Friday, December 18, 2015

Design - Exterior Walls

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.

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' walleven 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
Wall truss jig
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.

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. 

Rice Hull Insulation
As discussed in a prior post, we plan to use rice hull insulation, a cheaper and greener
alternative to conventional insulation. With 15" walls, rice hull insulation at R-3.2 per inch, we expect easily to exceed an R-factor of 45 and, with such minimal thermal bridging with the trusses, probably more like the higher '40s.

The logistics for transporting the hulls from the Mississippi delta area and getting them into the walls is still in flux. My tentative plan is to keep the garage un-roofed until the hulls have been dumped into it then use a homemade blower patterned after the one shown in the above link to the Rice hull house to get the hulls into the walls (and ceilings).  More on this later.

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