The Solar Carriage House

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[1] The Wallis Carriage House uses a post-and-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.
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[2] The second-story floor of the Wallis house is poured concrete on Epicore decking.
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[3] The Wallises have two well-insulated cold rooms on their first floor.
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[6] The spare simplicity of the Merkel house's post-and-beam frame.
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[5] The Merkels insulated with Homesmith Kit superinsulated panels.
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[4] The Merkel house has large windows on the south side for a valley view.

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.