Storm Windows vs. Replacement Windows: How to Decide

A low-e storm window added over a sound, working window cuts heating and cooling energy without removing anything. When the existing window is rotted, broken or no longer operates, replacement is the better fit. ENERGY STAR puts the saving from certified storm windows added over single-pane clear glass at about 10 percent of annual heating and cooling bills on its storm window page and about 20 percent on its Home Upgrade page, and a national laboratory study modeled savings of about a third of heating and cooling energy over single-pane windows in colder climate zones.

What a storm window is

A storm window is a second window attached to an existing one, either on the outside (exterior storm) or on the room side (interior storm or panel). The original window stays in place and keeps working.

ENERGY STAR’s storm window specification, Version 1.0, took effect on September 5, 2018. To qualify, a storm window has to meet requirements for emissivity, solar transmittance and air leakage that vary by climate zone.

Low-e glass. Certified storm windows use low-emissivity glass, which carries a thin, nearly invisible, durable coating that reduces heat loss through the glass in cold weather.
Climate-matched labels. Each certified product carries a label with a map showing the part of the country it is designed for, and ENERGY STAR says certified storm windows are designed to let through the right amount of solar heat for that climate.
Drainage. Exterior storm windows without weep holes, or another way to drain moisture from between the two windows, cannot be certified.

A new window, by contrast, replaces the whole unit. ENERGY STAR describes certified primary windows as having a full frame and at least two layers of glass.

What the savings research shows

Both options save energy, and the size of the saving depends on the windows already in place and on the climate. The figures below come from different studies with different baselines, so they are not a head-to-head comparison.

ENERGY STAR’s estimate for storm windows: about 10 percent off annual heating and cooling bills, on a national average, when certified low-e storm windows go over single-pane clear glass that has no storm window. ENERGY STAR calls the estimate conservative because it leaves out savings from reduced air leakage. Its Home Upgrade page gives about 20 percent for the same case.
Swapping old storms: replacing existing clear-glass storm windows with low-e ones saves about 2 percent more, by the same estimate.
ENERGY STAR’s estimate for new windows: upgrading to certified windows lowers household energy bills by an average of 12 percent.
The laboratory model: a 2015 Pacific Northwest National Laboratory study for the Department of Energy modeled low-e storm windows in 22 cities across all eight climate zones. Over single-pane wood-framed windows in climate zones 4 through 8, average source-energy savings for heating and cooling ran from 30 to 36 percent. Over double-pane wood- or vinyl-framed windows in zones 6 through 8, they ran from 16 to 19 percent.
Where the savings come from: in the same study, reduced air leakage accounted for roughly a quarter to a third of the savings over single-pane windows.

The pattern is consistent: the poorer the window underneath, the larger the gain from adding a storm window. The study’s figures are simulations, and ENERGY STAR notes that actual savings depend on local climate, the house and utility rates.

When a storm window is the better fit

A storm window fits when the existing windows are sound and still operate, and especially when they are single-pane. ENERGY STAR’s buying guidance lists the conditions:

The existing windows are in good condition. Storm windows are meant to go over working primary windows.
The existing windows are single-pane, or double-pane with clear glass. The savings potential is higher over single-pane glass without low-e. Over double-pane clear glass, ENERGY STAR says to expect a longer payback period.
The house is historic, or sits in a neighborhood with preservation requirements. Check first that those requirements allow storm windows.
The street is noisy. ENERGY STAR notes that adding storm windows can make a home somewhat quieter.

The National Park Service makes a similar case for older wood windows. Its Preservation Brief 9 calls exterior storm windows thermally efficient, reversible, and a way to keep the original windows. It adds that a historic wood window with a high-quality storm window should thermally outperform a new double-glazed metal window that lacks thermal breaks.

When replacement is the better fit

Replacement is the better fit when the existing windows are rotted, damaged beyond reasonable repair, painted shut or leaking air at worn sills and sashes, and in very hot climates where a storm window could trap heat. ENERGY STAR’s guidance in detail:

Rot or damage. If the current windows are rotted, do not operate properly, or are damaged beyond reasonable repair, it says storm windows may not be appropriate and suggests considering replacement with certified windows instead.
Painted-shut or drafty windows. Its list of reasons to replace includes windows that do not open or are painted shut, and windows with air leaks or degraded sills and sashes.
Very hot climates. In places where temperatures often exceed 115°F, low-e storm windows over windows that are already double-pane or low-e can trap heat between the layers. ENERGY STAR points to awnings, solar screens or replacement with low solar heat gain windows instead.

Three points to check either way

Whichever option you choose, check escape openings, moisture between layers and metal frames.

Escape openings. A bedroom or basement window may be the room’s required emergency escape opening under the building code (Section R319 of the 2024 International Residential Code). ENERGY STAR advises choosing a storm window that matches how the main window opens, fixed or operable, especially in bedrooms.

Interior storms and moisture. The Park Service’s Preservation Brief 9 warns that moisture trapped between an interior storm and the main window can condense on the colder outer window and damage it. Its approach is to seal the interior storm tightly while leaving some ventilation around the main window, which it notes is difficult to achieve in practice.

Metal frames. Storm windows over metal-framed windows need thermal breaks, small non-metal separators. Without them, ENERGY STAR notes, the storm window’s aluminum frame conducts heat and cold and reduces the benefit.

When to bring in a professional, and what to ask

ENERGY STAR notes that storm windows installed incorrectly may not deliver all of the possible energy savings, and that professional installers can install them correctly and point out local building code requirements. Every certified storm window must come with detailed installation instructions, or a link to them. For historic windows, the Park Service’s guidance generally ties any decision to replace to documented condition.

Questions for an installer or window contractor:

Is the storm window ENERGY STAR certified, and does its label cover my climate zone?
Interior or exterior, fixed or operable? Will bedroom windows still work as escape openings?
Does an exterior unit have weep holes to drain moisture?
My windows have metal frames: how will the storm window be thermally separated from them?
Is any glass in a location where the code requires safety glazing (Section R324), such as near stairs?
Before quoting replacement: which of my existing windows are rotted or not operating, and which could take a storm window instead?

A window-by-window answer to that last question can produce a mixed plan, with storm windows on sound units and replacement where the frame has failed.