Storm window guide · Cold climates
Low-E storm windows in Iowa, Minnesota and Wisconsin
In the cold climate zones that cover these three states, Department of Energy modeling found low-E storm windows cut heating and cooling energy by an average of 36% over old single-pane windows and 19% over double-pane wood windows. The colder the winter, the more energy that saves.
Savings by climate zone
Average heating and cooling energy savings from adding a low-E storm window or interior panel, from DOE-funded modeling of two model homes in 22 U.S. cities (PNNL-24826, 2015, Table 3). The figures average both homes, every city in the zone, and exterior storm windows and interior panels together.
| Climate zone | Example city modeled | Over single-pane wood window | Over double-pane wood window |
|---|---|---|---|
| Zone 5 | Chicago | 36% | 19% |
| Zone 6 | Minneapolis | 36% | 19% |
| Zone 7 | Duluth | 36% | 19% |
Savings are a share of the home's heating and cooling energy (source energy, not counting hot water or appliances), including the drafts the storm window stops. In the Chicago, Minneapolis and Duluth results, an exterior low-E storm window on the smaller, older model home saved about 24% to 26% over single-pane windows, and the larger, newer home saved about 43% to 46%. Your house, windows and fuel will move these numbers.
Where your state falls
Iowa, Minnesota and Wisconsin span climate zones 5 through 7, getting colder as you go north. Des Moines is zone 5, Minneapolis zone 6 and Duluth zone 7. The IECC climate zone map shows your county.
The research found the percentage saved was the same across zones 5 to 7, while the energy saved per square foot of window grows as winters get colder.
Why Low-E matters in the cold
In a heating climate, much of a window's heat loss is heat radiating out through the glass. A Low-E coating reflects that heat back into the room while letting light through. The PNNL report recommends regular low-E glass in zones 4 through 8, and says that if you are putting up a storm window anyway, it should be low-E.
ENERGY STAR estimates that certified low-E storm windows over single-pane clear-glass windows save about $350 a year, about 10% of heating and cooling bills, on a national average (ENERGY STAR). That figure is lower than the PNNL percentages because it averages warmer climates and measures bills, not energy.
What it costs to add
On our 30 × 60 Deluxe 240, Low-E adds about $57.17 over clear glass (November 2026). See prices by size.
Questions
Are Low-E storm windows worth it in Iowa, Minnesota or Wisconsin?
For older single-pane windows, yes. Department of Energy modeling found low-E storm windows over single-pane windows cut heating and cooling energy by an average of 36% in climate zones 5, 6 and 7, which cover these three states. Over double-pane wood windows the average was 19%, with a much longer payback, so they make the most sense there in the colder parts of the region.
Is Low-E better than clear glass in a storm window in a cold climate?
Yes. The same research recommends regular low-E glass in climate zones 4 through 8, and says that if you are putting up a storm window anyway, it should be low-E. How fast the extra cost pays back depends on the price of the upgrade and your heating bill.
What climate zone am I in?
Iowa, Minnesota and Wisconsin span climate zones 5 through 7, getting colder as you go north. Des Moines is zone 5, Minneapolis zone 6 and Duluth zone 7. The IECC climate zone map shows your county.
Do storm windows cause condensation?
A little frost or fog on the storm glass on the coldest days can happen. Heavy or lasting condensation usually means warm, moist room air is leaking past the inside window, so keep the inside window as tight as you can.
Add Low-E to your order.
Low-E glass is available on our storm doors and storm windows.