Windows Reduce Cooling Loads

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Glazing

Most large commercial buildings are dominated by cooling loads so window selection for commercial buildings is usually an exercise in maximizing daylighting and keeping summer heat out. Today's best windows block heat transfer more than five times better than single-pane glass, the standard windows of just two decades ago. High-performance windows are not only a wise investment for new construction but sometimes can be cost-effectively retrofitted especially when timed with planned replacement of HVAC equipment. Proper selection of glazing can actually enable the installation of smaller mechanical equipment.

To understand the principles of window selection, refer to Figure 1, which shows the spectral distribution of solar energy. About half of the sun's energy is at wavelengths visible to the human eye while the other half (longer-wavelength near-infrared or shorter-wavelength ultraviolet) is invisible contributing only heat to the building interior. To reduce cooling load, the ideal window would transmit as much of the visible portion of the sun's energy as is desired for the particular application while rejecting the rest of the solar radiation. That is, ideal windows provide light not heat.

Figure 1: The solar spectrum The ideal window for cooling load reduction transmits visible light and blocks ultraviolet and near-infrared radiation. Source: Platts

Reflective glazings

In hot climates, one effective way to reduce heat gain while letting in some light is through the use of semitransparent metallic coatings which can be applied to surfaces of clear or tinted glass. Some reflective coatings are available for single-pane applications and others must be sealed inside double-glass units. In general, most reflective glazings block daylight more than solar heat. Typical products offer solar heat gain coefficients from 0.13 to 0.35 with typical daylight transmittances of 5 to 35%. Solar heat gain coefficient, which has replaced shading coefficient as the standard window metric, is defined as the fraction of the incident solar energy transmitted through the window.

Reflective glass products have achieved their greatest market penetration in hot-climate applications where a high level of solar control is critical. However, they reduce cooling loads at the expense of daylight transmittance and that reduction is offset somewhat by the heat of the additional lighting required.

Spectrally selective glazings

In 1983, Southwall Technologies introduced the first glazing products designed to provide enhanced solar selectivity without significantly shifting the transmitted or reflected color. Now offered by a number of manufacturers under a variety of trade names, these spectrally selective coatings are a variation on earlier "low-emissivity" ("low-e") glazing coatings, which were designed to improve the insulation performance of windows while maximizing solar gain. Now selective glazings are available that can maximize or minimize solar gain or can produce anything in between.

Retrofittable window films

For retrofit applications, many of the benefits of solar control glazings are available through the application of after-market window films to existing glazing areas.

• Typical films have a total thickness of 0.001 to 0.004 inches.
• The films are made with a variety of adhesives and can be applied on-site to single or double-glazed windows usually to the inner surface facing the room.
• Besides reducing cooling loads, adding solar control films to windows also improves shatter resistance and blocks up to 99% of incoming ultraviolet radiation.
• Reducing daylight transmittance as well as solar heat gain reduces glare as well.
• Because of reduced solar radiation, local temperatures are reduced and comfort is improved near the windows. Anecdotal evidence indicates that air temperatures near windows can drop by 6° to 8° Fahrenheit with the addition of window films.

Retrofittable window films have been a relatively low-cost energy-saving method for many years and their performance is well documented by utilities and facilities managers. Historically, problems have included film fading and color shift, installation difficulties and poor adhesive performance. Most of these problems have been solved through advances in film and adhesive technology and through the use of better application techniques.

Window shading

Window shading devices reduce solar heat gain while transmitting enough daylight to help reduce the use of artificial lighting and its resulting cooling loads. Intelligent use of structural overhangs is a time-honored practice of solar engineering and is worth considering in a new building. However, unless there is a very specific need for window shading devices using advanced glazings is a better way to cut window heat gain—and a way to block solar radiation reflected from the ground, not just from the sky. Shading opaque areas of large buildings has little effect on cooling load since the large majority of solar gain comes through windows.

Eliminating windows—the most extreme shading option—removes all direct solar gain into the conditioned space and lowers conduction heat gains through the shell. Some buildings that house telecommunications equipment or electronics production facilities, for example, may be designed without windows in an attempt to keep temperatures within a very small range. However, for occupied offices, the aesthetic value of windows makes them worth keeping.

Several options in window shading are available:

• Window shading elements can be exterior, interior or mounted inside a double-glazing unit.
• Shades can be movable or fixed.
• Movable shading control can be manual or automatic.

Shading systems save money several ways:

• For new construction or in concert with HVAC renovation, capital and operating costs can be reduced if shading is extensive enough to allow downsizing of cooling and air-handling equipment.
• Also, sharply reducing perimeter heat gain may eliminate or reduce the need for HVAC zoning.
• In buildings that use terminal reheat, the smoothing out of cooling load between zones can result in less heating for more-efficient overall operation.
• Radiant comfort benefits in the perimeter zone may result as well.

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