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Thermal Envelope
Houses
The thermal envelope house, also known as the
double envelope house, is sometimes called a "house-within-a-house,"
which graphically describes the construction principles involved. The design
employs a double envelope with a continuous airspace of at least 6 to 12 inches
(152 to 305 millimeters) on the north wall, south wall, roof, and floor. You
achieve this by building inner and outer walls, a crawl space or sub-basement
below the floor, and a shallow attic space below the weather roof. The east and
west walls are single, conventional walls. (A buffer zone of solar-heated,
circulating air warms the inner envelope of the house.)
The south-facing airspace usually doubles
as a sunspace or greenhouse. During the day, the sun's rays pass through the
glazed, outer wall, heating the air in the sunspace. The warm air circulates
around the airspaces in a "convective loop," giving up heat. The air
can move from the sunspace, up and along the roof airspace, down into the
north-wall airspace, into and along the crawl space, and back into the sunspace
through slots in the floor. This completes the loop. Although there is generally
no definite, continuous cycling of the air, there can be a shift in air movement
patterns between day and night. During the day, heat is also stored in the crawl
space, which uses a concrete slab floor, a bed of gravel, or simply the earth
itself as "thermal mass" to hold the heat. At night, this stored heat
escapes, circulating through the airspaces to keep the house warm.
Although thermal envelope houses were a
popular design theory in the late 1970s, few thermal envelope houses have
actually been built. Brookhaven National Laboratory conducted tests on one house
in the winter of 1980. Although the house performed well, the convection of heat
by gravity alone did not occur evenly throughout the house. Temperatures on the
three living levels were stratified. The house did have low energy needs, due to
the high thermal insulation value of the double shell. Standard superinsulated
houses perform about as well as thermal envelope houses, but thermal envelope
houses cost more to build. Although they differ widely from the original thermal
envelope concept, hybrid designs show an improved performance over the double
envelope.
A drawback to the thermal envelope house
is the potential for catastrophic fire damage. The connecting airways
surrounding the inner shell provide a conduit through which flames may spread
rapidly. The structurally protected chaseway limits exterior access. Under such
conditions, firefighters may have difficulty containing a blaze in a thermal
envelope house. Anyone seriously considering building a thermal envelope home
should install a protective fire sprinkler system.
Bibliography
For further information, including a detailed
discussion of design variations, operating principles, and the possible
advantages and disadvantages of the thermal envelope house, consult the
following publications and articles. This bibliography was updated in June 1997.
Books
Ekose'a Homes, L. Butler, Ekose'a
Integrated Technologies Inc., 1980. Available from Ekose'a Integrated
Technologies Inc., 620 Biscayne Drive, West Palm Beach, FL, 33401, (407)
833-9007 or (407) 659-7656, E-mail: LButler@emi.net.
150 pp., $25.00.
Sun Earth Buffering and Super
Insulation, D. Booth, Community Builders, 1983. Available from Community
Builders, Canterbury, NH 03224, (603) 783-4743. 224 pp., $12.95 (softcover),
$15.95 (hardcover).
Super-Insulated Houses and Double
Envelope Houses, W. Surcliff, Brick House Publishing Company, Andover, MA,
1981. Out of print.
Reports
An Electric Coheating Experiment to
Determine the Heat Loss Coefficient of a Double-Envelope House, J. Ortega et
al., Solar Energy Research Institute, 1981. Available from the National
Technical Information Service (NTIS) (see Source List below). 7 pp., $12.50,
Report No. DE81027937.
Solar Project Description for Arno
Kahn/Builders and Laborers Commonwealth Single Family Residence Duluth,
Minnesota, D. Moore, Boeing Company, 1982. Available from NTIS (see Source List
below). 48 pp., $24.50, Report No. DE82014973 (or Solar/1103-82/50).
Proceedings
"Ground Coupling and Single Blow
Thermal Storage in a Double Envelope House," H. Ghaffari and R. Jones, Proceedings
of the 1981 Annual Meeting of AS/ISES, Philadelphia, PA, May 1981. Volume
4.1, pp. 530-34. Available from the American Solar Energy Society (ASES) (see
Source List below). $4.00 plus shipping.
"Heating and Cooling Performance of a
Thermal Envelope House," J. Akridge et al., Proceedings of the 5th
National Passive Solar Conference, Amherst, MA, October 1980. Volume 5.1,
pp. 492-96. Available from the American Solar Energy Society (ASES) (see Source
List below). $4.00 plus shipping.
Articles
"An Affordable Solar Envelope
Home," T. de Vries, Solar Living and Solar Greenhouse Digest, (3:2)
pp. 4-5, April/May 1980.
"The Double Shell Envelope Faces a
Death Blow," J. Hughes, Solar Age, (10:4) pp. 20-21, April 1985.
"Double Shell: An Owners
Perspective," T. Smith, New Shelter, (1:6) pp. 82-85, September
1980.
"Double Shell Houses," L.
Stains, New Shelter, (1:6) pp. 72-82, September 1980.
"Double-Shell Houses: At Last We Know
Why They Work," E. Smay, Popular Science, (222:2) pp. 56-59,
February 1983.
"Envelope Homes: A Climatic
Study," P. Henshaw, Alternative Sources of Energy, (44) pp. 18-21,
July/August 1980.
"Progress Makes Perfect," T.
Smith, Mother Earth News, (82) pp. 159-60, July/August 1983.
"Shakedown for the Envelope House-A
Physical Exam," V. Reno, Solar Age, (5:11) pp. 14-21, November 1980.
"Shakedown for the Envelope
House-Trial by Computer," J. Kohler and D. Lewis, Solar Age, (5:11)
pp. 22-27, November 1980.
"Thermal Envelope Home," R.
Freudenberger, Mother Earth News, (56) pp. 120-22, March/April 1979.
"Which Is For You? The Double
Envelope or the Superinsulated House," J. Russell, Mechanix Illustrated,
(No. 5) p. 50, October 1982.
Source List
American Solar Energy Society (ASES)
2400 Central Avenue
Suite G-1
Boulder, CO 80301
Phone: (303) 443-3130; Fax: (303) 443-3212
Internet: (E-mail) ases@ases.org; (World Wide
Web) http://ases.org/solar
National Technical Information Service (NTIS)
5285 Port Royal Road
Springfield, VA 22161
Phone: (800) 553-6847 or (703) 487-4650; Fax: (703) 321-8547
Internet: (E-mail) orders@ntis.fedworld.gov;
(World Wide Web) http://www.fedworld.gov
NTIS adds charges for shipping and
handling. Check the price and availability before placing an order.
EREC is operated by NCI Information
Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of
Energy. The statements contained herein are based on information known to EREC
at the time of printing. No recommendations or endorsement of any product or
service is implied if mentioned by EREC.
Energy Efficiency and Renewable Energy
Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
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