Monday, February 16, 2015

Timeline - Design Evolution - Stick Built Exterior Walls

Past Five Years

Ruling Out Alternatives for Exterior Walls
One of the first decisions we needed to make was whether to use an alternative to conventional stick building for the non-earth-contact walls.  The options are plentiful -- rammed earth, straw bale, Earthships, adobe, cob, cordwood, earth bags -- to name those that Daniel Chiras covers in his "The Natural House -- A Complete Guide to Healthy Energy-Efficient, Environmental Homes".  

My first choice however was structural insulated panels even to the extent of buying Michael Morley's book on SIPS and getting preliminary quotes from a couple of vendors.
The panels are structural enough to eliminate conventional framing, have built-in insulation, allow no air infiltration, are constructed off-site to cad-cam precision and go up so fast that the house is under roof in a matter of days.  SIPs work especially well with timber framing (most timber-framed houses today are enclosed with SIPs)  but the two together would have exceeded our total budget!  Time to look for an affordable alternative.

Stash of Salvaged Lumber
We are sitting on quite the stash of lumber that I salvaged from old buildings -- three houses, two garages and several outbuildings, plus freebies picked up through Craigslist. Most of the dimension lumber is 2 x 4s which, if installed in the usual way, would not provide the R-45+ wall we need for a super-insulated house.

An important but poorly understood reality is that, irrespective of the R-13 the manufacturer prints on the insulation, the whole wall R- factor for a 2 x 4 wall is no more than R-10 when thermal bridging through the studs and plates is factored in. And that's even before air infiltration enters the equation.  Similarly, due to thermal bridging, the actual R-factor for a 2 x 6 wall is R-14 instead of the  R-19 printed on the batts.

So, how do we build R-45+ walls using the salvaged 2 x 4s?

Double Wall Construction
Early on, I had considered using a double-wall construction which is basically two separate walls tied together at the top and bottom plates with the amount of space between the walls dictated by the R factor goal.  And for the double wall, I was thinking about using 2 x 4s rotated 90 degrees from normal studs in order to reduce thermal bridging and to better manage inconsistencies in the salvaged lumber. However, before I had worked out the details, I made a lucky strike.

Truss Walls
While searching for a cheaper alternative to conventional insulation, I remembered Don Stephens mentioning rice hull insulation in his article on Annualized GeoSolar.  When I Goggled rice hulls, I found a blog that detailed the construction of truss walls in conjunction with rice hull insulation. The blog was posted by a Louisiana company building low-cost, Habitat for Humanity-like, housing using wall trusses filled with rice hulls for insulation (Rice hull house with wall truss design).  The important difference between their trusses and my double-wall idea was that the trusses can be made individually in a jig and assembled in the wall as if they were studs.  This approach is much simpler than building and raising two separate walls then joining them at the plates, particularly for someone working alone.

At the time of this posting, the wall truss jig was nearly assembled in the workshop in anticipation of building the 65 or so wall trusses ahead of time on days too inclimate to work outside. The jig will yield trusses that are highly standardized despite the inevitable inconsistencies in salvaged lumber.

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