Priorities
Being on a strict budget will precipitate a lot of
compromises. However, one issue is not
negotiable and that is a super-tight and super-insulated envelope for the
house. Probably this means we will have laminate or wood counter tops instead of natural or man-made stone, or site-made interior doors instead of
prehung, but that's okay. Energy
conservation will remain our highest priority.
Stick-Built Walls Using Salvaged Lumber
In a perfect world, we would be using structural insulated
panels or insulated concrete forms for the exterior walls but they far exceeded our
budget and are limited on the amount of insulation they can provide. And we have a lot of free
salvaged dimension lumber that would go to waste if it were not used for the exterior walls. The trouble with salvaged 2 x 4s however is that the nails often split the ends during the salvage process, especially when toe-nailed. Typically, a couple of inches must be sacrificed from one or both ends of the boards during the denailing process, resulting in many studs that are too short for typical 8' wall construction.
salvaged dimension lumber that would go to waste if it were not used for the exterior walls. The trouble with salvaged 2 x 4s however is that the nails often split the ends during the salvage process, especially when toe-nailed. Typically, a couple of inches must be sacrificed from one or both ends of the boards during the denailing process, resulting in many studs that are too short for typical 8' wall construction.
Wall Trusses
Early on, I decided to use wall trusses instead of individual
two-bys as "studs" for several important reasons.
One, with trusses, the salvaged 2 x 4s could be utilized for an 8' wall even if shortened as long as
they remained at least 93" long. Two, some of the 2
x 4s were not entirely straight (but neither is a lot of new lumber these days) but could still be utilized quite well for trusses. Third, trusses would essentially eliminate thermal
bridging. The last and most important reason for using trusses is that R-value for the walls is directly proportional to the thickness of the insulation and trusses can be designed to house whatever thickness of insulation needed to hit a given R-value target.
Truss Design
The 2 x 4s in the trusses will be turned 90 degrees from the
way a stud normally sits in a wall and arranged in pairs, one facing outward
and the other inward. They will be tied
together with short 2 x 4s at the top and the bottom and braced with three pairs of gussets cut from 3/8" or 1/2" plywood or OSB. When arranged in the wall on 24"
centers, the result will be a wall filled with 15" of insulation and virtually no thermal bridging. What bridging does occur will be limited to the short 2 x 4s at the tops and bottoms of the trusses and through the skinny gussets.
As far as straightening the boards, my original idea was to
assemble them at the tops and bottoms with the bows pointed away from each
other. Then I planned to use clamps to
pull them together until they fit the precut gussets. Once the gussets were attached, the truss
would be straight.
Using Jigs
However, while researching rice hulls as insulation, I came upon a most interesting
slide presentation (Rice hull house) showing the use of wall trusses similar to what I envisioned but being built in jigs for ease of construction and for standardization. At the time of this writing, a jig was already in service (the subject of another post) that facilitated any straightening that needed to be done.
However, while researching rice hulls as insulation, I came upon a most interesting
Wall truss jig |
Top and Bottom Plates
Characteristically the top and bottom plates are as wide as the wall is thick, e.g., 2 x 4s for 3 1/2" walls and 2 x 6s for 5 1/2" walls. Unfortunately, the plates then become conductive thermal bridges because they are exposed simultaneously to both the exterior and interior environments. In order to solve this problem, our 15" walls will have double 2 x 6 mudsills side-by-side and 2 x 6 top plates that are double in a side-by-side sense as well as two courses on top of each other in the typical fashion. The side-by-side configuration of the mudsills and the top plates will allow 4" of insulation between them, thereby arresting thermal bridging.
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