The Solar Carriage House

Image Gallery [1] The Wallis Carriage House uses a postand-beam frame with insulation fitted between the posts. Large windows on the ground floor and a sun porch on the second story provide solar gain, which is stored in concrete, floors on both levels.

A blend of superinsulation and earth sheltering is a promising option for owner-builders.

by Angus W. Macdonald

I've been designing and building "underground" homes for over seven years now, and the energy efficiency of these buildings has often surpassed my wildest expectations. Nonetheless, my experience with earthsheltered structures has led me to look for ways to simplify construction for ownerbuilders and to reduce costs wherever possible. The result, my latest thinking on energysaving design, is a hybridization of earth tempering, superinsulation, and solar direct gain.

Now, I'm no less convinced than I ever was that earth contact is an excellent way to reduce heating and cooling loads. In my area of Virginia, temperatures eight feet below grade vary from 49°F in April to 64°F in October—a much more friendly environment than ambient air temperature, which typically goes as low as 0°F and as high as 100°F. What's more, earth sheltering practically eliminates convective wall losses from wind and thoroughly controls infiltration. Depending on a house's design, the earth surrounding it may also serve as a heat storage medium through the seasons.

There are some valid reasons, though, to question going completely underground. First, many of my clients aren't comfortable with the idea of underground living. They prefer the profile of a more conventional building and like the airy feeling of being above grade. Also, covering a roof with earth requires that the structure be capable of withstanding a load of 250 pounds per square foot and have a carefully installed, quality waterproofing system. The waterproofing alone may add $1.50 per square foot to the cost of the roof, and waterproofing on a horizontal roof seems to be more prone to problems than that on the walls. True, a covering of earth does reduce convective heat loss and limit infiltration. But even a two-foot-deep layer of soil will fluctuate from 40°F in February to 73°F in August, so heavy insulation is still required.

But what if, instead of building with a single story entirely below ground, we were to use two levels, with the lower one bermed on at least three sides? The below-grade walls must still be built to withstand a load of 350 pounds per square foot, and they must be waterproofed and properly drained. But this cost can be justified, since the walls serve several purposes. Masonry, which is highly conductive, provides an intimate thermal link with the earth. Thus we have mild temperatures outside the walls and plenty of thermal mass for solar storage. As long as the cool temperatures of the walls don't combine with high summer humidity to produce condensation, there's little need for insulation at depths greater than four feet.