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Lee Porter Butler's 1975 Double Envelope (Shell) design [1] 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 [2] magazines.
Butler was an artistic/ecological building designer. A self proclaimed "Ekotect." [3] He did not hold formal qualifications as an energy engineer.
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. [4]. This phenomenon is observed particularly in designs with inadequate thermal mass, poor cross ventilation and excessive polar facing windows.[1]
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 flow loop in the cavity between the two building envelopes and through a sub-floor or via earth cooling tubes.[2]
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 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.
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.[2]
While the design can perform better than a conventional home, formal performance monitoring suggested there were some problems with the original design. [5].
Commentators have criticised the design on various grounds:
Subsequent modifications have attempted to address these issues.[6]
A modification based on Butler's original design has been described[3] 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.
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.
Passive solar design concepts
Solar-designers