Double envelope house - AbsoluteAstronomy.com

History

Lee Porter Butler's 1975 Double Envelope (Shell) design http://www.motherearthnews.com/Green-Home-Building/1982-03-01/The-Double-Envelope-House.aspx received wide publicity after the U.S. solar energy tax credits were created in 1978. Versions were on the cover of Better Homes and Gardens and Popular Science http://web.archive.org/web/20091026191932/http://geocities.com/betadolphin/ magazines.

Butler was an artistic/ecological building designer. A self proclaimed "Ekotect." http://www.webcitation.org/query?url=http://www.geocities.com/betadolphin/LeePorterButlerVitae.html&date=2009-10-25+13:28:55 He did not hold formal qualifications as an energy engineer.

Design and theory

Butler's experimental design was a form of isolated passive solar design that incorporated a passive heat distribution system. It attempted to address the problem of unequal distribution of heat that was associated with some direct gain systems. http://users.chartertn.net/dhrivnak/first_home.htm. This phenomenon is observed particularly in designs with inadequate thermal mass, poor cross ventilation and excessive polar facing windows.

Butler's design essentially composed of a house within a house. Thermal energy was captured from a south-facing solarium and heat was circulated by a natural convection

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

flow loop in the cavity between the two building envelopes and through a sub-floor or via earth cooling tubes

Earth cooling tubes

A ground-coupled heat exchanger is an underground heat exchanger loop that can capture or dissipate heat to or from the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses...

.

A recirculating air flow path resulted from the warm (less-dense) air rising in a south-side solarium, and cooler (denser) air falling on the north side to create pressure differentials that automatically moved excess solar thermal gain from the south to the north side of the building without forced convection systems. Air flow

Stack effect

Stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or...

was proportional to the differences in temperature between the two convection paths.

In the summer, shading devices eliminate all direct solar gain. Vents are opened at the top of the to exhaust hot air. Fresh air intake uses ambient temperature Earth to cool and dehumidify replacement air at the base.

In winter, the air in the cavity is buffered by warm ambient-temperature Earth under the floor (which is partially recharged by the natural convection flow loop during each winter day). In the summer, the convective flow is replaced with cooler near-ambient-temperature Earth replacement air, and the warm air exhausts by natural convection.

Criticisms

The original explanation provided for its efficiency was the thermal buffer that existed in the double envelope cavity. However, observers have also commented that the overall insulation of the design is higher with two walls instead of just one.

While the design can perform better than a conventional home, formal performance monitoring suggested there were some problems with the original design. http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6367356.

Commentators have criticised the design on various grounds:

The rock bed under the house interfered with the geothermal night time effect and slowed warming in the morning.
Roof-angled glass (See sun path
Sun path
Sun path refers to the apparent significant seasonal-and-hourly positional changes of the sun as the Earth rotates, and orbits around the sun. The relative position of the sun is a major factor in the heat gain of buildings and in the performance of solar energy systems...

) made it difficult to control summer heat gain and winter heat loss.
An equator-side porch excessively shaded the lower floor from solar gain
The additional cavity on the north wall was excluded from the conditioned area (and created a potential fire hazard).
Dark roofs and absent radiant barrier was not adequate to inhibit summer solar gain
The presence of a fireplace, chimney and clothes dryer were a source of convective heat loss
The design was not adequate to function in polar climates.
Concerns about condensation and moisture issues forming in the convection loop leading to maintenance or health issues.
The additional materials and thermal mass used in construction
Warm are rises by natural convection. It does not go down and around. They are fire traps and are against building codes in places like Aurora CO.

Subsequent modifications have attempted to address these issues.http://100khouse.com/2008/01/05/zero-energy-design-binge/

Thermal buffer zone house

A modification based on Butler's original design has been described but not yet formally evaluated. It attempted to eliminate some of the initial issues.

An important difference is the northern limb of the convection loop (thermal buffer zone/TBZ) is employed as a utility area e.g. laundry room, closets, pantry, and storage space. The laundry room also doubled as an area for clothes drying.

An external window is located on the north side which can be opened to admit mild air into the TBZ during summer. An additional internal window separates the south side living quarters. This can be opened to admit warm winter air from the solarium to enter directly into northern rooms.

The designer states that on cold winter days, the TBZ tempered with solar-heated air could be often above 85 degrees F, while the outside air was below freezing.

Performance evaluation

Current technology makes it difficult to model the effects of these convective flow designs because computer performance simulation software does not presently incorporate these processes in their calculations.

History

Design and theory

Criticisms

Thermal buffer zone house

Performance evaluation

See also