Daylight Savings: Window Systems Deliver Light and Reduced Energy Costs

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Sponsored by Guardian Industries Corp., MechoShade Systems, Inc., Pella Corporation, and Technical Glass Products (TGP)

Winning Combinations

Often the best daylighting systems are custom designed for the building, its user requirements and location and climate. "What we've learned from many architects and our own research is that the most effective daylighting relies on a combination of interior and exterior design techniques," says Razwick, noting that for TGP's office and manufacturing facility, designers used a three-story atrium, butterfly roof with clerestory windows, an open interior design, and sunshades and light shelves. "They also maximized the glazing area with high-strength, narrow-profile steel frames and used channel glass in select locations. Together, these features create a light interior, while helping manage heating and cooling costs," says Razwick.

Hoffman Corporation, an architectural firm in Appleton, Wisconsin, also incorporated several sustainable strategies in the design of its corporate offices with particular attention to window position for maximizing daylight and glare control. Exterior sunshades and interior light shelves combined with light-colored ceilings direct daylight deep into the open office environment. Clear Low-E insulating glass above the light shelf provides maximum visible light transmission, and gray Low-E insulating glass below the light shelf controls glare. Between-the-glass blinds were used in the east- and west-facing windows to block unwanted heat gain, while triple glazing with multiple Low-E coatings reduces heat loss on the north side of the building.

Light shelves on the interior of the Hoffman Corporation's offices direct daylight deep into the building interior.

Hoffman Corporate Offices, Appleton, WI; Hoffman Corporation; Courtesy of Pella Corporation

 

Part of a Process

Proper daylighting is a multidisciplinary process that drives the entire design of the building and involves architect, engineers, interior designers and considers a wide range of factors including building orientation and footprint, availability of natural light, floor to ceiling ratios, integration with electric lighting systems, and interior design - as well as fenestration systems. Berman stresses the importance of understanding the daylighting system very early in the process, and finding a partner to execute the desired goals before the window details are locked in. Failure to do so can significantly drive up the cost of the system. "Early on, the design team may be able to show two or three design options that disappear down the line when details have been finalized," says Berman, noting that with motorized shades, the key cost is motor density. "At the New York Times building, one motor typically serves six windows over 30 linear feet, which is highly efficient - and that's attributable to their early and thorough due diligence."

Guardian's Dolan concurs on the advisability of knowing what you are buying. "What comes as a surprise to many is that not all Low-E glass products are created equal," says Dolan, "But unless the correct coated glass is installed, there can be occupant discomfort, increased energy consumption and a costly strain on cooling systems."

He points to a case in which architects on a Tennessee medical plaza that involved two identical 45,000-square foot buildings specified a pyrolytic Low-E coated glass, which eventually proved to be too warm for comfort. Seeking to achieve a neutral appearance and accommodate the relatively warm location, architects specified double-paned pyrolytic Low-E glass for Building 1 applied to a #3 surface instead of the #2 surface of the insulating glass unit, which is the norm for commercial buildings to reduce heat gain.

The pyrolytic glass transmitted too much heat to the building's interior, and occupants in the western and southern elevations of Building I complained of discomfort especially spring through fall, claiming that daily tasks were taxing in such an overheated environment, despite the air conditioning running full blast. The window glass was warm to the touch on the western and southern sections of the building.

Tests showed that energy costs were higher than they should have been due to the extra burden on the cooling system. For building ll, architects went with a transparent Low-E glass that would more effectively reflect heat energy; it was attached to a #2 surface with a 68 percent visible light transmission percentage combined with a very low solar heat gain coefficient (SHGC) of .38. That product was also specified in the new building and recommended to replace the glass in the western and southern elevations of the original building. "Once the glazing of Building I was replaced and the tenants reported a significant improvement in their comfort level and less burden on the cooling system," says Dolan

So important is initial understanding of a product that Berman advocates linking owner education to contracts. "It is critical that the owner fully understands the capabilities of the automated shade control system in order that they may derive the full benefit of the system as well as be a part of a successful implementation," Berman says.

Glazing Terms

The type of glazing selected can gain heat or lose it, and thus affect other building systems. With ever expanding choices in glass, it's advisable to be familiar with basic terminology used in describing today's glazing systems.

U-Value. The U-Value measures how well a product conducts heat - that is, how efficiently it keeps heat out in summer, and how well it keeps heat in the building in winter. U-factor ratings generally fall between 0.20 and 1.20, with the lower number meaning the less heat lost through the window.

R-Value. The R-Value measures how well a material resists heat flow. The higher the R-value, the more the material resists the movement of heat.

Solar Heat Gain Coefficient (SHGC).This rating indicates how effective the product is at blocking the heat caused by the sun. The lower the number, the less heat it allows in, and the greater its shading ability, hence its effectiveness in reducing cooling loads during the summer. High SHGC products are more effective in collecting solar heat gain during the winter. Specifying the proper SHGC depends on factors such as climate, orientation, and external shading.

Visible Light Transmittance. The amount of visible light that passes through the glazing material of a window, expressed as a percentage. VT is expressed as a number between 0 and 1, with heavily tinted products having a relatively low VT.

Light to Solar Gain Ratio. A measure of the ability of a glazing to transmit daylight while blocking heat gains. It is the ratio between the visible transmittance of a glazing and its solar heat coefficient. The higher the ratio number, the brighter the room will be without excessive heat.

 

A Bright Future

The future of daylighting looks bright - with a plethora of products and solutions. Manufacturers are constantly expanding and improving their offerings to help with daylighting goals, in the process turning out greener products. "Thanks to technological advances that make glass stronger and more durable, there more manufacturing plants throughout the U.S, meaning glass is more readily available, which cuts down on transportation costs, says Dolan. "Local production adds to sustainability and eligibility for LEED credits."

The potential for effective daylighting is present in nearly every type of climate - solutions to achieve the daylighting goals exist for gray overcast coastal areas and bright southern areas alike, and throughout each season and at all times of day.

 

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Originally published in Architectural Record
Originally published in June 2009

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