American Architectural Manufacturers Association

Energy Considerations for Windows

As the global community grows, the need for energy also increases. In a recent presentation at the AAMA Southeast Region Fall Meeting, Jeff Sonne, Senior Research Engineer from the Florida Solar Energy Center, discussed the approaching peak production of oil, new and innovative energy solutions and various developments in energy considerations for the future.

Sonne began his presentation with statistics on specific building and cooling loads. Of particular interest was the topic of heat flow through windows, given that it comprises the largest percentage of cooling load for a given residence. A window absorbs solar radiation as it is conducted through the frame. Natural sunlight and heat are directly transmitted as solar radiation through the glazing of the window, and then is reflected as solar radiation back through the opposite side. The glazing absorbs the solar radiant heat and directs it throughout the entire window frame; an inward and outward flowing fraction of glazing becomes conversely absorbed as radiation. Finally, heat is conducted through both the glass and the frame, contributing to the overall window heat flow and the solar heat gain coefficient (SHGC).

Sonne indicated that the SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. This measurement is expressed as a number between zero and one; the lower a window’s solar heat gain coefficient, the less heat that it transmits. According to the Efficient Windows Collaborative, the nationally recognized rating method by the National Fenestration Rating Council (NFRC) is stated for the whole window, including the effects of the frame.

Sonne also distinguished Visible Transmittance (VT), described as an optical property that indicates the amount of visible light transmitted. The NFRC’s VT includes the whole window rating and the impact of the frame, which does not transmit any visible light. Similar to SHGC, the higher the VT, the greater light that is transmitted through the window, thereby maximizing the daylight and overall view.

Sonne also referenced his book published in 2007, titled Effectiveness of Florida’s Residential Energy Codes, which discusses Florida Building Codes and Standards and the annual energy use in kilowatts per year for cooling, heating, hot water and all other sources. Through a comparison of the 1979 Codes and standards to those of 2007, the energy use for cooling decreased from approximately 14,000 to less than 5,000 respectively. Possibly attributing to this variance is the change in energy code Executive Order 07-127 which aimed to “convene the Florida Building Commission for the purpose of revising the Florida Energy Code for Building Construction to increase the energy performance of new construction in Florida by at least 15 percent.” The goal, therefore, was to consider incorporating standards for appliances and standard lighting in the Florida Energy Code.

Another legislative change in energy is HB 7135, related to environment and natural resources, requiring that “the 2009 IECC become the foundation code for the FEC and that the 2010 FEC achieve a 20 percent increase in new home energy efficiency relative to the 2007 FEC.” Other items included 405.3 Performance Based Compliance based on simulated energy performance requirements, and the Florida Building Energy Rating System (BERS), which incorporates appliances and lights, in addition to space heating, cooling and water heating.

For more information on energy considerations, visit the Florida Solar Energy Center and their publications and the Database of State Incentives for Renewables & Efficiency. 

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