[GSBN] [nbne] no Timber frame for Alberta house

[Robert Riversong]

Response follows:
--- On Sun, 9/18/11, Derek Roff <derek at unm.edu> wrote:
I aslo agree with Sarah that the ground will be warmer than the air in Alberta's winter.  But that doesn't make the ground near the surface a source of heat.  Maybe I don't understand this use of "earth coupling".  While Alberta probably has a lot of local variation, I suspect that in many places, the frost line is at 5 to 8 feet in depth.  Even going deeper than that, or using a shallow frost-protected foundation design, the earth under the building is still going to be well below the desired indoor air temperatures.  Hence, any earth coupling is going to be a heat sink and an energy loss for the house.  Are you suggesting that insulation under the floor will not be necessary/prudent in an "earth coupled" design?   
In any wintry climate, even shallow ground temperature is warmer than average winter air temperature. Deep ground temperature is usually within a degree or so of average annual air temperature. With snow cover (insulation) and heat loss from an earth-coupled house (any house on a full perimeter foundation - crawl space, basement or slab-on-grade), the soil isotherms will demonstrate a warming of the soil under and around the foundation - with increments depending on amount of heat in the foundation, amount of insulation between foundation and ground, and depth of snow cover (see attached measured isotherms for uninsulated basement and slab/frost wall for Saskatoon in March).
Of course, this is the principle of a shallow, frost-protected foundation: heat loss downward from the conditioned space warms the ground sufficiently to prevent frost penetration below even a 12" deep perimeter wall - and, in northern Vermont's climate (8500 HDD), with only 24" of R-10 foam board extending laterally around the perimeter (not the formerly assumed 48" necessary to protect from a 4' frost depth). A maximum of R-28 is allowed under a slab so that sufficient heat loss downward is available to keep the shallow ground above freezing.

Of course winter ground temperatures are below indoor set point temperatures. The earth is not normally a heat source, but the delta-T (and consequent heat transfer rate) is a good deal lower downward than to the winter air, particularly once the soil reaches thermal equilibrium from the conditioned house.
A house on piers is exposed to winter air temperatures on all six sides, and if air-permeable floor insulation is not properly protected from wind wash it will lose insulating value. Even an unconditioned but sealed crawl space will offer a good deal of protection from winter air temperatures, and a conditioned crawl space with insulated walls has far less surface area to the outside air than does an elevated floor on piers (typically about 25% as much).
Additionally, a house that's reasonably well thermally earth-coupled (with some but not too much insulation to the ground) is more likely to be freeze proof if it's left unoccupied and unheated. Assuming sub-slab soil equilibrates to about half way between normal indoor temperature and deep ground temperature [in N. Vermont = (44°+68°)/2 = 56°], then once the house temperature drops below 56°, the ground IS a heat source.
The first Larsen Truss house I built in Western MA, for a couple of potters who had to leave for three weeks every February to go to a big Florida craft fair to sell their wares, had to remain freeze-proof with no heat source (woodstove only plus passive solar which is about nil in February). It remained above 45° and the water pipes never froze. If it were built on piers, they would have come home to burst pipes and a holy mess.
- Robert
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