Laying The Ground For Our Foundations

What qualifies as a good foundation? Qualities that might come to mind are rigidity, strength, and longevity. How about hygrothermal performance? The term “hygrothermal” may seem like a complicated word, but it really is the sum of its parts, “hygro,” meaning water, and “therm,” indicating things to do with heat. Together, hygrothermal characteristics define how humidity and temperature affect the assembly.

In the case of foundations, we work with two primary types: slab on grade, and crawlspaces with suspended floors. In both cases, the primary structural elements are concrete and steel reinforcement (rebar). Steel is of course an excellent conductor of heat, but concrete wicks and holds moisture in addition to being a poor insulator. This means that moisture and heat flow are very important to consider.

Let’s consider heat first. For the residents of the home we build, thermal comfort is essential. When we think of comfort criteria, it includes more than just reaching a number on the thermostat. A well-constructed house will also minimize temperature stratification—that is—the zonal variation in temperature. Some of this has to do with proper ventilation and air circulation, but a large part of achieving the comfort criteria is outside the raw numbers. If we have a room with an air temperature of 68 degrees with a floor that’s at 60, the occupant is still going to feel cold because of the discrepancy between surface and air temperatures.

Crawlspace with suspended floor (left) and slab on grade with floating slab (right).

Regardless of climate, sub ground temperatures are around 55 degrees Fahrenheit. This means that things in contact with the ground will tend toward the path of thermal equilibrium. To avoid thermal discomfort, we need to isolate the floor from the thermal effects of the ground. In the case of a slab on grade, we have designed our walls to be placed on top of a “floating slab” that has continuous insulation underneath and around, thermally isolating it from the outside. In the case of a suspended, vented crawlspace, the air inside the crawlspace is the same as the outdoor air temperature, which can be much below the 55-degree ground temperature. In this case, our suspended floor uses dense packed cellulose between floor joists. This insulation is sufficient to mitigate the cold seeping in, as well as thermal bridging in the transitions between the stem walls and our wood framed panels.

While proper control of heat can be achieved with continuous insulation, a continuous moisture barrier is also necessary to prevent ground moisture from entering the interior of the building through the capillary action of concrete, or through evaporation from the ground through the suspended floor.

Protecting against both heat loss and water ingress through our foundation assemblies ensures the longevity of our product and that the occupants of our houses are comfortable.