It used to be easy to pick a window. Did it look good? Did wind and rain blow through? Now, it's more complicated. Windows have improved in many ways over the last ten years, especially in their energy performance. Manufacturers have increased insulating value with multiple lites, gas fill, new spacers and heat-reflecting low-e coatings.

Low-e was originally developed to reduce heat loss--a job it does well. Newly developed low-e coatings can selectively enhance solar heat gain or block it. Picking the wrong one could have a major impact on heat and cooling costs.

"Sunlight" falling on a window is made up of visible light, near infrared energy (heat) and ultraviolet light. This energy is either absorbed by the glass, reflected back to the outside or transmitted into the building. (See "Solar Energy Flow.") Far infrared energy is heat radiated from objects.

Selective Windows

About two-thirds of the heat escaping through double-glazed windows does so through radiation. Originally, low-e was intended to reduce heat loss by blocking heat radiating from objects inside the house. This is called far infrared energy. Scientists have devised new "selective" coatings that also help block solar heat streaming through the glazing. Solar heat includes energy from visible light and invisible heat called near infrared radiation.

While solar heat gain is helpful for some buildings, it's a problem for others. Blocking solar heat gain has been important in commercial buildings for decades. You've probably seen the bronze, mirrored office towers. Those windows block much of the sun's heat, but also much of the light and views. Selective glazing allows more visible light to penetrate. (See "Light and Heat") This means that daylight can be used instead of electric light and the occupants of the building can see the view.

Researchers haven't found the perfect window, but selective glazing is a dramatic improvement. As manufacturers offer more products, you must ask more questions. Should the windows gain heat or reject heat? How important is visible light?

Most manufacturers give you a choice of low-e types. For example, Marvin Windows allows you to choose from "northern" or "southern" low-e. Several manufacturers offer only one type, unless you request something different. What do you ask for? It's all in the numbers.

The Numbers

In most product literature, you'll find the following specifications.

U-value describes the rate of heat loss. So, lower numbers mean less heat loss. Most window manufacturers now give the U-value for the entire window unit, including the frame. Good windows are U-0.40. Super windows are U-0.25 and some products go down to U-0.15. (U-value is the inverse of R-value. To calculate R, divide 1 by U. (1÷ U 0.25 = R 4.0) To calculate U, divide 1 by R.) The best source of information is The Certified Products Directory published by the National Fenestration Rating Council.

Shading Coefficient (SC) indicates how much solar heat is blocked. It compares how well a particular glazing reduces solar heat gain to a single pane of 1/8-inch double-strength clear glass. SC values range from 1.0 for clear glass (without low-e) to 0.08 for heavily reflective glass. A standard insulated glass unit (IGU) has an SC value of about 0.87.

Solar Heat Gain Coefficient (SHGC) also indicates how much solar heat is blocked by the window, but it is slightly different from SC. SHGC expresses the amount of solar heat that penetrates the window compared to the amount that strikes the outside. Lower numbers mean less heat entering the building. SHGC will eventually replace SC in manufacturer's literature. The NFRC Directory should start listing SHGC starting in January 1995.

Visible Light Transmittance (VLT) is the amount of visible light that penetrates a window expressed as a percentage. This affects daylighting and the ability to see objects and views outside. The human eye can adjust to wide variations in visible light, so it compensates to some degree for low VLT numbers. Clear glass has a value of 90 percent. Reflective glass would be anything below about 20 percent VLT. For most residential applications, VLT should be between 40 and 70 percent.

Reflectance describes how much visible light is reflected back toward the source. The numbers are listed for inside, outside or both. To avoid the "mirrored" look keep outside reflectance lower than 10 -15 percent. A few people find interior reflectance a problem, too. Be sure to distinguish between visible reflectance and solar reflectance, which may also be listed in the product literature.

Light-to-Solar Gain Ratio (LSGR) is one way to evaluate the relationship between VLT and heat gain. When LSGR equals 1.0 the same fraction of light and heat gain pass through the window. For climates where cooling is more important, ratios above 1.0 are better, because light transmittance is higher than heat gain. It's not difficult to find residential products with LSGRs between 1.0 and 1.4. You probably won't find LSGR in product literature, but it's easy to calculate: divide VLT by solar heat gain (preferably SHGC).

Ultraviolet (UV) resistance is often promoted by window manufacturers because of its role in fading colors of furnishings. Some low-e coatings boast UV transmission under 1 percent. However, about 20 percent of sunlight's fading power comes from visible light. So even completely blocking UV doesn't stop fading altogether.

Mechanical Savings

Since glazing can be a big heat gainer or heat loser, glazing choices affect other building systems, especially heating and cooling. If you reduce heat gain in summer, you can reduce the size of the cooling system. For a heat pump, cutting one ton of capacity can shave about $300 off the equipment cost. If you disregard the impact of glazing on the mechanical system, costs will be higher than necessary. Plus, customers may be uncomfortable and unhappy.

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Light and Heat

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Other considerations

Heat flow (loss or gain) is only one issue. There are many other items to consider in choosing windows.

Daylighting is important too. In your haste to reduce solar heat gain, don't pick a product that cuts too much visible light. If the occupants of the building have to turn on the lights to see during the day, some energy savings will be lost. Plus, people love natural light and will not feel comfortable if it's too dark.

Appearance is often critical. Mirror finishes and obvious colors may not be acceptable in residential buildings. The new selective glazing systems reduce the need for highly reflective coatings and strong tints.

Windows are not the first line of defense for heat rejection. Exterior shading from trees, shrubs, trellises, overhangs and awnings can be more effective. By using appropriate shading strategies, you can use a glazing system with a higher SC that allows more visible light.

Glazing Options

Mirror films are commonly used in tall office towers to reject heat. Standard reflective coatings aren't choosy, they block visible light and heat. The SC can be as low as 0.11. Mirror coatings have lost some appeal. Reflections can be annoying and dangerous, if they blind drivers. The reflected heat can scorch landscape plants.

Tinted glass absorbs energy, which turns to heat. Some of this heat flows back outside, but some continues inward. Bronze and gray tints block much of the visible light as well as heat. Blue and green tints are more selective for blocking solar heat (near infrared). PPG makes a product called Azurlite™ and Libbey-Owens-Ford (LOF) makes one called Evergreen™. While tints give the windows a noticeable color, many people like the appearance of tints better than mirror coatings. The SC ranges from .5 to .15.

Low-e coatings, originally developed to decrease heat flow (far infrared) through windows, have been "tuned" to cut solar heat gain (near infrared), too. The selective low-e coatings block more solar heat without paying a high penalty in visible light. They are highly reflective to heat, but not visible light, so they don't look like mirrors.

Combination products aimed at cutting solar heat gain often use both tinted glass and a selective low-e coating. Normally, tinted glass is used for the outside lite of an IGU with a low-e coating. Combination products give excellent insulating value, low SCs and keep visible light above 50 percent.

Goals

Here are two examples of how you might go shopping by the numbers.

Situation: You want windows to capture solar heat for a building in a heating-dominated climate. In this case, the goal is to maximize transmission of visible light and solar heat.

Situation: You want to minimize solar heat gain. This could apply to all windows in cooling dominated climates, such as the southeast and southwest United States. This situation also may occur in cooler climates where many large windows face west into the setting sun. In this case, the goal is to reduce near infrared as much as possible without cutting too much visible light.

Of course, there are many other situations. Fortunately, you now have more glazing options than ever. These performance numbers can help you sort through the options to make the best choice for each construction project.

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Selective Glazing Products

All the major window manufacturers offer selective glazing options. Here is a small sampling of advanced low-e and selective tint products. Notice the relationship between VLT and SC.