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By: Bill O’Brien

July 28, 2020

Windows are a common building envelope component that cause significant thermal energy (heating/cooling) loss in buildings. During cooler months, interior heat can be transferred through the glass, especially through windows that are not well insulated. This can lead to discomfort on the interior spaces that require adjustments to the thermostat to compensate for thermal energy losses; this type of energy loss leads to increased utility bills. According to the article, Window Film: A Cost Effective Window Retrofit, by Jeffrey R. Plummer, “between 10% and 15% of the energy [used in an average sized home] is lost through windows, and windows can be responsible for up to 25% of a heating bill. Nearly 75% of existing windows are not energy efficient.” Selecting a window system with substantial insulating qualities is crucial to provide comfort for residents and tenants in a climate controlled environment, especially where there are four seasons.

There are two key terms that the construction industry uses to describe the energy efficiency of a given material: R-value and U-value. Choosing the right window system for a building starts with understanding these two terms and their relationship to a window’s overall energy efficiency. R-value represents resistance to the flow of heat; the higher the R-value, the greater the resistance and the insulating value. U-value represents the amount of heat that escapes through a material. The lower the U-value, the slower the rate of heat flow and the better the insulating quality. Both of these values are determined by The National Fenestration Rating Council (NFRC); however, they only list the U-value rating on a window label. A good U-value for windows in cold climates is between 0.17 and 0.39, which is between R-6 and R-2.5. U-value in a hot climate is optimal between 0.17 and 0.30.

The NFRC takes a number of variables into consideration when rating these units, such as window glazing, frame type, distance between the glass and insulation, and the number of layers of glass and the vacuum quality of the space between glass panes. They are determined by a series of tests performed in a controlled test laboratory. Once the window meets the NFRC’s standard rating, it receives a verification of energy code compliance. The NFRC window label displays the energy properties of the whole window system and appears on all fenestration products that are part of the ENERGY STAR Program. Double-hung windows used to be the preferred window system by architects and engineers due their high water infiltration ratings. However, many major manufacturers have discontinued their double-hung models because they often cannot achieve the proper U-value ratings. Thanks to improvements in technology and years of research, there are a myriad of choices in glass, coatings, and window frames available for selection. The options for these window system can provide enhancements for the optimal window system that is tailored to each project.

The aforementioned, insulation, is a key component to a window system and can either cost or save a building owner a lot of money. Below are a few of the vital factors that should be considered when selecting the right insulation for a window system.

  • Coating:A coating is often installed over a window glass, which is an important factor in determining U-value. Windows with more substantial coating, called low-emittance or low-e coating, have lower emissivity, which is the unit’s ability to radiate heat through itself. Low e-coating can contribute to a significant reduction in U-value.
  • Gas Fills: Double and triple-pane windows are made with an inert gas fill between each pane. Since gas is a low conductor of energy, it slows down the passage of heat and prevents it from seeping out. Gas filled windows provide a better U-value, promote soundproofing, can be used in all climates, and do not corrode the window material like oxygen. Argon gas was previously used for this type of window, but krypton gas, xenon, nitrogen, or a combination of them, can also be used. Since each compound has pros and cons, it is important that the window system with gas fill is best suited for the application.
  • Framing Materials:Heat loss through the window frame can be significant. Frames are made from an amalgamation of materials, including: steel, aluminum, wood, fiberglass, and vinyl. Some of these materials retain heat more efficiently than others. For instance, standard steel and aluminum frames have higher U-values compared to wood or fiberglass frames, because aluminum transfers heat more rapidly; a wood or fiberglass frame may only half a window’s U-value when installed on a double-pane window. Additionally, many wood and vinyl frames do not provide sufficient structural support, as required in many of high-rise buildings in NYC. This is also the case with the fireproof capabilities of the windows. The building code for a commercial application and larger residential structures will require steel or aluminum frames. It is important to pair windowpanes with the proper frames in order to ensure code compliance and to obtain optimal energy efficiency.
  • Window Spacers:Windows that contain more than one pane are separated by spacers. Spacers are typically aluminum but can made from a variety of substances and improve the window’s ability to retain heat around the perimeter. The more space between the panes of glass, the more heat that can be retained in between the panes.
  • Size and Thickness of the Glass:The number of glass layers in a window system is important. Double and triple-pane windows prevent heat loss more effectively than single-pane windows. However, if the glass is thicker or there are multiple glass panes, the additional weight may not be supported by the structural frame or hardware, which would also have to be updated. Glass thickness plays a big role in reaching the desired U-value, but also contributes to noise cancellation from the exterior of the building to the interior. The thickness of window glass can range from a few millimeters to over 10 millimeters. In some cases, larger coated-windows with gas fills have better U-values than smaller windows with thick glass; it all depends on the combination of window component features.

Whether you are replacing old window units or installing windows in a new building, the options for window systems are abundant. A qualified building enclosure consultant or window specialist should be able to provide additional guidance to ensure optimal energy efficiency within your building or home. Although the upfront cost may be higher, there is savings in energy (heating/cooling) bills when the window units are efficient; this is in comparison to utility costs that are ever increasing due to rising energy rates.

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