FMJ July/August 2016 : Page 72


US Perspectives on High-performance Windows

John Bendt

Today’s facility managers face myriad challenges in managing interior environments to provide a comfortable workplace for building occupants. The challenge is even greater in buildings that have window systems installed prior to 1980. These windows were designed without the benefits of low-emissivity glass and improved frame thermal barriers that help reduce heat flow into and out of the workplace.

Without modern window performance, building occupants are prone to complain about being too hot or too cold. This discomfort can contribute to a loss of productivity for occupants, as well as increased operating costs for facility managers. New high-performance window systems can significantly improve occupant comfort, reduce energy costs, and minimize capital costs for upgrading HVAC and lighting systems.

Impact of inefficient windows

According to the U.S. Environmental Protection Agency (EPA), inefficient windows account for 50 percent of the cooling load in warm climates. During warmer months with periods of direct sun, older window systems can allow too much solar heat gain, leading to uncomfortable temperatures. This can in turn challenge facility managers to balance airflow and ambient temperature to keep all occupants comfortable.

In buildings with older glass technology, occupants will often close the blinds or shades, reducing the amount of natural daylight coming into the building and causing more people to turn on electrical lights generating even more heat. Excessive heat can even result in some occupants bringing in electric fans in to cool their work areas, which adds to energy costs through increased plug loads.

The EPA also has noted that inefficient windows account for 25 percent of a typical building’s heating load in cold climates. In buildings that have window systems with poor insulating values, occupants that sit near them often will plug in their space heaters and rely on additional layers of clothing to stay warm. It can be very challenging for facility managers in buildings with old windows to regulate temperatures throughout the space.

According to the U.S. National Building Sciences’ Whole Building Design Guide, “In 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings is one-fourth of all the energy used for space heating and cooling in the United States.”

Measuring heat gain

The industry benchmark of solar heat gain coefficient (SHGC) is used to help measure and control unwanted heat through windows.

This is the fraction of incident solar radiation (both heat and light) admitted through a window, including radiation that is directly transmitted as well as that which is absorbed and subsequently released inward. An SHGC of 0.2 indicates that little incident solar radiation is entering a building in the form of heat or light. An SHGC of 0.8 indicates that much of the incident solar radiation is transmitted through the window.

The best way to balance visible light transmittance with an appropriate SHGC depends upon the climate, building orientation, shading conditions and other factors. Typically, windows with low SHGC values are desirable in buildings with high air-conditioning loads, while windows with high SHGC values are desirable in buildings where passive solar heating is needed. Solar heat gain of glass in commercial window systems generally ranges from above 0.8 for uncoated water-white clear glass to less than 0.18 percent for highly reflective coatings on tinted glass.

Improvements in window technology

Through the energy crisis of the 1970s, commercial buildings largely relied on tinted or reflective glass to manage unwanted solar heat gain. In the early 1980s, the first glass to use microscopically thin, low emissivity (low-e) coatings to reflect solar infrared heat energy was introduced into the commercial building market.

Today, this technology has greatly improved. Manufacturers now can “stack” multi-layer coatings on the glass to improve the performance. These coatings total only one-ten-thousandth the thickness of a human hair, yet provide significant improvement in both solar heat gain coefficient and thermal transmittance (U-factor). U-factor represents conductive and convective heat flow per unit area, expressed in units of British Thermal Units per hour, per square foot, per degree Fahrenheit.

To quantify a commercial window system’s thermal performance, U-factor is the accepted measurement. The higher the U-factor, the more heat is transferred (lost) through the window in winter. U-factors usually range from a high of 1.3 for a typical aluminum-framed, singlepane window to a low of around 0.16 for a multi-paned, high-performance window with low-emissivity coatings and expanded thermal barriers in the aluminum frames. Window manufacturers can provide test reports to show the insulating value of their products. To ensure an accurate product comparison, be certain to check that the test specimen sizes are the same.

Many studies have shown that health, comfort and productivity are improved for building occupants with well-ventilated indoor environments, access to natural light and a view to the outside environment. One measurement that can help in evaluating these benefits is visible light transmittance (VT or Tvis). This is the amount of light in the visible portion of the spectrum that passes through the glass. A higher VT means there is more daylight in a space.

With proper design and specification, glass with the appropriate VT can offset electric lighting and its associated cooling loads. The type of glass, the number of glass panes and any glass coatings influence the VT. Ranges for VT can be above 90 percent for uncoated water-white clear glass to less than 10 percent for highly reflective coatings on tinted glass. A typical doublepane, insulated glass unit has a VT of approximately 78 percent. This value decreases somewhat by adding a low-e coating and is decreased substantially when adding a tint.

New glass technology can provide the appropriate VT and SHGC, while reducing glare. Glare can cause discomfort for occupants when too much bright light enters a building. Glare is most uncomfortable when there is a large difference between the entering sunlight and the area on which the occupant is trying to concentrate. Discomfort occurs when the eye attempts to even out the contrast between the task and the surrounding surfaces.

Other test reports and data that aid in selecting an optimal window system include: air, water and structural performance, acoustic performance, condensation resistance, and special performance, such as hurricane impact resistance or blast hazard mitigation.

Retrofitting an existing building’s window system can often noticeably improve air infiltration. Not only does this help overcome occupants’ complaints about drafty, old windows, but it also makes maintaining a constant interior temperature and relative humidity easier, and places less demand on mechanical systems.

Facility managers may choose to complete on-site pressure chamber testing to get actual air infiltration rates. As a guideline, ASHRAE’s “Handbook of Fundamentals” indicates that older, existing windows may experience air infiltration of:

• 2.5 cfm*/square foot at 1.56 psf+ for existing, nonweather- stripped, hung or sliding windows;

• 1 cfm/square foot at 1.56 psf for existing, weatherstripped hung or sliding windows, or for nonweather- stripped awning or casement windows; and

• .5 cfm/square foot at 1.56 psf for existing, weatherstripped awning or casement windows.

Today’s window technology with improved weatherstripping, hardware and precision machining has significantly ameliorated air infiltration rates. The American Architectural Manufacturing Association, which establishes the window industry’s standard testing procedures and performance ratings, requires that architecturally rated window systems meet:

• .1 cfm/square foot at 6.24 psf for new fixed windows or curtainwall;

• .1 cfm/square foot at 6.24 psf for new awning, casement or other operable windows; and

• .3 cfm/square foot at 6.24 psf for new hung or sliding windows.

These rates are substantially better than older window system technology and the previously published ASHRAE guidelines.

Evaluating whether to retrofit

Replacement window systems may be a large capital decision. When evaluating initial cost, remember to consider the projected savings that new windows can have on a facility’s energy costs, downsizing of HVAC and lighting capacity, and lowered maintenance. When properly specified and installed, modern window systems can reduce lighting and HVAC costs by up to 40 percent.

While lower operational energy costs are always welcome, even a small increase in employee productivity can have a significant positive financial impact, as salary costs are generally 10 times higher than energy costs in U.S. office buildings. Energy-efficient, high-performance buildings provide occupants with access to daylight, comfortable temperatures and better air quality. These environmental characteristics are correlated with lower absenteeism and higher productivity, and can save up to US$2,000 annually per employee.

Demonstrating their energy efficiency, more than 28,000 facilities have been certified by the EPA’s ENERGY STAR program. Associated benefits include up to a 26 percent increase in property value, up to an 11 percent increase in occupancy rates and up to a 15 percent increase in lease rates. Improving both the performance and the appearance of existing buildings, a new energy-efficient window system is proven to enhance the building’s overall value.

*Cubic feet per minute, + Pounds per square foot


• National Institute of Building Sciences’ Whole Building Design Guide,

•‚‚ Efficient Windows Collaborative, index.php

‚‚• ASHRAE, handbook-online

• American Architectural Manufacturers Association,

‚‚• American Institute of Architects and Rocky Mountain Institute, “Deep Energy Retrofits,” 2013,

John Bendt serves as vice president of Apogee Enterprises, Inc.’s Building Retrofit Strategy Team. He assists facility managers in evaluating the benefits of energy-efficient building envelope renovations and upgrades, including by offering free energy modeling, product selection and design assistance, and a network of installers covering North America.

Previously, Bendt served in leadership roles at two Apogee companies — as vice president of sales and marketing for Wausau Window and Wall Systems, and as vice president of service and special projects for Harmon, Inc. Prior to joining Apogee, he worked in numerous general management positions for Otis Elevator Company, a unit of United Technologies Corporation.

With more than 25 years in the commercial building industry, Bendt has led many teams responsible for upgrading building systems to enhance the value of commercial buildings.

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