In north Florida's subtropical panhandle region—where
summertime humidity can average a clammy 82% and
temperatures in the 90's aren't even considered noteworthy
enough to comment on—cooling a dwelling is
usually of greater concern than is warming it. On the other
hand, though, heating requirements can't be ignored
altogether, since the region is subject
to nearly 1,600 heating degree-days during its cold season.
Understandably enough, designing a passive solar home to
suit such fluctuating conditions could present quite a
challenge. However, the folks at Tallahassee's Mad Dog
Design and Construction Company felt that, by coupling modern thermal
conditioning methods with "tried and true" locally popular
building techniques, they could come up with a house that
would rely on natural cooling and heating yet be comfortable enough throughout the year to require little or no utility-supplied power.
A Well-Planned Design
Essentially, the Florida builders used several
passive methods—backed up by two active
systems—to insure a pleasant interior climate during
the hot summer months. First and foremost, the
earth-sheltered structure relies on natural ventilation,
combined with heat-gain prevention, to maintain a
comfortable internal environment.
Borrowing from traditional local designs (which, of course,
evolved to suit the area's climate), the Sunshine Staters
utilized high ceilings, continuous attic-linked soffit
vents, and strategically placed windows to encourage the
flow of air throughout the house.
For example, the
south-facing solarium/greenhouse area incorporates ceiling
mounted outlets that can be opened, in case of overheating,
to allow warm air to duct into the attic and out the soffit
vents. Similarly, in the house itself—which is
separated from the solarium by a quartet of sliding glass
doors—unwanted warmth can be vented around four
movable 8' X 8' insulated ceiling shutters.
These horizontally hinged overhead "flaps" are mounted in
light wells which are framed into the front of the attic
and faced with acrylic glazing. When the electrically
operated sky shutters are three-quarters open, pockets are
formed, which accumulate warm air and direct it upward while permitting plenty of indirect light to brighten the
Other heat-controlling elements in the unique design
include  roof overhangs above the skylights,  solar
screening over the greenhouse windows during the warmer
months,  insulated draperies on the sliding glass doors,
and  ceiling insulation with an R-value of 28.
Additionally, the builders have taken advantage of
deciduous trees to help shade the structure in the summer
Another factor that contributes to the energy
efficiency of the Tallahassee residence is the dwelling's
earth sheltering itself. Calculations made prior to
construction had indicated that a large portion of the
necessary tempering could be accomplished by using the soil
as a massive heat sink, since about two-thirds of the
building's wall area—as well as its entire
floor—were slated to be below ground level.
However, data gathered later showed that earth temperatures
low enough for cooling purposes could be found only at a
depth greater than five feet . . . so, to compensate for
the fact that the earth-bermed structure was not
wholly engulfed in soil, the Mad Dog designers
merely insulated its surrounding earthen ramparts with a
two-inch-thick Styrofoam beadboard skirt that extends
outward eight feet from the home's concrete walls.
This protective "collar"—after being covered with
waterproof sheeting and 12 inches of
backfill—serves to minimize the effect of the sun and
ambient air on the soil below (creating stable and
comfortable temperatures at a depth of only four feet, even
during the hottest months). It also provides an effective
watershed that directs surface flow away from, rather than
along, the sides of the building.
And the innovative
designers didn't stop there:
The Sunbelters also took the opportunity to bury a Givoni
air tube (so named after its developer, Dr. Baruch Givoni)
and a temperature sensor deep within the berm,
allowing the home's residents to force cool evening air
through the system to offset the effects of any heat gained
during the day.
Several backup methods of comfort control
were incorporated into the dwelling, as well.
A whole-house attic fan—located in a central
corridor—supplies additional positive ventilation as
required. A 1 1/2-ton heat pump can be activated if
air conditioning is needed (the unit is also capable of
operating, more economically, as a non-cooling dehumidifier
When the winter months arrive, many of the same
features that prevent heat accumulation in the
summer can act to retain warmth.
Because the cold season's sun is low and the deciduous
trees have long since shed their leaves by then, the house
receives ample insolation through the unshielded solarium
glazing and the fully opened ceiling shutters ... which
can be closed at night to hold the day-gathered warmth in.
Furthermore, the contact between the earth berm and the
massive structure provides a 45-day "thermal lag," which effectively permits the earth-stored temperature
extremes of the late summer to be dissipated over a period
that extends about a month and a half into the cool season,
thus helping with the heating load.
Engineering Cuts Costs
Normally, an earth-sheltered structure will start its
"life" as an assembly of filled concrete block walls
resting upon a reinforced monolithic slab,
or—alternatively—8- to 12-inch poured
bulkheads, on footings, surrounding a "floating"
However, because the soil found at the building site has
low permeability and is potentially unstable, the designers
decided to utilize post-tensioned components,
which would stand the effect of lateral loads or twisting
resulting from shifting earth much better than would
conventional methods of construction.
Post-tensioning, a comparatively new technique in the
home-building industry, is in many ways ideal for use in
earth-sheltered construction. Most important, it imparts
additional compressive and tensile strength to any member
upon which it's used.
Unlike regular reinforced concrete construction (in which
the steel rebar can begin to resist a load only after the
concrete itself has failed ), a post-tensioned
component incorporates a network of interwoven
cables—encased in lubricated plastic sleeves and cast
in the middle of the slab on three-foot centers—which
are pulled to a tension of about 30,000 pounds after the
pour sets, and locked in place by means of engineered
wedges mounted in stressing anchors.
This fantastic force puts the concrete under constant
compression and increases its strength without the need for
excessive mass (the four-inch floor slab is designed to
support 3,500 pounds per square inch, a 40% greater
load than a conventionally built pad of the same thickness
Equally significant, post-tensioning can
be accomplished right at the building site, using little
This do-it-yourself factor, in turn, allows for some
creativity on the part of the builders—as
evidenced in the Tallahassee home by the exposed
aggregate finish on the solarium's interior walls: They
were poured in forms lined with a one-inch layer of river
rock, then removed with the stone facade intact.
Finally, the use of the "pulling after pouring" technique
can help cut materials costs. Recent studies of
post-tensioned structures and "typical" earth-sheltered
buildings show that only about half as much steel, and
perhaps 40% as much concrete, will be required when a house
is tensioned rather than conventionally poured and the resulting shell is less likely to crack.
So, although waterproofing is still necessary with any
post-tensioned wall, the use of exotic (and costly)
materials can be eliminated.
Because the north Florida
dwelling was primarily designed to provide passive cooling
rather than heating,
the people at Mad Dog chose not to insulate the concrete
mass. Instead, they merely poured their floor slab over a
bed of gravel and waterproofing, installed footing drains,
then tilted up the previously poured-and-tensioned wall
panels and positioned them in perimeter slots that had been
cast into the foundation.
The "box" thus created was held together with steel corner
brackets, and laterally supported by a series of stout
cables which are strung between interlocking top plates on
the rear wall and concrete "deadmen" sunk in the ground
behind the structure. All the bulkhead surfaces were
covered with a continuous layer of "Tu-Tuf" flexible lining
material, then the house was bermed as described
And, rather than covering the building's top
with a layer of earth (which would have required a whole
slew of additional engineering computations and upped the
materials expense), the designers merely installed oversized roof trusses and built a conventional—but well-insulated—asphalt-shingled "lid," accepting what they feel are only minor thermal losses as a result.
Energy Savings at Standard Prices
The bottom line, though, is cost. This
1,750-square-foot dwelling was constructed for about $35
per square foot, which is a good price even for above-ground
houses in today's market.
After living in it for nearly a full year, the owner states
that the home works even better than she expected ...
thanks largely to the fact that she thoroughly understands
how it functions, and willingly takes an active part in
matching its capabilities to the fluctuations of the
Of course, one of the beauties of this particular home is
that it works well in either a passive or an active
So, although an occupant who's insensitive to its design
would be penalized by higher utility costs, one who works
with the passive systems will enjoy considerable
savings, and a comfortable passage through the
seasons as well!