Windows may account for a major source of unwanted heat loss or gain in a structure. The purpose of windows is to bring in daylight and allow ventilation. A completely windowless structure would prevent heat loss, but would run up the lighting bill. There is a direct connection between daylight and ventilation and personal health. Daylighting is also a consideration of many codes. Current technology allows for windows to be more energy efficient than in the past.
Energy transfer through a window may take place by: conduction—direct transfer; convection—sunlight heating air in a room; and radiation—sun heating objects in a room. The U factor indicates the heat flow due to conduction, convection and radiation. The U factor is also the mathematical inverse of the R factor. The relationship between U-factor or R-value and thickness is not always exactly inverse, and therefore R-value cannot be precisely extrapolated for a material of different thickness. The SHGC, or solar heat gain coefficient, is the fraction of incident solar radiation that enters a building as heat. The SHGC accounts for the glass and frame of the window. The SC, or shading coefficient, is defined as the ratio of solar heat gain through a particular glazing to the solar heat gain through a single lite of 1/8” glass.
Low-E refers to a low emissive coating that reflects invisible long-wave radiant heat sources. Clear glass has an emissivity of .85, so a low number is better. Spectrally low-e reduces heat loss and heat gain.
To reduce heating costs in northern latitudes, select the highest SHGC you can find with the U factor required for colder areas to gain heat in the winter months. In the southern latitudes a low SHGC is most important to restrict the sun all year and lessen cooling requirements. For areas in between, you must balance your heating and cooling requirements.
Visible transmittance (TVIS-glass) indicates the percentage of the visible portion of the solar spectrum that is transmitted through a given glass product. Tints, films and coatings (low-e) will determine the amount of transmission. There are a wide range of coatings and tints to cover a myriad of applications. One example would be to protect interior objects from sun light damage. Films may be applied as a retrofit.
Multi-glazed windows incorporate two or more layers of glass into a single window sash. These layers are typical sealed and may have inert gases injected between the layers to reduce conduction and convection heat transfer. This reduces the U value without affecting the shading or visible transmission of light. The exact mix of gases is typically a proprietary item. The glass pane surfaces are numbered starting from the outside to the inside of the structure. This is used as standard convention to keep track of the location of the coatings applied to the panes.
Some glazing has the ability to block light transmission whether or not an electrical charge is applied to the glass.
Changes coming to affect the above coefficients will be the use of ceramics and the possibility of the use of glazing as a vehicle for solar power.