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A Model for Solar Access

The model for the solar envelope is found in the ancient settlements of North America. here, a thousand years ago, settlements were laid out for solar access. Acoma Pueblo, located on a plateau about 50 miles (80 km) west of modern Albuquerque, New Mexico, exemplifies such early planning. Rows of houses are stepped down to the south. Walls are of thick masonry. Roofs and terraces are of timber and reeds, overlaid with a mixture of clay and grass.

Individual houses at Acoma are well suited to a high-desert climate. The sun’s low winter rays strike most directly their south facing masonry walls where energy is stored during the day, then released to warm inside spaces throughout the cold nights. In contrast, the summer sun passes high overhead, striking most directly the roof-terraces where the sun’s energy is less effectively stored. What’s more, east and west walls are covered by adjacent houses thus further reducing harmful summertime effects. A small roadway separates each row of houses. The space between the rows is wide enough so that winter shadows cast by any one row of houses covers only the adjoining roadway. Terraces and heat-storing walls remain exposed to the warming rays of the winter sun. It is this critical relationship of building height to shadow area that presents a model for the solar envelope.

What Is a Solar Envelope?

The solar envelope is not a physical thing. It is a set of imaginary boundaries, enclosing a building site, that regulate development in relation to the sun’s motion. Buildings within this envelope do not overshadow their surroundings during critical energy receiving periods of the day and year.

The idea of an imaginary envelope is common to all zoning in the united States. Conventional zoning mostly uses an envelope shaped like a simple box with four sides and a top to establish setbacks and heights. In contrast, the solar envelope is shaped more like a multifaceted crystal or even a series of warped surfaces, generated to follow the moving rays of the sun. Adjacent envelopes can be quite different, depending on their site and particular surroundings. Consequently, buildings made within the solar envelope are more likely to have unique shapes than to repeat box-like designs.

The solar envelope is a construct of space and time: the physical boundaries of surrounding properties and the period for which access to sunshine is assured. The way these measures are set decides the envelope’s final size and shape.

First, the solar envelope guarantees sunshine to others by preventing shadows above designated boundaries along neighboring property lines; these boundaries have been called shadow fences. A shadow fence is an imaginary wall that rises from a property line. The solar envelope is then configured to meet the top of the fence the solar envelope rather than the ground, thus allowing the solar envelope to rise and gain volume. Different heights of shadow fences will affect the shape and size of the envelope.

Shadow fences, being imaginary, do not actually cast shadows but instead allow shadowing of adjacent properties within limits set by community values. The height of the shadow fence can be set in response to any number of different surrounding elements, such as windows, party walls, or courtyards. The height of the shadow fence may also be determined by adjacent land-uses. For example, housing may have lower shadow fences, and thus less overshadowing, than some commercial or industrial uses where rooftop access for solar collectors may suffice.

Second, the envelope provides the largest possible building volume within time constraints, called cutoff times. The envelope accomplishes this by defining the largest theoretical container of space that would not cast shadows on neighboring properties between specified times of the day. Cutoff times that are specified very early in the morning and late in the afternoon will result in smaller volumes than would result from later times in the morning and earlier times in the afternoon. When shadow fences are set at all property lines (sides as well as front and back), including any adjacent streets or alleys, solar envelopes are shaped with tilted facets defined by the sloping rays of the sun. Each separate face of the envelope is defined by a different time of day or season of the year. And because the wintertime sun angles are lowest, they are usually the main determining factor of envelope form.