Fits and Starts over Past Eight Years
Decision on Amount of Earth Sheltering
Our original intention was to heed the advice of Hiat, Stephens and Carmondy/Sterling and earth shelter as much of the house as was practical for our site. Accordingly, the north wall, the north half of the roof and most of the west wall would be sheltered. The
east wall would remain mostly unsheltered because it was an attached garage and entryway. According to the prevalent philosophy on earth sheltering, maximizing the amount of earth contact would benefit the thermal performance of the house in two ways -- by increasing the size of the thermal mass and decreasing the amount of the envelope exposed to the environment. Less exposure means fewer opportunities for air infiltration and fewer walls needing conventional insulation.
east wall would remain mostly unsheltered because it was an attached garage and entryway. According to the prevalent philosophy on earth sheltering, maximizing the amount of earth contact would benefit the thermal performance of the house in two ways -- by increasing the size of the thermal mass and decreasing the amount of the envelope exposed to the environment. Less exposure means fewer opportunities for air infiltration and fewer walls needing conventional insulation.
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| Green roof and earth contact west wall (all concrete) |
As recently as last year, after our engineer had produced the first set of drawings, based on my amateur drawings and a model of the house, it became apparent that earth on the roof was not doable because the timber frame to support it would be too expensive. And, in the absence of a concrete roof, there was always the issue of termites. However, the amount of earth sheltering for the west wall remained in flux until the final construction drawings when it was reduced by one-half in order to reduce cost and simplify construction.
Earth Contact Walls in Conjunction with AGS
The only practical option for earth contact walls is concrete of some sort. If an earth contact wall is continuous with earth on the roof, as advocated for AGS (see second post on AGS) the entire wall is protected by the insulation-watershed umbrella and could remain uninsulated. However, in our case, at least 8" of the top of the wall will be exposed below the roof (for termite protection). So part of the wall has to be insulated and part not, creating two zones. The bottom zone is uninsulated so heat can move freely back and forth through the wall as part of the AGS system. The top zone is insulated and starts a foot or so under the level of the insulation that lies horizontally as part of the insulation-watershed umbrella and it ends at the roof line.
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| Upper area of back wall is insulated (black); lower wall is not (brown) |
Advantage of a Two-Story House
Fortunately, our two-story house will have a tall back wall. This allows for an uninsulated zone almost as large as the back wall of a typical single story with earth on its roof which should make the AGS system just a functional as the pure design Stephens advocated. The biggest challenge will be to create stick built exterior walls and a conventional roof that will not waste the precious heat after the system produces it. However, as I will detail in a separate posts, the 15" thick truss walls and 16" deep cathedral ceilings filled with rice hull insulation will more than compensate for the lack of earth contact on the east and west walls.
Fortunately, our two-story house will have a tall back wall. This allows for an uninsulated zone almost as large as the back wall of a typical single story with earth on its roof which should make the AGS system just a functional as the pure design Stephens advocated. The biggest challenge will be to create stick built exterior walls and a conventional roof that will not waste the precious heat after the system produces it. However, as I will detail in a separate posts, the 15" thick truss walls and 16" deep cathedral ceilings filled with rice hull insulation will more than compensate for the lack of earth contact on the east and west walls.
Complete Block System
In order to reduce costs, the design of the concrete walls went through several iterations ranging from DIY poured concrete to DIY dry-stacked cinder blocks to a new system called "Complete Blocks" (Complete Block Company) to professionally poured concrete. The most ideal solution would have been the Complete Block System in that its precision concrete blocks come both insulated and uninsulated so they could be mixed and matched as necessary to satisfy the two zones. However, the cost was budget-busting and the test data the start-up company was able to provide for our structural engineer was insufficient. So we went to plan B -- professionally poured uninsulated concrete.
