Improving Home Energy Performance With Super Insulating Windows

Improving Home Energy Performance With Super Insulating Windows

Understanding the ConceptsSuper-insulating windows can help reduce energy bills in two ways. They can reduce heating and cooling by reducing the amount of heat conducted through the windows as well as the amount of heat leaked around the window termed "air infiltration".The ability of a window to block heat from going through it is described by its U-factor or the inverse metric, R-value. R-value stands for Thermal Resistance, and it is commonly used to describe the thermal performance of insulation materials like fiberglass and foam. As with insulation, the higher the R-value, the better the window is at keeping heat in during the winter and out during the summer. A typical dual pane window with low-e coating has an R-value of approximately 3.0. A high-performance, energy-efficient window can have an R-value of approximately 4.8 to 11.Independent of R-value, the ability of a window to retain heat by reducing air leakage through gaps in or around the window is described by its Air Infiltration and is typically measured in cfm/ft (cubic feet of air per minute/square footage of the window). The lower the Air Infiltration, the less air can leak in or out. A common replacement window has an air infiltration value of approximately 0.3 cfm/ft2. A high performance window can have an air infiltration value of as low as 0.01 cfm/ft2, allowing approximately 95% less air to pass through and around the window. The Air Infiltration performance of any National Fenestration Rating Council (NFRC) rated window appears on the NFRC label. Modeling the DifferenceIn order to quantify the incremental energy savings realized by improving R-value and lowering air infiltration of windows, an engineering team at a top manufacturer of energy efficient windows, modeled the natural gas consumption for heating a multi-family residential building. The first scenario modeled an average, existing residential building with single-pane, low R-value, high air infiltration windows. In the second scenario, the model represents the same building but with high R-value windows (still with high air infiltration). Finally, the third scenario modeled the building with high R-value, low infiltration windows. Each of these scenarios was modeled in three different heating-dominated climates: New York, Chicago and Pittsburgh. The results of the model show that in each of the three cities, upgrading from low R-value to high R-value saved 26% on natural gas consumption. With the further improvement of low air infiltration, the savings increased to 36-38% compared to the baseline scenario. Thus the improvement in air infiltration performance generated an additional 10-12% savings on top of the savings generated by improving the insulation value of the windows. SummaryEnergy-efficient windows can reduce energy bills in two complimentary ways: by improving insulation and reducing air infiltration. Based on the models constructed to simulate this effect, increasing the R-value of a window in an existing multi-family home can reduce energy consumption for heating by 26% in heating-dominated climates. By reducing air infiltration around those same windows, an additional 10-12% of heating energy can be saved, amounting to a total savings of 36-38% energy savings.

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