A Building Envelope is a concept that is relatively new to building technology, if we look back over history. It came to be because of our ability, with modern construction materials and engineering, to separate the function of "enclosing" from the structural function of "holding up the walls and roof". That means that walls and roof have no role to play in the work of fighting gravity - the resistance to compression the need for rigidity can therefore be entirely absent from the modern Building Envelope. Instead the envelope can be a flexible skin that is lightwieght that is "stretched", in tension, over a compression frame. This technical advancement is expressed in its most efficient form as a Tensegrity Structure. The Wide Span and Eco Sphere are examples of Tensegrity Structure design.
What is a tensegrity? Fuller explained, "Tensegrity describes a structural-relationship principle in which structural shape is guaranteed by the... continuous, tensional behaviors of the system and not by the discontinuous exclusively local compressional member behaviors" (Quote from http://www.angelfire.com/mt/marksomers/133.html)
In other words, the tensegrity concept diverges from traditional construction design in that that the structure’s shape is defined by a continuity of members in tension, like a spider web. Compression members (structural elements carrying weight by “squashing” force rather than being pulled on) are placed here and there to “stretch out” or support any variety of cable or net-like arrangements, but are not the defining elements for the form.
Why tensegrity design? Tension is the most efficient use of construction material in resisting force. James Gordon pointed out in “The New Science of Strong Materials or Why You Don't Fall Through the Floor” that mankind is the only species on the earth, other than ants, that build their shelter using compression structures.
At least the ants build in a stout cone form. We stack a series of identical weighty floors on top of columns. When the wind blows or the earth shakes, the heavy mass at the top acts like a pendulum to bend the columns and twist them off of the floor beams they support.
In traditional Western home construction, the critical structural role of exterior sheeting is to transmit torque through the beams and columns to the foundation (with many, many nails) to resist this twisting and bending and, hopefully, keep the whole structure from tipping over like dominoes. It takes enormous amounts of material and effort to handle this torque. And as soon as nature exceeds the building’s design criteria, the rigid structure cracks up like egg shells.
The Sola Roof envelope is not required to be rigid and heavy - instead, it is flexible and light. It will absorb forces in tension and the deflection of the "skin" of the building will not cause any problem. Rather it will stengthen the whole systems response to forces that act upon the structure. Since the area of the building envelope is the greatest factor in the cost of a structure, we are able to greatly increase the affordability of all buildings that use these methods.
The building envelope is designed and fabricated to work in tension and thus it will use a material for the covering skin that has a fabric or open mesh "srim" as the substratum of the skin to give strength and dimensional stability to the skin. A simple film is too stretchy to work well but a composite of woven or formed tensile material like fibers and yarns are used to build up a composite material by lamination of film to the substratum or by coating a scrim or fabric to form a weather proof covering skin.
These non rigid skins are ideal materials to use to form the transparent building envelope, which is the key concept of Blue Green design. No rigid glass or plastic can be as safe and strong and resistant to damage by forces that may act upon a structure - strong wind, heavy rain, the wieght of snowfall, earthquake, tsunami - our non rigid structure and envelope can be designed to resist and survive with minimum damage.