Storm Windows: Exterior vs Interior and Energy Savings

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Older homes in Denver, Boulder, Colorado Springs and Fort Collins often still have their original single-pane wood windows, many of them handsome, well-built and difficult to replace without changing the character of the house. Storm windows offer a middle path. Added to the outside or inside of an existing window, storm windows create a second layer of glass and a still air space that cuts drafts and heat loss, often at a fraction of the cost of full replacement. Modern low-e storm windows go further, and ENERGY STAR now certifies them. This guide explains exterior and interior options, what the ENERGY STAR criteria and savings estimates actually say, why weep holes matter, what condensation between the panes means, how lead paint rules affect work on old sashes, and what storm windows typically cost compared with alternatives.

Part of our Structure & Exterior guide — start there for the full picture →

What Storm Windows Are and How They Work

A storm window is a secondary window mounted in front of or behind an existing primary window. It does not replace the original; it adds to it. The air space between the storm and the primary window slows heat transfer, and the storm itself blocks much of the air leakage that makes old windows drafty.

Older aluminum “triple-track” storm windows are familiar on many mid-century Front Range homes. They hold two glass sashes and a screen in separate tracks so the owner can slide glass up or down for ventilation. Newer products include fixed panels, operable units that match the primary window, and low-e glass or plastic glazing.

ENERGY STAR describes storm windows as attachments to existing primary windows, distinct from certified replacement windows, which have a full frame and at least two layers of glass. Its buying guidance says storm windows work best when installed over working primary windows that are in good condition, and that if existing windows are rotted, not operating properly or damaged beyond reasonable repair, low-e storm windows may not be appropriate and replacement should be considered instead.

There are side benefits as well. ENERGY STAR notes that adding storm windows can be an inexpensive way to reduce outside noise, and that low-e storms can suit neighborhoods with historic preservation requirements, provided local rules allow them.

Exterior vs Interior Storm Windows

Both types add a layer of glazing, but they differ in appearance, maintenance, ventilation and moisture behavior.

Exterior storm windows

Exterior storms mount to the outside of the window casing or blind stop. They protect the primary sash from weather, hail and UV, which can extend the life of old wood windows and their glazing putty. Triple-track and fixed exterior units are common, usually with aluminum frames that can be painted or ordered in colors to blend with trim.

Drawbacks include changing the look of the house from the street, the need for ladders on upper floors to clean or service them, and the requirement for proper drainage at the bottom, covered below. Historic districts may also restrict how exterior storms look.

Interior storm windows

Interior storms fit inside the window opening on the room side. Many use acrylic or polycarbonate glazing in a frame with compression seals, so they press into the opening and can be removed seasonally. They leave the exterior appearance untouched, are easy to install and remove from inside, and can be very effective at stopping drafts.

Interior units need careful fit, and they can make it harder to open the primary window. Because they sit on the warm side, a tight interior storm generally helps reduce condensation on the primary window, while a leaky one can let humid indoor air reach the cold primary glass.

Operability and emergency escape

ENERGY STAR notes that both interior and exterior storm windows come in operable and fixed models, and advises choosing a storm window that matches the operability of the current window. That is especially important in bedrooms and other locations where emergency escape and rescue openings may be needed or required by code. A fixed storm panel over a bedroom egress window can create a serious safety problem.

Low-E Storm Windows and ENERGY STAR Criteria

ENERGY STAR certified storm windows use low-emissivity (low-e) glass. ENERGY STAR describes low-e glass as having a microscopically thin, almost invisible coating that reduces heat loss through the glass when it is cold outside. Some low-e glass also has a low solar transmittance rating that blocks incoming solar heat and can help keep a house cooler in summer.

The program’s Key Product Criteria set requirements by climate zone. For all four zones, emissivity must be 0.22 or lower. Solar transmittance requirements differ:

• Northern zone: solar transmittance greater than 0.55, letting more winter sun through.
• North-Central zone: any solar transmittance.
• South-Central and Southern zones: solar transmittance of 0.55 or lower.

The criteria also set air leakage limits: 1.5 cfm per square foot or less for exterior storm windows and 0.5 cfm per square foot or less for interior storm windows. ENERGY STAR says every certified storm window carries a label with a map showing the part of the country the product is designed for, so checking that map for a specific Colorado address is the simplest way to choose the right product.

What ENERGY STAR says about savings

ENERGY STAR’s storm windows page states that EPA estimates, on a national average, ENERGY STAR certified (low-e) storm windows can save homeowners $350 a year on heating and cooling bills, about 10 percent, when installed over single-pane clear glass windows without existing storm windows. It estimates the incremental cost can be paid back in about three years. For homes that already have clear glass storms over single-pane windows, it estimates replacing them with certified low-e storms saves an additional $50 a year, about 2 percent.

The page describes these figures as conservative national averages that exclude air-leakage savings, and says actual savings depend on local climate, home characteristics and utility rates. ENERGY STAR also notes that low-e storms offer the most savings potential over single-pane windows without low-e glass; savings over clear double-pane windows exist but come with a longer payback.

Other ENERGY STAR buying points

• Metal-framed primary windows: ENERGY STAR calls it very important to use thermal breaks when installing storm windows over metal-framed primaries, since aluminum storm frames can conduct heat and cold.
• Tinted windows: adding low-e storms over tinted glass may darken the view further.
• Safety glass: state or local codes may require it in some locations, such as near stairs.
• Installation: certified products must include or link to detailed installation instructions, and poor installation can reduce savings.

Weep Holes on Exterior Storm Windows

Exterior storm windows must let water out. Rain, melting snow and condensation can collect in the space between the storm and the primary window, and if that water cannot drain, it sits against the wooden sill and lower sash, where it can cause peeling paint and rot. For this reason, exterior storm frames have small drainage openings, called weep holes or weep slots, along the bottom rail.

ENERGY STAR treats this as a requirement: its specification excludes from certification exterior storm windows without weep holes or other features that allow moisture to drain from between the storm and primary window. Homeowners and installers sometimes caulk the entire perimeter of an exterior storm, including the bottom, to stop drafts. That seals in moisture. The usual approach is to seal the sides and top while leaving the bottom edge and weep openings clear.

During a walk-through, look at the bottom rail of each exterior storm. Weep holes clogged with paint, caulk, dirt or insect debris, peeling paint on the sill below, and soft wood at the lower corners of the primary sash are all signs of trapped water. The exterior window caulking guide covers where sealant belongs and where it does not.

Condensation Between the Storm and the Primary Window

Fog, frost or water droplets on the inside surface of an exterior storm window in winter usually means warm, humid indoor air is leaking past the primary window into the cold space between the panes. The primary window is the “warm side” and should be the tighter of the two. If the storm is tighter than the primary, moisture gets trapped.

Common fixes include weatherstripping and repairing the primary sash, lowering indoor humidity during cold snaps, and making sure the storm’s weep holes are open. Condensation on the inside of the primary glass, facing the room, is a different issue tied mainly to indoor humidity and the surface temperature of the glass.

This is different from fogging inside a sealed insulated glass unit in a modern double-pane window, which indicates a failed seal. That problem cannot be fixed by adjusting a storm window; the double-pane window repair guide explains the options.

Lead Paint on Old Sashes

Many homes old enough to need storm windows were built before 1978, when lead-based paint was banned for residential use. The U.S. Environmental Protection Agency says approximately three-quarters of U.S. homes built before 1978 still contain some lead-based paint, and that old or worn lead paint on friction surfaces such as doors and window sills can produce chips and dust that become a hazard when inhaled or ingested.

Installing storm windows can involve scraping, sanding, cutting or drilling painted casings and sills. EPA’s Renovation, Repair and Painting (RRP) Rule requires that anyone paid to perform work that disturbs painted surfaces in homes, childcare facilities and preschools built before 1978 be certified and use lead-safe work practices. The rule generally does not apply to homeowners working on their own homes, but it does apply to those who rent out all or part of the home, operate a childcare center in it, or buy, renovate and sell homes for profit. EPA recommends that homeowners planning such work hire a lead-safe certified contractor. Colorado does not appear on EPA’s list of states authorized to run their own RRP program, so EPA administers the program there.

Before hiring, ask whether the installer is an EPA lead-safe certified firm and how they will contain and clean up dust around painted window components.

Installing and Maintaining Storm Windows

Fit drives performance. Old window openings are rarely square, especially in houses that have settled for a century, so careful measurement at several points across the width and height matters. Many manufacturers recommend measuring the opening in three places and ordering to the smallest dimension, then using expanders or seals to close the gaps. Custom-made units cost more but usually seal better in out-of-square openings.

A typical exterior installation looks something like this:

• Inspect and repair the primary window first: loose glazing putty, failing paint, sash cords and rotted sills.
• Clean and prepare the blind stop or casing where the storm frame will sit.
• Set the storm frame and fasten it per the manufacturer’s instructions.
• Seal the sides and top while leaving the bottom and weep openings free to drain.
• Check that operable sashes and screens slide smoothly and lock.

Interior units are simpler. Compression-fit panels press into the jamb; magnetic or track-mounted panels attach to a thin frame screwed to the casing. Labeling each panel by room and window makes seasonal removal and reinstallation much easier.

Ongoing maintenance is modest but worth doing. Once or twice a year:

• Vacuum and wipe out tracks on triple-track units so sashes slide freely.
• Clear weep holes of dirt, insects and paint.
• Check gaskets and seals on interior panels for compression set or tears.
• Look for cracked glass or acrylic, especially after hail on exterior units.
• Watch for peeling paint or soft wood on sills, which can signal trapped moisture.

Acrylic and polycarbonate glazing scratches more easily than glass, so manufacturers usually recommend soft cloths and mild cleaners rather than paper towels or ammonia-based products. Following the specific product’s care guidance protects both clarity and any warranty.

Storm Windows vs Other Window Upgrades

Storm windows are one tool among several. Depending on the condition of the existing windows, budget and goals, alternatives include:

• Weatherstripping and air sealing. Often the cheapest first step. The drafty windows guide walks through finding and fixing leaks.
• Window film. Low-e or solar control film applied to existing glass can reduce heat gain and glare; see the window film guide.
• Full replacement. When frames are rotted or windows do not operate, replacement with efficient units may make more sense. The energy-efficient windows guide explains ratings, and the window replacement cost guide covers pricing.

Doors have a parallel option: a storm door adds a second layer at entries and protects the primary door from weather. The storm door installation guide covers types and fitting.

Typical Costs and What Inspectors Check

Prices vary with size, glazing, frame material, operability, custom sizing and installation. As rough 2026 Front Range estimates rather than quotes:

• Interior acrylic or polycarbonate inserts: often around $100 to $400 per window, depending on size and seal design.
• Standard exterior aluminum storms, installed: commonly about $200 to $500 per window.
• Low-e or custom-fit units, installed: frequently $350 to $800 or more per window.

Lead-safe work practices, repairs to sills or sashes, upper-story access and historic-district approvals can raise costs. Some utilities offer rebates on ENERGY STAR products, and the ENERGY STAR Rebate Finder can show local programs.

Home inspectors typically inspect a representative number of doors and windows under the American Society of Home Inspectors (ASHI) Standard of Practice. The same standard says inspectors are not required to inspect screening, shutters, awnings and similar seasonal accessories, or the coatings on and hermetic seals between panes of window glass. Storm windows may be noted but not evaluated in detail, so asking about scope helps. The guide to hiring a home inspector explains what to ask, and the structure and exterior hub collects related guides on windows, doors and exterior trim.

References

• Storm Windows — ENERGY STAR (U.S. EPA)
• Storm Windows Key Product Criteria — ENERGY STAR (U.S. EPA)
• Lead Renovation, Repair and Painting Program — U.S. Environmental Protection Agency
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Weighing storm windows against replacement on an older home you are buying? Send us a note and we can connect you with an inspector familiar with older Front Range windows.