The Solar Carriage House

A solar carriage house is a blend of superinsulation and earth sheltering, a promising option for owner-builders. Includes information on Wallis carriage house, Merkel carriage house, the future of carriage houses and photographs.
By Angus W. Macdonald
July/August 1985
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[2] The second-story floor of the Wallis house is poured concrete on Epicore decking.
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Learn why a solar carriage house is energy efficient, simplifies construction for owners, and reduces building costs. (See the carriage house photos in the image gallery.)

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 earth sheltered structures has led me to look for ways to simplify construction for owner-builders and to reduce costs wherever possible. The result, my latest thinking on energy saving 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 degrees Fahrenheit in April to 64 degrees Fahrenheit in October—a much more friendly environment than ambient air temperature, which typically goes as low as 0 degrees Fahrenheit and as high as 100 degrees Fahrenheit. 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 degrees Fahrenheit in February to 73 degrees Fahrenheit 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.

Above grade, as dictated by the site topography, we use wooden framing—I'm fond of heavy-timber construction for both aesthetic and economic reasons—to erect a superinsulated cap. Because the south side of both the first and second floors is unbermed, there's a large potential area for solar gain, and the apertures can be sized to suit the mass in each area.

A little more than two years ago, I amalgamated these concepts into a design I call the Solar Carriage House, and in the last two years two homes have been built using modified versions of this plan. Though the implementation is somewhat different in the two buildings, the basic ideas remain the same.

WALLIS CARRIAGE HOUSE

Jean and David Wallis's version of the Solar Carriage House is actually just one of the structures these owner-builders plan to erect on the steep, rocky hillside overlooking their private stream and valley in central Virginia. Jean and David, with help from relatives and friends, constructed the 1,456-square-foot building for use as a temporary residence while they build a larger hybrid earth-tempered home about 100 feet away. They intend to use the smaller home as a guest house once the main residence is done, but the little structure has proven so comfortable that they now feel no urgency to move.

David and Jean worked closely with mason Adrian Stillson to erect the lower floor of cast concrete and block on a 26 foot by 28 foot foundation. This area incorporates a large workshop, two root cellars/cold storage rooms with heavily insulated doors of David's design, and a woodstove. The intervening floor is reinforced cast concrete on Epicore decking. This masonry floor provides a fireproof barrier between the workshop and living area, and offers thermal mass for the sun porch and direct gain living area.

The above-grade heavy-timber structure was prepared and assembled by George Allman's company, Timber Smith Kits of Gordonsville, Virginia, and David and his two sons, Allan and Paul, framed and superinsulated the walls and roof. Many other local craftsmen contributed to the completion of the Wallises' home, which at times resembled a community project.

During the winter of 1984-85, the performance of the Wallis Carriage House was excel lent. Backup heat from the woodstove was rarely needed, even when the temperature plunged to -10 degrees Fahrenheit, and the living area temperatures ranged from 64 degrees Fahrenheit to 78 degrees Fahrenheit. Jean and David regulate the interior temperature by operating the sliding glass doors that connect the solar porch and living area, and natural convection has been sufficient to provide all the circulation needed.

MERKEL CARRIAGE HOUSE

Richard and Deborah Merkel chose 30-foot-square version of the Solar Carriage House plan, with an earth-bermed ground floor for an office and two stories above Richard erected the masonry-shell first floor and roofed it with a timber deck . . . then George Allman's crew assembled the two-story timber frame on top. Richard worked with Chip Bond, a contractor in the Rapidan, Virginia, area, to complete the shingled roof and apply Homesmith Kit R-25 panels and siding to the outside of the post-and-beam frame. The superinsulated panels—which consist of one-by wood frames with expanded polystyrene insulation applied inside were nailed through their structural rims to the heavy timbers with pole nails.

A major advantage of the timber-kit-and-insulating-panel approach is that the shell of a home can be assembled very quickly. Richard completed the masonry in a week, the timber frame went up in four days, the roof took another week, and the insulated panels took three men only two days to apply. All this was done without scaffolding. For an owner-builder who's as pressed for time as for money, this approach allows a home to be "dried in" in an amazing hurry.

The Merkels' main floor is one open living space, with the exception of a bath and laundry on the north side. They'll reach the uppermost floor—which includes the master bedroom, a study, a walk-in closet, and a large bathroom—by a prefabricated Piedmont Products pine spiral stair. All told, the finished area of the house will be 1,400 square feet, with an additional 900 square feet on the ground floor.

Richard and Deborah's house is tucked into a grove of pine trees on a south-facing slope overlooking a rural valley. Deciduous trees offer summer shading for the large glass area on the south side but won't significantly impede winter solar gain. To protect the interior from major nighttime heat loss through the windows, the owners plan to use Window Quilts by Appropriate Technology Corporation. Backup heat will be from a woodstove on the ground floor.

THE FUTURE

I'm particularly excited about the potential of timber kits and superinsulated panels for owner-builders—because of the speed and ease of assembly. This approach is rapidly growing in popularity, and superinsulated panels are widely distributed by Homesmith Kits, Homasote Corporation (which has a polyurethane panel, with a higher R-value per inch, called TUPS), and Radva Corporation of Radford, Virginia. The time has already arrived when you can order panels sized to fit your frame from a nationwide manufacturer-distributor. But perhaps the most exciting development for owner-builders on a budget is the emergence of local producers. Before long, you may be able to buy panels from a company only a few miles away; better yet, methods may evolve that will allow you to make them yourself at your building site!

EDITOR'S NOTE: The Solar Carriage House is one of several hybrid designs that are featured in Angus Macdonald's new collection of standard plans for low-cost, passive solar, earth-tempered, and superinsulated homes. The collection is available for $5.00 from Survival Consultants, Rapidan, VA. In addition, Angus markets—also through Survival Consultants—Homesmith Kit panels that are designed to fit specific applications. The architect's book, Building Your Own Earth Tempered Home, is available from MOTHER's Bookshelf, Hendersonville, NC, for $9.95 plus $1.50 shipping and handling.  


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