Storm Windows: Interior vs Exterior, Low-E Storms and What They Cost

A storm window is a second window fitted to the inside or outside of an existing one ⁴. It does not replace the window; it adds a layer of glass or plastic and a pocket of still air, and it seals some of the leaks around the old sash. ENERGY STAR describes storm windows as an affordable option for homes where full window replacement may be difficult ¹.

Storm windows are still made and sold, and ENERGY STAR has a specification for them, in effect since September 5, 2018 ¹. The modern version is often a low-E storm: a storm panel with a heat-reflecting coating. The U.S. Department of Energy's Building America Solution Center says low-E storms are about 50% more energy efficient than traditional uncoated storm windows ². They make the most sense over single-pane or older double-pane windows that are still in good shape.

Exterior vs interior storm windows

Exterior storm windows are installed on the outside of your primary windows ¹. They also shield the old window from rain and sun. Angi notes that modern storm windows have movable sashes that work much like a regular window, where older fixed styles were swapped in and out each season ⁶.

Interior storm windows and panels are installed on the inside of your primary windows ¹. They leave the outside look of the house unchanged, which can matter on an older facade. DOE's Building America Solution Center calls them a cost-effective way to upgrade existing windows, with improved comfort and sound control among the benefits ². Angi notes that they leave the existing window itself unprotected on the outside ⁶.

What low-E storms add

A plain storm window adds a layer of glass and still air. A low-E storm adds a coating that reflects heat back toward its source, the same idea as low-E glass in a new window (see low-E glass explained). Both plastic and glass storm products reduce air leakage ², and Building America notes that low-E storm windows cost about the same as standard glass storms ².

EPA estimates that, on a national average, ENERGY STAR low-E storm windows can save homeowners $350 a year on heating and cooling, about 10%, when installed over single-pane clear glass windows, and about $50 a year (about 2%) when they replace clear glass storms ¹. A 2015 DOE article put the reduction in heating and cooling costs at 10% to 35% when a low-E storm is installed over a low-performing window ³. These are averages from EPA and DOE; your result depends on climate, the window underneath and how well the storm seals.

To qualify for the ENERGY STAR label, storm windows must meet emissivity, solar transmittance and air leakage requirements set by climate zone ¹. Building America also suggests looking for products rated by the Attachments Energy Rating Council (AERC) ².

When a storm window makes more sense than replacement

A storm window is worth considering when:

• The existing windows are sound. The sash operates, the frame is not rotting, and the glass is intact. A storm cannot fix a failing window.
• You want to keep original windows, for example in an older house where the original sash is part of the character.
• The windows are single-pane or older double-pane units. That is where DOE's and EPA's estimates come from ¹³.
• Budget is the constraint. In the 2015 DOE article, a consultant to the project said low-E storms cost about a quarter of what full replacement costs and bring similar energy savings ³. That is one person's estimate, not a survey.

If the windows are rotting, fogged between the panes or will not open, start with repair or replace windows instead.

What storm windows cost

The two surveys measure different things:

Fixr prices a set of eight standard storm windows, installed. Angi gives per-window figures by frame material and reports about $55 per square foot for exterior storms and about $24 per square foot for interior storms ⁶. Angi's page treats storm windows partly as protection from severe weather, with tempered glass, hurricane film and hurricane shutters among the options it lists, so its figures may include heavier-duty products than a standard storm ⁶. For low-E storm units, Fixr lists $125 to $400 per window before labor ⁵.

For comparison, HomeAdvisor reports that a full replacement window costs $300 to $2,100, with an average of $850 (updated June 17, 2026) ⁷. For whole-house replacement prices, see window replacement cost.

Downsides of storm windows

• Another layer to open, clean and maintain. Operable storms have their own tracks and screens.
• Condensation between layers. An interior storm reduces condensation on the room side, but if it is not airtight, moisture can collect between the storm and the original window. Building America suggests sealing the storm well and reducing indoor moisture ². More in window condensation.
• Looks. Exterior storms change how the window reads from the street; interior storms show from inside.
• No new features. A storm does not make an old window easier to open, add a tilt-in sash or fix a rotted sill.

Questions to ask before you buy

• Is the storm low-E, and does it carry the ENERGY STAR label for your climate zone ¹?
• Is it interior or exterior, fixed or operable, and how is it sealed to the existing frame?
• Who measures and installs it, and is the installer licensed?
• What does a comparable replacement window cost for the same opening?

You can compare quotes from window installers for both storm windows and replacement windows. For the numbers on a window label, see energy-efficient windows explained.

About these figures

• ¹ Official record: ENERGY STAR storm windows page: interior and exterior definitions, specification effective September 5, 2018, criteria by climate zone, and EPA's national-average estimates of $350 (about 10%) over single-pane clear glass and an additional $50 (about 2%) over clear storms, checked October 22, 2026.
• ² Official record: DOE Building America Solution Center guide on interior storm windows: low-E storms about 50% more efficient than uncoated storms, similar cost to standard glass storms, air leakage, comfort and sound, and condensation risk, checked October 22, 2026.
• ³ Official record: DOE article (September 30, 2015) reporting 10% to 35% lower heating and cooling costs over low-performing windows, and a consultant's estimate that low-E storms cost about a quarter of full replacement, checked October 22, 2026.
• ⁴ Official record: NFRC glossary definition of storm windows, checked October 22, 2026.
• ⁵ Reported: Fixr $2,090 average for 8 storm windows installed ($1,505 to $2,737), unit and labor ranges, and low-E storm units at $125 to $400, updated August 29, 2025, checked October 22, 2026.
• ⁶ Reported: Angi per-window ranges by frame material ($90 to $400), labor of $30 to $200 per window, $55 and $24 per square foot for exterior and interior storms, movable sashes, and its framing of storm windows as severe-weather protection, updated October 2, 2026, checked October 22, 2026.
• ⁷ Reported: HomeAdvisor replacement window range of $300 to $2,100 with an $850 average, updated June 17, 2026, checked October 22, 2026.