The Solar Hemicycle is semicircular in plan, featuring a single concave arc of fourteen-foot high glass spanning the two stories both vertically and horizontally, and opening southward to a circular sunken garden and the Wisconsin prairie beyond. The north, east, and west sides are bermed up to the height of the clerestory windows on the second floor, protecting the house from cold winter north winds, while the sunken garden in front combines with the rear smooth berming to create am air pressure differential that deflects snow and wind up and away from the large south-facing windows. (This author has personally experienced this remarkable phenomenon many times.)
The interior lower level features a concrete floor slab for direct absorption and conversion of the incoming radiant solar energy. Imbedded within the floor is a radiant boiler-heated system for back-up heating that emulates and supplements the solar-warmed floor. All interior walls are Wisconsin limestone, providing an irregular and enhanced mass surface area for thermal energy exchange and interior temperature stabilization. There are no dividing walls throughout the entire width of the downstairs, allowing for air and heat distribution evenly throughout.
The second floor is a five-bedroom balcony, suspended from the roof joists and hence not requiring obstructing support from below. The front of the balcony is pulled away from the south glazing by several feet, enabling the solar-heated air from below to rise up onto the second floor and into the bedrooms over the full balcony width. The air return of this convective loop is completed by a large circular stairwell connecting the two floors.
The semicircular plan actually reduces the solar gain by about 8% in comparison with a straight south-facing plan, but the arc provides support for the north wall, reducing structural costs, while the bermed arc serves to channel cold winds around and away from the south glazing to reduce heat losses. The semicircular shape also provides for a sense of room separations and even gives visual privacy as one moves along the arc through the interior undivided spaces.
The house was constructed originally according to the norm of the day, with single-pane windows and minimal insulation above the berm levels and in the ceiling. In the 1980s a new owner replaced all single-pane with double-pane glass, re-roofed with exterior insulation, and installed thermal night curtains (all of which we may assume that Wright would have specified, had they been available at the time).
Careful energy-use records kept by the third (and still present) owners, combined with this author’s thermal load calculations, enabled a determination of the passive solar performance of the house. The passive solar features provide an average 53% furnace energy saving (“solar savings fraction”) for the entire Wisconsin winter. The monthly savings coincide very closely in percentage with the average available solar radiation (“percent sun”), suggesting that the house on the average gains heat on a sunny day in amount to that which is used over a 24-hour period.
This is a very successful and exemplary “climate responsive”passive energy design, now in its 63rd year of operation.
Frank Lloyd Wright and the “Solar Hemicycle”, Proceedings of the 14th National Passive Solar Conference, Denver, Colorado, M.J. Coleman, Ed., American Solar Society, Inc., 14, 3-17 (June, 1989);
Frank Lloyd Wright’s “Solar Hemicycle” Revisited: Measured Performance Following Thermal Upgrading, Proceedings of the 17th National Passive Solar Conference, Cocoa Beach, Florida, 52-57 (1992); and
The “Solar Hemicycle” Revisited: It’s Still Showing the Way, Wisconsin Academy Review, 39, No. 1, 33-37 (1992-93).