Understanding U-Factors in Architectural Metal Products

by Jonathan McGaha | March 31, 2014 12:00 am

Energy-efficient building designs must comply with codes and meet owner expectations. To meet owners’ needs, architects increasingly are concerned with misrepresentation of energy savings. Understanding energy requirements is an essential starting point, but concerns often arise in the product selection process. Close collaboration between architects and manufacturers will help achieve the desired results.

U-Factor (also called U-Value) is a measure of thermal transmittance that applies to such architectural metal products as windows, curtainwalls and entrance systems. U-Factor is the reciprocal of the familiar “R-Value” cited for insulation and residential products.

With U-Factor: the smaller the number, the better the thermal performance. Systems using conventional thermal barrier aluminum window and curtainwall frames can achieve overall U-Factors below 0.20 (R-5), meeting needs for best-in-class envelope performance in any of the eight U.S. Climate Zones.

Climate Zones are noted within ASHRAE 90.1 Energy Standards for Buildings Other Than Low-Rise Residential Buildings, and the commercial requirements of the International Code Council’s International Energy Conservation Code (IECC). These documents are updated in three-year code cycles to reflect the need for more energy-efficient and environmentally responsible building envelopes.

Under prescriptive compliance options within these codes, minimum performance requirements are mandated for a number of important parameters, including window-to-wall ratio, Solar Heat Gain Coefficient (SHGC), air infiltration rate, and of course, U-Factor.

U-Factor is determined using testing and modeling standards of the National Fenestration Ratings Council (NFRC). NFRC labeling is a compliance option for most state and local codes. Many high-performance window products are listed in the NFRC Certified Products Directory. Some manufacturers also have on-staff design engineers certified as approved calculation entities for NFRC’s component modeling approach, addressing curtainwall and window wall for non-residential applications.

Because of the residential origination of the NFRC organization, NFRC operable window test sizes are based on small, residential windows.

These small, residential sizes apply to operable windows only; hence, NFRC U-Factors of fixed windows, window wall and curtainwall are realistic for non-residential and commercial applications. Test size is critical in assessing overall window U-Factor validity. If using published product NFRC U-Factors, be sure to review the project sizes and configurations.

Center-of-glass U-Factors are commonly cited in misinformation, instead of citing the overall window U-Factor required by codes, which includes frame and edge-of-glass areas. The center-of-glass is the best-performing component of a non-residential window assembly. Frame U-Factor includes heat transfer through surfaces perpendicular to the glass plane.

At a basic level, heat can transfer in three ways: conduction, convection and radiation.

• Conduction is heat transfer through a solid, liquid, or gaseous material via molecular contact. Example: Touching a hot stove. To reduce conduction in windows, add frame thermal barriers.
• Convection is the transfer of heat through the movement of liquids or gases. Example: Facing into a cold north wind. To reduce convection in windows, add enclosed air spaces.
• Radiation is the transfer of heat through space without relying on an intervening medium. Example: Facing into the heat of the sun. To reduce radiation in windows, add low-E glass coatings.

Unsightly residential insulated glazing spacer systems are often the basis of other misleading U-Factor claims. Many of these spacer systems cannot be structural silicone glazed or used on large lites. Stainless steel, warm-edge spacers-while marginally lower in thermal performance than non-metallic spacers-are a better choice, since they are similar in breadth of application and warranty to conventional, mill-finished, aluminum spacers. Some manufacturers even turn windows “inside out,” with removable glazing beads placed to the exterior, to make U-Factors look better.

U-Factors are reduced significantly by using triple-insulating glass, with corresponding improvements in condensation resistance. Keep in mind that triple glazing requires hardware packages designed to carry the additional weight of these heavier units.

No discussion of fenestration U-Factors would be complete without emphasizing the importance of balanced design. U-Factor is just one aspect of overall energy performance-air infiltration, SHGC, visible light transmittance, condensation resistance and durability also must be optimized. When all design requirements are considered in balance, aluminum remains the framing material of choice for non-residential applications. Involving the manufacturer early in the design process will help achieve the intended balance of performance, aesthetic and energy-saving results.

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Steve Gille is the education market manager at Wausau Window and Wall Systems, Wausau, Wis. For more information, visit www.wausauwindow.com[1].

Source URL: https://www.metalarchitecture.com/articles/understanding-u-factors/

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