4.8 Glazing Selection
As suggested by "Tips for Daylighting" from Lawrence Berkeley National Laboratories, "... glazing selection should be a careful process of evaluating and weighing trade-offs."
Choose between dual-pane and single-pane glazing.
This is the critical first decision in glazing selection. Although higher in first cost, dual-pane, or insulating, glazing typically improves comfort in perimeter zones, offers greater flexibility in product selection, improves acoustic performance, and reduces mechanical loads. Most new energy-efficient buildings should use insulating glazing. Single-pane glazing with exterior shading can be effective in mild climates if there is significant solar radiation.
Choose a spectrally selective glazing.
Select a moderate visible transmittance for glare control (50-70% is a good starting point). The optimum visual transmittance depends on visual tasks, window size and glare sensitivity; the larger the windows or the more critical the glare control, the lower the desirable visible transmittance. Examine manufacturer literature for good glazing candidates. Find the product tables for insulating or single-pane units, depending on your initial selection, and look for products with your desired visible transmittance and the lowest possible solar heat gain coefficient.
Balance the conflict between glare and useful light.
A physical model studied outdoors is a good tool to qualitatively assess glare. If glare is an anticipated problem, and if an architectural solution to glare is not possible (moving windows out of the field of view, using deep reveals, shading systems, and other physical modifiers), then select a glazing visible transmittance that is a compromise between glare and daylight. A visible transmittance as low as 25% may still provide adequate daylight.
Window size and glazing selection can trade off with each other.
Use the effective aperture approach when making these decisions: Larger window area requires lower visible transmittance; smaller windows require high visible transmittance. A good target value for effective aperture is between 0.20 and 0.30.
Big windows require better glazing.
The bigger the window, the lower the required solar heat gain coefficient and visible transmittance. The bigger the window, the greater the need for insulating glazing. Large areas of inefficient glazing cause major comfort and energy cost penalties, increase cooling system size, and may not be permitted by building codes.
Don't assume that dark glass provides good solar control.
Many dark glazings block more light than heat, and therefore only minimally reduce cooling load. Dark glass can produce a gloomy interior atmosphere and may affect productivity and absenteeism. Dark glass not only reduces daylight, it also increases occupant discomfort on a sunny day, particularly in single glazed form. The glass absorbs solar energy and heats up, turning it into a virtual furnace for anyone sitting near it. Today, solar control is available in much clearer glazings.
Don't count on glazing alone to reduce heat gain and discomfort.
If direct solar beams come into the building, they still create a mechanical cooling load and discomfort for occupants in their path. Exterior shading combined with a good glazing selection is the best window strategy. Interior shading options can also help control solar heat gain.
Vary glazing selection by facade, if possible.
A lower solar heat gain coefficient on the south, east and especially west windows will reduce the cooling load.
Check Building Codes.
Some codes restrict the allowable area of glazing or thermal properties or both. Often tradeoffs are possible: more area is permitted if better glazing is specified.