January, 1995
from: Glass Magazine
By Susan Reilly

Solar Glazing For Homes

Residential housing located in climates where cooling is the main function of the HVAC systems can reduce cooling costs by reducing the solar heat gain through windows. Tinted and coated glazing and sunscreens can reduce the solar gain, as well as overhangs, draperies, blinds and shutters. We focus here primarily on glazing technology and sunscreens as cost-effective alternatives for minimizing solar loads.

Technologies which enhance the energy performance of windows include low-conductivity frame materials, frame design, warm-edge spacers, tinted and coated glazing and sunscreens. These technologies affect the thermal performance or U-factor of a window. In Figure 1, the average reduction in total window U-factor is shown for various frame materials as compared to an aluminum frame. The annual energy savings from the lower U-factor windows with clear glass in a cooling-dominated climate are negligible through sun control glazing and sunscreens.

Sunscreens have solar heat gain coefficients between 0.1 and 0.5 when placed on the exterior of double-pane, clear glass. The visible transmittance is usually less than the solar heat gain coefficient and thus they have a dark appearance. The screens also require replacement every five to 10 years. Screens are a cost-effective option and are particularly suited to the retrofit market. However, for the new construction market, we focus on glazing options because glazing has a life equal to that of the window and is more aesthetically flexible.

Figure 1.

Reduction In Total Window U-Value Due To Frame Type
T/B Alum=-0.20
AL Clad Wood=-0.25
Wood/Vinyl=-0.27
Fiberglass=-0.33

Average reduction in total window U-values for different frame types as compared to aluminum frames. (T/B Alum is thermally broken aluminum and AL Clad Wood is aluminum clad wood.)

There are literally thousands of glazing options currently on the market that can reduce cooling loads. Table 1 gives the center-of-glass solar heat gain coefficients (SHGCc) of a sampling of products available from glass manufacturers today that are used in the residential market. The spacing between the glass layers has a negligible effect on the SHGC. There are many products which can achieve much lower solar heat gain coefficients (0.1-0.3) at a comparable cost. Most of them are commercial glazing, which are either too reflective or too dark to be considered for typical residential applications. Consumers are accustomed to clear glass in homes so glass with a higher visible transmittance is desirable.

From a home buyer’s perspective, switching from clear glazing to glazing with a lower solar heat gain coefficient reduces energy costs and increases the comfort of the home. Assuming electricity costs of $0.092 per kWh for heating and cooling, a window with a solar heat gain coefficient (SHGC) of .55 (shading coefficient, SC of 0.63) reduces cooling costs by $0.6 per square foot of window area over a house with windows with clear insulating glass (SHGC of 0.78 or an SC of 0.9) in Phoenix, AZ.

If the upgraded window package adds $1,200 to the cost of the 1,500 square foot house with 300 square feet of windows, the mortgage is increased by approximately $120 per year at 9 percent over 30 years. The cooling energy savings is $180 per year with a solar heat gain coefficient of 0.55 an a U-factor of 0.75 Btu/hr-ft 2-F. The annual energy cost savings easily pay for the increased cost of the windows at current prices. These calculations do not take into account the costs to the utility that were avoided, the cost of money, the potential for downsizing cooling equipment or the benefits to society from reducing the need for new power generation facilities.

As an aside, the 1992 Model Energy Code for residential energy use does not consider solar heat gain from windows, either in terms of offsetting heating requirements or increasing cooling loads. A performance approach to compliance could be taken (MEC, Chapter 4) to show the benefits of solar control to reduce cooling loads. Note that in taking the tradeoff approach to compliance (MEC, Chapter 5), the window area may be decreased as a cost-effective approach to meeting the code and this is based solely on window U-factors.

The estimate of cost savings to the homeowner is a convincing argument for using high performance glazing. The challenge is to increase the use of high performance windows in cooling-dominated climates. An example of how the market can change is the market transformation in window products which occurred between 1990 and 1993 in Phoenix. Pre-1990 window sales were dominated by single-glazed aluminum windows. The driving force has been attributed to energy efficient mortgages in combination with Good Cents energy-efficient home programs and electric utility rebates (ADOC-Energy Office 1994).

To achieve even a fraction of the potential demand reduction, the market for high performance windows needs to be stimulated. Our recommendations emphasize education of window manufacturers, distributors, real estate agents and lender, builders, architects and consumers. We advocate the use of the performance approach with the 1992 Model Energy Code to take advantage of the benefits from solar control devices. Window manufacturers and glass manufacturers should also consider promoting the load-change equation for windows used in ASHRAE 90.2 as an addendum to the Model Energy Code. This would give designers more flexibility with windows and provide them with a simplified tradeoff approach rather than using the performance approach.

Table 1

Center-of-Glass Solar Heat Gain Coefficients (SHGCc)

 #  GLAZING SYSTEM                                      SHGCc

*SHGC's for a window are 5-20% lower depending on the operator type, size, and frame material.
SHGC calculated using WINDOW 4.1. Shading Coefficients can be found by dividing SHGC by 0.87

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