Different buildings have differing needs for aesthetics, performance, and functional operations. Few building materials have as great an impact on all three of these areas as glass since it plays a unique and important role in building design and the environment. The use of glass in buildings affects design, appearance, thermal performance, and occupant comfort. Historically, glass was used mainly for windows to admit air and light, but with advanced manufacturing options and the need for high-performance buildings, it is now integral to interior and exterior architecture. From facades, skylights and walkways to revolving doors and glass box extensions, glass is being used to do much more than just let light in. Therefore the selection of the right types of glass is a crucial element of the design process to create solutions that address thermal control, energy efficiency, views, and lighting quality as well as light quantity. Architects who understand the full range of possibilities available from glass manufacturers are able to use them as a complete palette to create designs that excel in all areas.

Glass and Glazing Overview

The main ingredient of glass is SiO2 (silica sand). During the first half of the 1900s the predominant glass manufacturing technology was the plate glass process. Plate glass was created from molten SiO2 which moved along rollers while still heated. This created imperfections in the surface of the glass that needed to be ground and polished to produce parallel surfaces that were optically clear which required extensive labor. During the 1950s and 60s Sir Alastair Pilkington invented and perfected the float glass process which has become the current world standard for the production of high-quality glass. Float glass is manufactured by melting sand, soda ash, dolomite, and limestone, along with other minor batch materials to produce a continuous glass ribbon. The molten glass flows from the furnace and “floats” over a bed of molten tin where it spreads out to form a level sheet with virtually parallel surfaces. It is then carefully cooled to anneal the glass—a process that minimizes the internal stresses enabling it to be cut.

Variations in the float glass manufacturing process or additional actions immediately after allow for the creation of different types of glass and glazing. Each general type is produced to create different properties of light transmission, thermal characteristics, or other inherent structural properties. Several of the common types can be briefly summarized as follows:

Coated Glass

Coatings can be applied to glass creating a wide variety of products. These products typically combine low emissivity, solar control, low or high reflection, and self-cleaning properties. There are two processes used to apply coatings to glass: pyrolytic and sputter. These are sometimes referred to as hard coats (pyrolytic) and soft coats (sputter).

Pyrolytic coatings are called a "hard" coat as they are added into the glass while it is still in a molten state during float glass process by depositing microscopically thin layers of metallic oxides using a process known as chemical vapor deposition (CVD). Having this type of hard pyrolytic surface fired on at over 640°C (1,200°F) make pyrolytic products more durable than sputter coating which is a low pressure technique that deposits coatings using physical vapor deposition, typically without applied heat. The pyrolytic process creates extremely durable coated products that can easily be handled, transported, processed and won't degrade over time.

Heat Strengthened

Annealed glass is subjected to a special heat-treatment in which it is heated to about 680°C (1256°F) and afterwards cooled. When it is cooled slowly, the glass is twice as strong as annealed glass. If it does break the fragments of the broken glass may remain in the frame but are large. Heat strengthened glass is not recognized as a “safety glass” by typical building codes.

Tempered Glass

Tempered glass is at least four times stronger than annealed glass. When broken, it shatters into many small fragments which reduces the potential for major injuries. This type of glass is intended for glass facades, sliding doors, building entrances, bath and shower enclosures, and other uses requiring superior strength and safety properties.

Laminated Safety Glass

Laminated glass comprises two or more layers of glass bonded together with a plastic or resin interlayer. If broken, the interlayer is designed to hold the glass together. Virtually all glass types can be laminated and the thickness and types of interlayer can be varied to provide ballistic, bomb or physical attack resistance. Laminated glass also provides attenuation of sound and can typically be cut and further processed.

Low-e Glass

This type of coated glass provides thermal control and enhanced insulation, as well as control of solar heat gain when combined with a solar control glass in either a monolithic or insulating glass unit. Low-e coatings reduce the emissivity of the glass surface meaning the glass provides greater insulation by reflecting heat back towards its source and can also be designed to absorb or reflect solar energy. As such, low-e coatings are useful for reducing both solar heat gain and heat loss. For a sense of context, uncoated glass has a typical emissivity of 0.84 while a low-e coated glass could have an emissivity of 0.15. This means only 15 percent of heat is absorbed and re-emitted while the rest is reflected. Different combinations of low-e coatings can be used in an insulated glass unit to provide the desired performance.

Insulating Glass Units

Insulating units are two or more panels of glass bonded to a perimeter spacer material with a hermetically sealed airspace. The primary benefit is to improve thermal performance with better U-factors as well as solar control by influencing the Solar Heat Gain Coefficient (SHGC). Most types of processed glass can be incorporated into an insulating glass unit to adjust and fine tune its overall properties accordingly. Double glazed units are the most commonly used type of insulating glass unit, however in some climates the use of multi-cavity triple glazed units is increasing in response to tightening energy codes.