The Solar Carriage House

By Angus W. Macdonald
Published on July 1, 1985
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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.
[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.
PHOTO: MOTHER EARTH NEWS STAFF
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[2] The second-story floor of the Wallis house is poured concrete on Epicore decking.
[2] The second-story floor of the Wallis house is poured concrete on Epicore decking.
MOTHER EARTH NEWS STAFF
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[3] The Wallises have two well-insulated cold rooms on their first floor.
[3] The Wallises have two well-insulated cold rooms on their first floor.
MOTHER EARTH NEWS STAFF
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[6] The spare simplicity of the Merkel house's post-and-beam frame.
[6] The spare simplicity of the Merkel house's post-and-beam frame.
MOTHER EARTH NEWS STAFF
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[5] The Merkels insulated with Homesmith Kit superinsulated panels.
[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.
[4] The Merkel house has large windows on the south side for a valley view.
MOTHER EARTH NEWS STAFF

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.

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