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Windows have long been used in buildings for day lighting and ventilation. Many studies have even shown that health, comfort, and productivity are improved due to well-ventilated indoor environments and access to natural light. However, windows also represent a major source of unwanted heat loss, discomfort, and condensation problems.

In recent years, windows have undergone a technological revolution. High-performance, energy-efficient window and glazing systems are now available that can dramatically cut energy consumption and pollution sources: they have lower heat loss, less air leakage, and warmer window surfaces that improve comfort and minimize condensation. These high-performance windows feature double or triple glazing specialized transparent coatings, insulating gas sandwiched between panes, and improved frames. All of these features reduce heat transfer, thereby cutting the energy lost through windows.

DESCRIPTION

Window systems are comprised of glass panes, structural frames, spacers, and sealants. In recent years, the variety of glass types, coatings, and frames available for use in window systems has increased dramatically, as has the opportunity to fine-tune and optimize window selection on a project-by-project basis.

Careful specification of window and glazing systems is essential to the energy efficiency and comfort of all buildings. In residential, skin-load dominated structures (such as housing) optimum window design and glazing specification can reduce energy consumption from 10%-50% below accepted practice in most climates. In internal-load dominated commercial, industrial, and institutional buildings, properly specified fenestration systems have the potential to reduce lighting and HVAC costs 10%-40%.

Window and glazing choices should be considered holistically. Once the design team and owner agree on the design problem, window and glazing options can be evaluated. Issues to consider include:

• Heat gains and losses

• Visual requirements (privacy, glare, view)

• Shading and sun control

• Thermal comfort

• Condensation control

• Ultraviolet control

• Acoustic control

• Color effects

• Daylighting

• Energy requirements

Ultimately, the optimum choice of window and glazing systems will depend on many factors including the building use type, the local climate, utility rates, and building orientation.

A. Specifying Windows and Glazing

To fully specify a window system, it is necessary to specify the following characteristics:

• Window U-value

• Window Solar Heat Gain Coefficient (SHGC), or shading coefficient (SC)

• Glass Visible Transmittance (Tvis-glass)

For specific aesthetic and performance objectives the specifier may also wish to specify:

• Tints (colors) and Coatings

The properties of a given glass can be altered by tinting or by applying various coatings or films to the glass.

Glass tints are generally the result of colorants added to the glass during production. Some tints are also produced by adhering colored films to the glass following production.

Tints are usually selected for aesthetic purposes. Some tints also help reduce solar gains.

Coatings, usually in the form of metal oxides, can also be applied to glass during production. Some of these coatings, called "low-emissivity" or "low-e," help reduce radiant heat transfer between panes of glass by blocking some or all of the IR wavelengths. These coatings can dramatically lower the window U-factor.

Care should be taken in specifying tints and coatings, as their application can dramatically impact window heat loss and heat gain. Mis-specification can result in the exact opposite of the desired performance.

B. Other Attributes

Other important attributes of window and glazing systems include:

• Gas Fills—Inert gases such as argon and krypton are often injected between panes of glass to reduce conductive and convective heat transfer. These low-cost, gas fills reduce U-values without affecting shading coefficients or visible transmittance.

• Fritting—Baked on ceramic coatings, or frits, can be applied to the surface of glass in many different patterns, colors, and densities.

• Safety and Security Glass

• Retrofit Films

• Switchable Optics—A new generation of dynamic glazings are available that change optical properties through changes in light, temperature, or voltage (i.e., photochromic, thermochromic, electrochromic.)

C. Opportunities and Cautions

Some design opportunities and cautions concerning the specification and application of window and glazing systems include:

Opportunities

Using high-performance windows can dramatically reduce heating and cooling loads, and eliminate the need for perimeter heating in internal-load dominated buildings due to the effect of increased Mean Radiant Temperature (MRT) on occupant comfort (see High-Performance HVAC).

Window systems with low-e and spectrally selective coatings can filter damaging UV wavelengths and increase the life of room furnishings.

Optimized fenestration systems for passive heating in residential buildings or for daylighting in commercial/industrial buildings will reduce loads and save O&M costs.

Cautions

Always specify certified, whole product energy performance values.

All low-e coatings are not the same! If selective coatings have been identified as a strategy to increase performance, make sure the coating is fully and properly specified for your application.

Always consider glare control, especially in commercial and industrial applications. Limiting contrast ratios and providing visual comfort in the field of view is critical, particularly in day lighting applications.

Avoid condensation problems. Condensation occurs when the glass surface temperature falls below the dew point of the room air. It can damage window and wall elements and obstruct views.

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