Friday, February 5, 2016

Construction - Pre-Made Trusses for Exterior Walls

"Super-insulated" seems to be the green building catch-phrase for exterior walls that greatly surpass the recommended R-factor.  For our passive solar design that eliminates convention HVAC, super-insulation is not an option.  We need to turn away as much heat as possible in summer and retain as much heat as possible in winter. Consequently, we are using wall trusses in lieu of "two-by" studs in order to minimize thermal bridging and maximize the space for wall insulation. (Reminder: click on any photo to enlarge it for detail.)

The trusses are thick enough to hold 15" of rice hull insulation at slightly over R-3 per inch for a total of R-45+.   By contrast, the R-value for fiberglass batts in 2 x 4 walls is 15 and, for 2 x 6 walls, 19.
Truss jig; the aluminum angle "iron" provides rigidity
when crooked salvaged 2 x 4s are forced to fit the
jig in order to give absolutely straight trusses; notice
the pre-cut truss parts in the background

Another important design feature from an energy conservation standpoint is that the trusses eliminate through-the-wall penetration of two-by structural members, thus minimizing thermal bridging (see prior post, whole wall R-value).   For the sake of consistency and reproducibility, the trusses are built in a jig that is not our original idea (as explained in a previous post, stick-built exterior walls and in the original reference for the trusses).

Truss Configuration
A finished truss has 2 x 4 vertical members arranged flat-ways, i.e., rotated 90 degrees from the usual.  The tops and bottoms are joined by short 2 x 4s. Then there are six gussets, three to a side, made from 3/8 or 1/2" thick OSB or plywood to impart structural integrity while limiting through-the-wall thermal bridging.  All components come from recycled lumber so a certain amount of twisting and bowing is to be expected (but not unlike new lumber today, right?).  And recycled lumber comes with many nail holes on the 1 1/2" side, so exposing the 3 1/2" side for new nailing is another advantage for using trusses.  The second photo below is a prime example of the nail hole problem.


On rainy or cold days and other such house construction downtime, I have been making trusses in front of the
Framing nailer used to fasten 2 x 4
components together; the long 2 x 4s are
the bowed ones shown in the photo below;
notice how straight they are after the short
2 x 4s are wedged to placew
double garage workshop next door to the building site. At the time of this writing, most of those that will be needed have been assembled and stored.

The components are pre-cut to master patterns.  The long and short 2 x 4s are placed edgewise in the jig and fastened together with a framing nailer -- one nail at each corner. Then, with a trim nailer, three gussets are attached with 2" nails -- 11 nails in the end gussets and 8 in the middle gusset.  Since trim nails are virtually headless, they are driven at various off-angles to provide more holding power. The truss is pried out of the jig, turned over and forced back into the jig for identical nailing and gusseting of the second side.

When the long 2 x 4s are bowed, they are positioned in the jig with the convex side against the jig.  Then the short 2 x 4s are driven to place at each end to straighten them. Because the trusses are held jig-straight by the gussets,  the future sheathing and the drywall will lay perfectly smooth.  When bowed in the other direction, straightening is unnecessary.  A typical 2-by stud with only a 1 1/2" nailing surface has to be straight in order to catch enough of the sheathing and drywall panels for secure nailing.  By
Driving the short 2 x 4 to place at
the far end of a bowed board pulls
it away from the jig on nearby end.
Forcing a short 2 x 4 to place on the
 nearby end creates a straight truss
 that the gussets fixate 
contrast, each truss offers 3 1/2" of nailing surface so almost any amount of bowing left or right is acceptable.

Other Green Features
As with 2 x 6 construction, a wall will be plenty rigid with trusses on 24" centers tied together with side-by-side 2 x 6 mud sills and side-by-side 2 x 6 top plates.  The span between nailing surfaces, by virtue of the 90 degree rotation of the vertical members. will actually be 3" narrower than with 2 x 6s.  The double sills and plates are necessary because there are no 2x boards wide enough for a 15" walls -- which is a blessing. Having to tandemize the mud sills and top plates leaves a sizable gap that can be filled with insulation and thereby provide two more breaks (mud sill and top plate) against thermal bridging.  

Doors, Windows, Corners and T-Walls
Trusses that frame openings for doors and windows will have to be modified to carry headers and, for windows, sills.   The plan for headers and sills is to make regular trusses, minus the gussets on the side facing the opening, then let the headers and sills into the truss 2 x 4s to a depth of 1 1/2" to give the same amount of support as a jack stud aside a king stud.  I plan to tie a window-supporting truss to the closest regular truss with horizontal 2 x 4s in line with the header and sills to support the header/sill truss in one direction.  The opening itself  will be lined with OSB or plywood, not only to provide anchorage for the window or door, but to support the header/sill truss in the opposite direction.

When a "T- wall" intersects a truss wall between trusses, horizontal 2 x 4 blocking will join the two trusses and provide fastening for the T-wall, much like what is done with advanced framing techniques.

Gussets are fastened with 2" nails using a
Truss walls intersecting at corners will utilize three trusses arranged so that the entire corner is accessible to 15" of insulation and thermal bridging is held to a minimum just like the rest of the walls.

Fire-Blocking, Electric Cables and Plumbing Pipes
Fire-blocking is not possible with trusses but rice hulls are virtually impossible to ignite (paper on rice hulls as insulation, page 3) thereby rendering fire blocking in exterior walls moot.

Running electric cables and plumbing pipes inside of trusses will be a joy since holes in studs are not necessary as with stud walls.  The worst case scenario is that an occasional gusset would require drilling. What's more, plumbing supply pipes can be held to the interior side of the wall to isolate them from the exterior with more than enough insulation to prevent freezing.

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