LBNL Report Number
Glass plays a significant role in the design of building envelopes today. Since its emergence during the last century as a major building material, glass has evolved into an ubiquitous and versatile building design element, performing functions today that would have been unimaginable a few years ago. The optical clarity and transparency of glass that we take for granted is one of its most unique features. Glass windows keep out the cold wind and rain without blocking the view, but also perform many more complex functions which require variable properties and tradeoffs between conflicting conditions. The glazing that provides view must also provide visual privacy at other times and must sometimes become totally opaques(for audiovisual shows, for example). Transparent glass admits daylight, providing good color rendition and offsetting electric lighting energy needs, but it can also create discomfort and disability glare conditions. The sun provides desirable warmth in winter but its heat is unwelcome in summer when it contributes to thermal discomfort and cooling energy requirements. And glass is an important element in the appearance and aesthetics of a building, both interior and exterior.
The problem confronting a building designer is one of tremendous variability in the environmental forces that impinge on the building exterior and the rapidly changing needs inside the building. For example, the intensity of sunlight varies on several timescales: by the seconds or minutes on a day with scattered clouds; by the hour as the sun follows its diurnal cycle; and over months as the seasons change. Since exterior daylight illuminance can vary by a factor of 10 to 20 during a day, from approximately 5000 lux (approx. 500 fc) under overcast sky to 100,000 lux (approx. 10,000 fc) in direct sun, optical controls must operate over this wide dynamic range.
There are many building envelope options to extend the intrinsic degree of optical and thermal control exhibited by glass alone. The glass selection will influence total transmittance, spectral properties, and directional properties. But these will be fixed properties and will satisfy some, but not all, of the performance requirements. In order to provide a wide range of performance responses, dynamic control of one or more of the glazing properties is essential. This is not a new thought; a vast array of window accessories can be added to glazing to alter its properties in response to changing external conditions and/or internal needs. Current research in a new field, large area chromogenics, is leading the way toward development of a new generation of glazing materials that will provide an intrinsic dynamic and responsive optical control capability within the glazing. A recent approach is to provide this dynamic control using thin film coatings deposited on the glazing.