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.3 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 boxlike 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

Acoma Pueblo avoids winter shadows by spacing the rows of houses.

Generation of Solar Envelope: (Top) A solar plane generated for the instant of noon, winter solstice, slopes to meet the top of a shadow fence on the north edge of the site. (Bottom) As the period of solar access is increased, the original plane lowers and other planes are added, one generated by the winter morning sun and another by the afternoon sun. Compass rose points north.

Generation of Solar Envelope: (Top) A solar plane generated for the instant of noon, winter solstice, slopes to meet the top of a shadow fence on the north edge of the site. (Bottom) As the period of solar access is increased, the original plane lowers and other planes are added, one generated by the winter morning sun and another by the afternoon sun. Compass rose points north.

Shadows: (Left) Without solar-access zoning, there is no guarantee of sunshine. (Right) The heights of shadow fences can be set to control the solar envelope, thus the shadowing impact on neighboring buildings, while maximizing buildable volume. (Drawings by Daniel Wright in Sun Rhythm Form by Knowles 1981,122f.)

Shadow Fences:The solar envelope is generated to meet shadow fences that are set according to surrounding land uses and community values. It can extend beyond its own site to meet shadow fences across the street.The shadow fences vary in height,for example, higher for parking and lower for a community park.

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.4

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

Space-Time Limits: (Left) Shadow fences on surrounding properties set the envelope's volume (gray). (Right) Varying the cutoff times will change the envelope's volume.

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.

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