Cost Analysis: Low-E Storm Windows
High-performance storm windows are a lower-cost alternative to window replacement and can save significant amounts of energy.
WINDOWS ARE A MAJOR SOURCE OF heating losses and gains in the building and can significantly drive up energy bills. Window replacement is not an option for many homeowners, whether the cost is too high or because they want to keep the historical integrity of their homes. Low-E storm windows can provide a cost-effective alternative to replacement; in particular, for moderate and low-income households. DOE has supported technology development, market assessment and early deployment of low-E storm windows through a variety of research, including climate-based modeling and field lab tests.
Over 40 percent of existing homes have single-pane windows. In addition, almost half of all the double-pane windows are not high-performing low-E windows, but are made of clear glass.
The PNNL Lab Homes Test
Building on previous research, the Pacific Northwest National Laboratory (PNNL) recently tested low-E storms on manufactured homes to determine the energy savings and payback periods. Side-by-side tests were conducted in two Lab Homes built in Richland, Washington. These manufactured homes were provided by Marlette, a manufactured housing manufacturer in Hermiston, Oregon. They were specified so that they would be representative of 1970s-era housing stock in the Pacific Northwest region. For example, insulation levels for floors, walls and ceilings are R-22, R-11 and R-22, respectively. The windows are double-pane, clear glass with aluminum frames. The performance of these baseline windows was compared to the performance of those same windows with exterior low-E storm windows attached, and to highly insulated windows retrofitted into the lab home.
Three Ways Low-E Storms Improve Windows
- They provide air sealing of the primary window, for up to 10 percent reduction in air leakage of the entire home.
- A low-E storm window creates dead air space, which reduces conduction and convective heat losses across the primary window.
- Low-E glass reflects radiant heat back into the home.
Performance measures. Windows with low-E storms showed significant reduction in the U-factor and solar heat gain coefficient (SHGC). The significantly lower SHGC for the highly insulated window would not pay off in the Northwest.
Whole-house energy savings. The homes with low-E storms saw an average whole-house savings of 10 percent, compared to 12 percent for triple-pane primary windows. The low-E storm windows saw better savings in the heating season than the summer. The opposite was true for the highly insulated windows, because of their low solar heat gain coefficient.
Air leakage performance. Manufactured homes are generally airtight, since factory environment is more controlled, compared to stick-framed homes built onsite. The lab homes did not see a significant benefit from decreased air leakage; this would probably not be the case in the field.
Low-E storm windows and interior panels installed over all single-pane windows, as well as all double-pane metal frame windows with clear glass, are cost effective in climate zones 3 through 8. In climate zone 3, solar control low-E storm windows are recommended.
Weep holes. For some experiments, weep holes at the bottom edge of the storm windows were sealed to see if this improved air leakage performance; results were not significant. Weep holes are designed to prevent air leakage into that dead air space, because they are only at the bottom and don’t have an escape at the top.
Peak energy use. During the heating season, the majority of savings occurred on sunny days at night, when it was the coldest outside. In summer, the greatest savings occurred on hot, sunny afternoons. This coincides with peak power periods for utilities experience, indicating that low-E storms can potentially decrease peak power use in the summer.
Cost and payback. The total installed cost of low-E storms ranged between $1,500 and $2,000, for a simple payback period of five to seven years. In comparison, R-5 windows showed greater annual savings, but with a payback of over 20 years.
Installing Low-E Storms
Window or general contractors can easily install low-E storm windows; the only tools needed are caulk and caulking gun, screw gun and measuring tape.
Storm windows, such as the interior unit shown here, can be custom ordered from window distributors and some big box retailers. Some common sizes are available off the shelf.
- Measure existing window opening. Exterior storm windows can be installed in two different configurations: an overlap installation or a blind stop configuration, in which the storm fits into the window opening. For a slider or any horizontal opening, measure the top, middle and bottom; use the smallest measurement to ensure the window will fit into the opening.
- Dry fit the window by holding the exterior window up to the window opening and checking that the screw holes all land on solid wood. Make sure that the storm window and the primary window open in the same direction.
- Caulk around the opening, then put the window back in place and screw it in. Caulk around the top and sides of the opening, but not around the bottom, as exterior storm windows have weep holes designed into the bottom. These help drain any condensation that occurs between the primary and exterior windows.
- Blind stop installation is recommended for interior storms. Typically, a trim piece is installed between the primary window and the interior storm window to ensure a good thermal break and sufficient air gap between the two windows. Make sure that the low-E coating is facing the right side. (The side with the low-E coating feels squeaky.)
- Caulk around the entire opening, as interior storms function as the primary air barrier. Interior low-E storm windows don’t have weep holes.
Presenters: Tom Culp, Owner, Birch Point Consulting, LLC, and Sarah Widder, Engineer, Pacific Northwest National Laboratory (PNNL)