Low-E and behold: Low-E storm windows provide a new way to solve the window conundrum

The Problem

Infrared images of buildings show that windows are often one of the largest heating and cooling sinks, or “holes” in the thermal envelope. Single-pane windows are the most egregious offenders, particularly in colder climates such as the Vermont climate. In the Northeast and Midwest, single-pane windows represent just 30 percent of all installed windows. However, they account for more than 50 percent of all heat loss from windows and are responsible for cumulative heat flows of 0.53 quadrillion Btu.

How Storm Windows Work

Storm windows create an insulating airspace that reduces both conductive and convective heat loss. The addition of a durable, transparent, low-emissivity (Low-E) coating to the glass reduces radiative heat loss, while permitting solar radiation to pass through the glass. This combination of attributes creates passive solar gain, which is beneficial in colder climates such as Vermont’s. Storm windows also reduce air infiltration and leakage through and around the existing windows.

Low-E storm windows have undergone the most field testing of all window attachment types. Field test results indicate that adding Low-E storm windows can generate significant energy savings for single-family and multifamily homes. Further, energy modeling suggests that a Low-E storm window upgrade also offers a relatively short payback period when installed over clear-glass, double-pane, metal frame windows in International Energy Conservation Code (IECC) climate zones four through eight.

The Pilot

Efficiency Vermont, a statewide energy efficiency utility, funded a research and development (R&D) pilot in 2015 to explore the respective and combined roles of greater consumer awareness of Low-E storm windows and the elimination of the incremental cost price barrier on purchase activity. Efficiency Vermont did not claim energy savings for this pilot, although the Evaluation, Measurement, and Verification (EM&V) team is now, as a result of this pilot, developing a TRM for Low-E storm windows. Efficiency Vermont collaborated with Larson Manufacturing, a storm window and door manufacturer, and D+R International, Ltd., an energy efficiency consulting firm, to design and implement the pilot.

Pilot Results

The markdown of the price of Low-E storm windows to or near to the price of clear-glass storm windows was highly successful in converting sales of clear-glass storm windows to Low-E storm windows.

During the pilot, overall storm window sales increased by more than 37 percent, and sales of Low-E storm windows increased by 337 percent, shifting the Low-E market share at participating retailers from 22 percent (2014) to 70 percent (2015). Retail locations that sell both stock and custom order Low-E storm windows sold, on average, 10 times more windows than locations that sell only custom windows.

The pilot was also effective in increasing the overall volume of storm windows sold. The pilot’s marketing messaging centered on Low-E storm windows as a lower-cost alternative to full window replacement and stopping drafts from old windows. These messages resonated with customers. Additionally, surveys revealed that customers had installed the Low-E storm windows themselves without problems, so future marketing efforts can emphasize that storm window installation can be a do-it-yourself project.

Lastly, this pilot tested a program with thermal savings that could reach low- to moderate-income customers in new, more cost-effective ways. If Efficiency Vermont moves forward with an expansion to a full program for Low-E storm windows, it will need to ensure broad coverage via retailer networks, including independent retailers, and full geographic coverage to reach all Vermonters. Please see the full report for additional information and recommendations regarding marketing, training, outreach, etc

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