Description

Storm windows are exterior windows added to cover and protect existing windows from weather and improve energy performance. High-performance, low-emissivity (low-e) exterior storm windows often have better seals, smoother long-term operation, and significantly better energy performance. Low-e coatings increase thermal insulation, which significantly reduces heat loss from the home’s interior during cold weather. Low-e coatings may also modestly reduce interior temperature increases from sunlight (solar heat gain). The lower the emissivity of the low-e coating, the better the energy performance of the storm window. The low-e coatings used on storm windows are highly durable and barely visible but give some texture to the window’s interior surface.

There are three basic types of exterior storm windows: triple-track (two operable glass panels and an operable screen), double-track (an operable lower panel and an operable screen), and fixed (one non-operable glass panel). More than 90% of storm windows are aluminum-framed because of the economy and durability of this design. Wood and vinyl storm windows are also available. Each type can be made with low-e glass panels. All modern low-e storm windows are more airtight than old-fashioned storm windows. Fixed panels are the most airtight, and double-tracks are more airtight than triple-tracks. Fixed-panel exterior storms may need to be removed seasonally to get full benefit when employed on operable windows.

Overall Thermal Performance

Exterior low-e storm windows, when properly installed and deployed, bring the overall thermal performance (airtightness and thermal insulation) of the window close to that of a new double-paned low-e window. Most current low-e storm windows are high solar heat gain, best suited for climates where the need for heating dominates. Low solar heat gain, low-e storm windows suited for climates where the need for cooling dominates are becoming available.

Key Benefits

• Significantly increase air tightness/insulating properties of window assembly
• Offer convenient, easy operation of storm, sash, and screen (unless fixed)
• Protect existing windows (less maintenance)
• Improve comfort near window

Key Drawbacks

• May conflict with historical codes or condominium regulations that prohibit changing windows’ exterior appearance
• Interfere with existing window operation (outswing casement and awning windows)
• Seasonal installation and removal (fixed panel exterior storms)

Aesthetics

• Low-profile and higher-quality storm windows possibly considered more attractive/acceptable than standard triple-track storm windows
• Higher-quality storm windows often available in variety of colors
• Interior storm panels possibly preferable if maintaining exterior appearance is important

Tips/Cautions

• Look for a specific term warranty (e.g. 5-year) or transferable lifetime warranty.
• Unless venting is required at the top of the window, choose double-track storm windows with just one operable glass sash.
• If living in a condominium or historic district, check with authorities before proceeding.
• Follow manufacturer’s cleaning instructions. Low-e coatings are tougher than plain glass, but some coatings may not be completely smooth, so a lint-free cloth is recommended.
• Keep weepholes open at sill to reduce condensation. Keeping the most airtight layer to the interior of the entire window assembly reduces condensation potential.
• Check with manufacturer for actual emissivity value of the low-e storm window. The lower the value, the better the insulation properties.
• Consider how storm windows will affect egress.
• Use caution in combining exterior low-e storms with double-pane windows that have spectrally-selective low-e or tinted surfaces because the combination may produce temperatures high enough to damage the insulated glazing unit.