Energy efficiency considerations in window replacement projects

Daylighting should limit undesired glare and heat.

A window replacement project may present an opportunity to improve user comfort by incorporating daylighting schemes. By introducing appropriate levels of natural light into a space, daylighting can reduce the need for artificial illumination, lowering electricity expenses and providing the health benefits of full-spectrum lighting.

Daylighting may be quantified in several ways, one of which is through the glazing factor—the ratio of exterior to interior illumination, expressed as a percentage. The architect or engineer may perform calculations to determine whether a minimum two-percent glazing factor is achieved for all daylighted spaces. Another method for determining daylighting requirements is to demonstrate through computer simulation at least 269 lux (25 foot-candles) of daylight is available for illumination.

Daylighting schemes usually need redirection or glare-control devices to maintain energy efficiency and user comfort. For windows receiving direct sunlight, interior shading may be required to manage glare and limit heat gain. Automatic photocell controls for light screens, blinds, or curtains can be programmed to adjust shading depending on incident light levels. Advanced glazing technologies, including electrochromic and photochromic ‘smart glass,’ can adapt light transmission levels in response to electric controls or sunlight.

Although it may add to project costs, increasing the window opening size to amplify natural light and expand views to the exterior may be considered as part of a window replacement project. Windows that are 0.8 m (2 ½ ft) at their base to 2.3 m (7 ½ ft) at their head above a finished floor (AFF) are considered optimal, as they are most effective at distributing daylight deep into spaces.

When designing daylighting schemes, it is important to consider not only the window dimensions and glazing, but also the channeling of light within and between rooms. Interior glazing allows borrowed light from exterior windows to reach inside spaces; low partitions and open-plan office layouts are other options for distributing natural light across large areas.

While daylighting schemes may add to the up-front cost of a project, providing better-quality natural lighting can pay dividends in improved user experience. From a return-on-investment (ROI) standpoint, a pleasing daylighting design can also add value to the building.

Craig A. Hargrove, CDT, RRC, AIA, LEED AP, is senior vice president and director of architecture at Hoffmann Architects Inc., a firm specializing in the rehabilitation of building exteriors. As manager of the New York City office, he leads project teams in developing building enclosure solutions for existing buildings and in consultation for new construction. A CSI-certified Construction Document Technologist, Hargrove received his architecture degree from New York Institute of Technology. With a focus on high-performance building envelope design, he is a LEED AP, RCI Registered Roofing Consultant, and member of the National Institute of Building Sciences (NIBS), New York Landmarks Conservancy, New York chapter of the U.S. Green Building Council (USGBC), and Building Owners and Managers Association’s (BOMA’s) Codes and Regulations Committee. Hargrove may be reached at

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