trusses are routinely used as rafters; talk to any roof truss manufacturer, and they can offer a range of truss configurations to frame any roof.
You'll need to consult an engineer before considering the use of open-web floor trusses or roof trusses as "studs." Katrin Klingenberg used engineering guidelines prepared by TrusJoist MacMillan when she used I-joist trusses as studs; the guidelines required OSB sheathing on the interior as well as structural sheathing on the exterior.
Onlooker,
If both chords of the truss are supported at the bottom, there's no reason you can't use them as vertical wall framing. It then becomes just a double-wall frame with diagonal gussets. If you're using cellulose for insulation, make sure that it contains only borates for fire resistance and not ammonium sulfate, which is corrosive to metal truss plates.
If your interest is in saving the time of building Larson trusses on-site you may be interested in looking at a double wall system where you tilt up a load bearing 2x4 or 2x6 - 24" oc exterior wall, toe nail on a 2 1/2" purlin / brace at 4' AFF and window sills and then tilt up a 2x4 - 24" oc non-load bearing interior wall with a 2x12 upper top plate.
On a recent 2,480 sf home profiled on this site we were able to have these walls built by a local panelization plant and delivered to the site and tilted up and roofed from slab up two stories to dried in for an added cost of $4,200 over the cost of 2x6 framed walls including the differential cost of the 2x12 upper top plate and of increasing the R-value of the Spider insulation from R-23 to R-46.
Unfortunately, in North Carolina this only results in $70 per year incremental energy savings over 2x6 with R-23, so, even at only $4,200 ($1,70/sf), not enough to break even on the increase in the mortgage payment at current energy cost.
We need the cost of energy to triple to get America on the right track to stop global warming.
https://www.greenbuildingadvisor.com/homes/nc-home-grabs-five-green-ratings
Michael,
Im curious how you came up with the $70 per year energy savings for your wall assembly.
Also, $4,200 additional costs over a 2x6 wall seems high... Was it the panelization? Did you price out framing a double wall (2x6 exterior, 2x4 interior with a gap and dual plates) assembly?
Robert,
Many thanks for an informative answer. I was planning to support the outer chord (whether Larsen or open-web truss) with a plate of 1" ply trapped between, and cantilevered out from, the inner sill plate and the concrete foundation. There would be a capillary break between the concrete and the ply and the underside of the ply would be insulated with rigid foam down to grade level. Thanks for the warning about avoiding ammonium sulfate.
Any advice on structural gypsum sheathing as part of a sealed drywall air-control scheme?
Brett
The additional costs included the installed cost of the interior wall panels, blocking and 2x12 upper top plate and the additional six inches of insulation on all the walls for the two story, 2,500 project. The walls did have separate top and bottom plates, only the upper top plate was a 2x12. Seemed pretty cheap to me compared to the ICF or AAC alternatives we also priced out. I think the portion of the cost that was insulation was about half.
The savings calculation came from a what-if analysis done by my HERS rater in the context of the rest of the energy saving features of the house. Neither the walls or the solar water heater penciled out very well in that analysis. I don't believe he was able to calculate the impact of the 37" roof overhang and shade structures.
We included both in the final project and were planning to hire Truvion to sub-meter the house to track energy performance but have run tight on money so are holding off in hopes of finding a grant to pay for the monitoring cost ($3,000 to $5,000) no luck on that so far.