Lessons in U-Factors
Energy efficiency and occupant comfort are two key criteria in designing green buildings, especially for projects seeking LEED® certification. Contributing to these goals, thermal performance will continue to be an essential factor in the upcoming International Green Construction Code (IgCC) and the 2012 International Energy Conservation Code (IECC), as well as the Architecture 2030 Challenge, which calls for the operation of all new buildings and major renovations to be carbon neutral by 2030.
According to Architecture 2030, the non-residential/or commercial building sector is currently responsible for almost half of the energy – consumption (49 percent) and green house gas emissions (47 percent) in the United States. The greatest percentage of the energy consumption and their associated emissions is attributed to buildings’ operations, such as heating, cooling and lighting.
Windows in these buildings are major contributors to the nation’s gross energy consumption. Energy-efficient building designs must comply with codes and meet owner expectations. Design professionals are concerned with misrepresentation of energy savings. Understanding thermal transmittance in window systems will help to correctly report energy savings.
U-Factor is a measure of:
• Conduction – heat transfer through a solid, liquid or gaseous material via molecular contact. For example, touching a hot stove. To reduce conduction in windows, add frame thermal barriers.
• Convection – the transfer of heat through the movement of liquids or gases. For example, facing into a cold, north wind. To reduce convection in windows, add enclosed air spaces.
• Radiation – the transfer of heat through space without relying on an intervening medium. For example, the heat of the sun on your face. To reduce radiation in windows, add low-E glass coatings.
U-Factor = BTUs / Square Foot / °F Differential / Hour
BTUs are British Thermal Units, the approximate heat required to raise 1 pound of water 1 degree Fahrenheit, such as from 590F to 600F.
U-Factor allows the HVAC engineer to calculate peak loads, as well as energy consumption, for any size window, in any climate. When comparing U-Factors, remember that lower is better. For example, 0.18 BTU/ft2-hr-ºF is better than 0.34 BTU/ft2-hr-ºF.
For an apples-to-apples comparison of commercial window systems’ U-Factors, be aware that there are two U.S. thermal testing protocols: American Architectural Manufacturers Association (AAMA) and National Fenestration Ratings Council (NFRC). The two programs yield similar, but not identical, thermal performance results. NFRC U-Factors are typically 10 percent lower than AAMA U-Factors. NFRC was residentially oriented, so NFRC operable window test sizes are small. Window area and configuration can significantly affect the overall window assembly U-Factor. Smaller test sizes make a huge difference in U-Factor for aluminum windows.
Aluminum remains the framing material of choice for non-residential applications, when all design requirements are considered in balance. Be certain to consider ALL factors – from structural integrity to longevity to stiffness to heat build-up – before deciding on alternative materials.
The three components used to calculate U-Factor for window systems are:
• Center of Glass (COG) – typical value is 0.29 BTU/ft2-hr-ºF (low-E IG)
• Edge of Glass (EOG) – typical value is 0.34 BTU/ft2-hr-ºF (aluminum spacer)
• Frame – typical value is 0.90 BTU/ft2-hr-ºF (thermal break). A frame’s U-Factor includes heat transfer through surfaces perpendicular to the glass plane.
COG is different than “whole window” U-Factor. Whole window U-Factor, which includes frame and EOG areas, usually is required by codes. To calculate total system U-Factor:
• First, we must determine what percentage of glass versus aluminum there is in the opening. Most storefront and curtainwall elevations have between 85 and 90 percent glass.
• Second, we need to know what the COG is. This information is obtained from the glass fabricator.
In the following graph, assuming 85 percent glass and a glass COG of 0.24, the total system U-Factor will be 0.45.
In addition to noting total system U-Factor versus COG, be cautious not to confuse U-Factor with R-Value. R-Value is the inverse of U-Factor and commonly used in residential applications. For instance, a U-Factor of 0.50 would equate to an R-value of 2.0. If commercial fenestration can get down to a U-Factor of 0.25, that is still only R-4. Equal to about 1 inch of fiberglass insulation.
The commercial glazing industry is faced with some real challenges in meeting the 2012 IECC. Most of the U.S. Climate Zones will require Total System U-Factors of 0.38 or lower. These new U-Factor requirements cannot be met by a standard, thermally broken, aluminum-framed storefront system, even one with a glass COG of 0.24. Some manufacturers are responding to this new code by developing new technology, including “double” pour and debridge thermal breaks. (Thermal barriers will be the topic of my next blog.)
In addition to reviewing Climate Zones’ criteria, remember to check that the specifications for U-Factor match the glass and glazing specifications, code requirements and building permit values. Clear specifications and accurate data are essential to optimizing window systems’ thermal performance and to achieving the intended results.