Superinsulation For the Masses
(Page 3 of 5)
September/October 1986
By the Mother Earth News editors
The Tricky Case of Earth Sheltering
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In general, superinsulation practices apply directly to other types of energy-efficient construction, but there are some areas where the degree of insulation may come into question.
In earth shelters, for example, some designers contend that the floor and the walls below frost line shouldn't be insulated. The theories cited to support this approach are that the mass of the earth outside the walls can be used to store heat gained from the sun and that the earth's comparatively cool temperature in summer will help keep the space comfortable during the dog days.
Before you decide to go along with these arguments, here are a few things to consider. In areas where summer humidity goes above 60% or 70%, condensation is very likely to be a problem on the inside surface of uninsulated, below-grade masonry walls and floors. Dew point temperatures can exceed 70°F in many parts of the U.S., and uninsulated below-grade walls are likely to be cooler than that.
In addition, if there is much need for heat in the winter, the floors and walls may be uncomfortably cool during that season. The earth temperature at a 25-foot depth is likely to stabilize at around the average annual air temperature for your locale. Above that depth, the situation worsens.
The supposition that the earth outside the walls can be used for heat storage is probably wrong in most cases, as well. Unless the design employs techniques to isolate the earth from the surface (as in "Passive Annual Heat Storage"; see issue 91, page 46), heat storage on an annual basis is probably negligible. And on a daily schedule, heat flux through masonry is limited to a useful thickness of about four inches.
If you're building an earth shelter, we recommend that you use at least an inch of rigid foam under the floor and outside below-frost line walls, unless you live in a desert climate. Insulation above frost line should be at least to code.
Building Techniques
Most superinsulated houses are built of wood, though competitive R-values can be achieved using special masonry systems. Because the masonry approach is largely pre-engineered for new construction and isn't widely applicable to retrofits, we'll stick to wood in this discussion.
Double walls: Traditional superinsulation, as it originated in Canada, employs two adjacent walls: an inner, structural one built from 2 X 4s and an outer, nonstructural one consisting of 2 X 4s, as shown in Fig. 1. The space between the walls is filled with insulation, so there's no wooden thermal bridge from inside to outside. Today, this technique is still used, but other less-expensive approaches are often preferred.
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