Many glass companies promote their solar control glass products as the answer to sun penetration problems. In many cases the use of such glasses can save significant amounts of energy. However, if specified or used incorrectly they can actually add to heat loads in a building. Therefore it is vitally important that designers properly understand how such glasses work and why. As background, you should first read the properties of glass topic - specifically the solar control glass section.

When sunlight hits a pane of glass, it is split into three components - that which is reflected, that which is absorbed and that which is transmitted through. The actual process is a little more complex than this as the three major components comprise a number of sub-components outlined below.

Figure 1 - The three major components of sunlight when it hits a pane of glass.

Reflection and Inter-reflection

The first sub-component is that which is reflected by the outer surface of the glass. For typical window glass at low angles of incidence this is in the order of 4%. Of the amount of the light that enters into the pane some of its energy will be absorbed by particles and impurities within the glass itself. Also, as the inner surface of the pane is also reflective, an additional 4% will be reflected back into the glass. Of this 4%, some will continue to bounce back and forth between the two surfaces with around 96% escaping to join the reflected or transmitted components each time. The longer this light stays within the glass, the more absorption by contact with impurities will take place.

Long-wave Radiation

The reflected and transmitted components remain as visible and short-wave infrared wavelengths. However, the energy from the absorbed component actually heats up the pane of glass. This energy does not stay within the glass, but is radiated back out into the environment as long-wave heat energy.

Radiant Gains

This is why it is important for the designer to understand solar control glass. Whatever solar energy is not transmitted or reflected is re-radiated as heat. The highest amounts of solar radiation are likely to occur on hot days when all the surrounding surfaces external to the building are also well heated by the Sun. If the building is air conditioned the inside air and all internal surfaces will be at a pleasant 20-24°C - so guess where most of the radiant heat ends up - and no, it isn't the outside. Depending on the geometric configuration of the building and the location of the window, it may even be that the window is receiving more long-wave radiant energy from the external surfaces than it gives out in their direction - in which case almost all the net long-wave radiation from the glass will be towards the inside.

In an urban environment, using a highly reflective glazing simply makes the reflected solar radiation someone else's problem. There have actually been cases where an existing building owner has successfully sued the developers of a new building and made them pay for additional air-conditioning because of reflected radiation. Thus, in many parts of the world where solar radiation is likely to be a problem, councils place restrictions on the maximum allowable reflectivity of external glazing.

With restrictions on reflectivity, all that is left for manufacturers to increase is the absorbed component. Thus the plethora of tinted and spectrally selective glazing available. Such glasses do work - in that they reduce the transmission of direct solar radiation - however, if used incorrectly you will end up gaining almost as much heat by long-wave radiation as you would have by short-wave if you had used normal glass.

Benefits and Pitfalls

In most buildings, energy-efficient window glazing can result in significant energy savings and improvements in comfort. However, the different types of glazing have characteristics which may or may not be appropriate for a particular situation. Both the benefits and pitfalls of each glazing option should be considered before choosing a particular glazing.

Benefits

• Tinted, reflective, and spectrally selective glazing can all reduce solar heat gain significantly.
• Spectrally selective glazing let in the most visible light, providing the greatest daylighting opportunity.
• A gas-filled multi-layered low-E window with a good frame that has thermal breaks or is vinyl can out perform a standard insulated wall in terms of overall seasonal heating loads by allowing solar heat in and not allowing room heat out.
• Tints and reflective glazing can give a building a pleasing appearance.
• Applied films reduce solar heat, glare, and UV; increase shatter resistance; and provide some improvement in insulation value.

Pitfalls

• Tints alone only achieve a modest shading coefficient because some of the heat absorbed eventually transfers into the space as radiant heat. Reflective glazing can have similar problems if installed on the inside of the glass.
• Reflective glazing may cause increased lighting requirements and indirect cooling load attributable to the lighting because of their very low light transmittance.
• Tints have a colour which may or may not be appealing. And, for consistent appearance they are applied to all faces of the building even though they may not be economic on all faces.
• Spectrally selective glazing can produce problems with glare, particularly in rooms with computer screens.

Comparative Performance

The following table gives some indication of the relative values for for different types of glazing systems, including solar control glasses. From more information on the SC and SHGC values, see the Shading Coefficients topic.

Related Links

Characteristics of Window Glass, CBD-60

http://www.nrc.ca/irc/cbd/cbd060e.html

Dockrell Glass - Glass Facts Website

http://www.dockrellglass.ie/

Energy Efficient Window Systems - Pacific Gas and Electric Company

http://www.pge.com/pec/inftoc/facglaz.html

The Efficient Windows Collaborative Website

http://www.efficientwindows.org/