Low-E Exterior Storm Windows

    Scope Images
    Image
    With modern low-E storm windows, manufacturers offer many frame colors and the option for custom color matching
    Scope

    When installing exterior storm windows,

    • Choose models that are certified by ENERGY STAR.
    • Choose models that are certified by the Attachments Energy Rating Council (AERC).
    • Install according to manufacturers’ instructions.

    See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home programENERGY STAR Certified Homes, and Indoor airPLUS.

    Description

    Modern storm windows include new energy-efficient technology and sleek design features that provide an easy-to-install, cost-effective method for upgrading the energy efficiency of existing windows, while offering additional benefits such as improved thermal comfort and better sound control (Figure 1). Unlike our grandmother’s old storm windows that had to be removed, stored, and replaced seasonally, modern storm windows are designed to be permanently installed and remain in place year-round. They come in both operable and fixed versions. Traditionally, storm windows have been single glazed with clear glass, but in recent years, many models have become available with low-emissivity (low-E) coatings, which significantly improve the thermal performance of the window. Low-E glass has a durable transparent silver coating that reduces energy loss through the window by reflecting the radiant heat back into the home. Tight-fitting modern storm windows also provide energy savings by helping to reduce the overall air leakage of the window. Low-E storm windows cost about the same as standard storm windows but are about 50% more energy efficient than traditional uncoated storm windows (Cort 2013). 

     

    Modern low-E exterior storm windows blend in with the original window and provide a year-round increased comfort and energy efficiency
    Figure 1. Modern low-E exterior storm windows blend in with the original window and provide a year-round increased comfort and energy efficiency (Source: Larson).

     

    For these reasons, the U.S. Environmental Protection Agency (EPA) now recognizes low-E storm windows in its ENERGY STAR® Program (Figure 2). ENERGY STAR certified storm window products may be found at on the ENERGY STAR Certified Storm Windows webpage.  https://www.energystar.gov/productfinder/product/certified-storm-windows/results.  

    The Attachments Energy Rating Council (AERC) (Figure 3) also provides information, certified ratings, and labeling for energy-efficient storm windows and other window attachments. See the Attachments Energy Rating Council website.

     

    The ENERGY STAR logo indicates products that have been verified to meet energy efficiency and performance criteria.
    Figure 2.  ENERGY STAR now offers low-E storm windows as a certified product category (Source: ENERGY STAR).

     

    The Attachments Energy Rating Council (AERC) certifies storm windows.
    Figure 3. The Attachments Energy Rating Council (AERC) now certifies storm windows for energy efficiency (Source: AERC 2021).

     

    Benefits of Installing Exterior Low-E Storm Windows

    Exterior low-E storm windows provide many benefits include the following:

    • Interior appearance of the window is maintained, although the exterior appearance will change as the framing will appear slightly wider (Figure 1).
    • The addition of exterior storm windows is typically acceptable for most historic preservation projects (Figure 4). 
    • Many frame colors are available, including the option for custom color matching (Figure 5). 
    • Installation is reversible, an important consideration for historic preservation projects (Figure 6).
    • Low-E storms can significantly improve the energy performance of the home by reducing both air infiltration and thermal conductance through the assemblies (Figure 7).
    • Low-E storm windows will reduce the potential for interior condensation on windows. There may be some risk of interstitial condensation between the original window and the exterior storm (but this is generally a temporary aesthetic concern, as opposed to a durability concern).
    • The cost is low to moderate depending on the system chosen.

     

    The addition of low-E permanent exterior storm windows is typically accepted for most historic preservation projects
    Figure 4. The addition of low-E permanent exterior storm windows is typically accepted for most historic preservation projects (Source: Larson). 

     

    With modern low-E storm windows, manufacturers offer many frame colors and the option for custom color matching
    Figure 5. With modern low-E storm windows, manufacturers offer many frame colors and the option for custom color matching (Source: Larson). 

     

    Modern Low-E storm windows are typically kept up year-round but can be removed without damage to the existing window frame, an important consideration for historic preservation projects
    Figure 6. Modern Low-E storm windows are typically kept up year-round but can be removed without damage to the existing window frame, an important consideration for historic preservation projects (Source: Larson). 

     

    Modern low-E storm windows can significantly improve the energy performance of the home by reducing both air infiltration and thermal conductance through the window assemblies
    Figure 7. Modern low-E storm windows can significantly improve the energy performance of the home by reducing both air infiltration and thermal conductance through the window assemblies (Source: Larson). 

     

    Installation of Low-E Storm Windows

    Installing an exterior low-E storm window is a simple project and the labor, time, cost, and waste is considerably reduced compared to a full window replacement.  If the existing window is in good shape, the installation of exterior low-e storm window is quick and easy, requiring only simple tools and less than about 20 minutes of time (Figure 8). It can be a do-it-yourself task, although contractor installation may be advised, especially for upper-story installations.  The existing windowsill must be in good condition, as it will continue to be exposed to the elements (although exposure will be significantly reduced). 

    The exterior storm window is fastened to the outer window casing, or in some cases directly to the window trim. The storms should be sealed with an exterior-grade sealant at the jambs and heads, but left unsealed at the sill, then screwed into place in accordance with the manufacturer’s instructions. Installation can be completed in less than 20 minutes.  Most storm windows have an adjustable bottom leg to account for variations in the sill height.  Exterior storm windows include small weep holes or notches in the bottom leg (Figure 9); these should not be caulked as they are included to provide drainage.  Windows with weep drainage openings are recommended and are required for ENERGY STAR® certified products.

     

    Installation of permanent exterior low-E storm windows is simple and easy, requiring a few basic tools and about 20 minutes of labor
    Figure 8. Installation of permanent exterior low-E storm windows is simple and easy, requiring a few basic tools and about 20 minutes of labor (Source: Larson).

     

    Permanent exterior low-E storm windows include weep channels in the bottom leg of the frame to allow for drainage.
    Figure 9. Permanent exterior low-E storm windows include weep channels in the bottom leg of the frame to allow for drainage (Source: Larson).

     

    Reducing the Potential for Condensation

    In order to minimize the potential for interstitial condensation, the original window must be made as air tight as possible (Wilson 1960).  Slight ventilation of the exterior storm is typically provided by the weep holes provided at the sill.  If the interior window is made sufficiently airtight, then the slight ventilation of the weep holes of the storm window should provide adequate air change to prevent condensation while also maintaining the improved thermal performance. 

    If condensation does appear on the storm windows, take the following steps:

    • Make sure the weep holes are clear.
    • Slightly increase the openings to allow for additional ventilation.

    If condensation persists as an ongoing problem, consider the following additional steps to control indoor sources of humidity:

    • Add timers or humidity sensors to bath exhaust fans and advise occupants to run exhaust fans when showering or cooking.
    • Recommend drying clothes outside or in the dryer; hang drying clothes indoors adds a considerable amount of moisture to the indoor air.
    • Ensure that the exhaust fans and clothes dryer duct outside, not into the attic or crawlspace.
    • Verify that the crawlspace floor is covered with a continuous vapor barrier that covers the entire floor and is secured to the walls and that the sump pump basin is covered with a tight-fitting lid. Or, address any basement moisture issues.
    • Run a dehumidifier if needed.

     

    Wintertime condensation potential for exterior storm windows can be reduced if the original window is airtight and weep holes in the storm window are open to provide a small amount of ventilation to the outside
    Figure 10.  Wintertime condensation potential for exterior storm windows can be reduced if the original window is airtight and weep holes in the storm window are open to provide a small amount of ventilation to the outside (Source: Building Science Corporation).

     

    How to Install Exterior Low-E Storm Windows

    1. Inspect and repair or replace original window, sills, weather stripping, if damaged or missing.
    2. Follow manufacturer’s installation instructions, typically:
      • Ensure proper size and fit in accordance with the manufacturer’s measurement guidelines.
      • Match current opening style of window to ensure operability and egress if desired or required.
      • Apply exterior-grade caulk or sealant to the existing window frame where the sides and top of the storm window will be mounted. Do NOT caulk the bottom, in order to not block the weep holes and drainage gap in the new storm windows.
      • Mount and screw the storm window into place.

    3. If the low-E storm window is being installed onto a metal-framed existing window, care should be taken to include a “thermal break” between the storm window and existing window frames.   This is simply a nonmetal separation such as a wood stop, rubber gasket, or even an air separation to minimize direct metal-to-metal contact and thermal conductance.

    For more information, see the video “Interior and Exterior Low-E Storm Window Installation."

     

    Ensuring Success

    Prior to considering exterior low-E storm windows, examine the condition of the existing windows. Replace or repair broken panes, broken hardware, and missing weather strippping. If the existing window shows excessive deterioration or frame rot, then full window replacement may be required. Additionally, the condition of the windowsill is critical to water management and to avoid water intrusion. Cracked or rotting sills need to be replaced prior to any installations being done.

    To reduce the potential for interstitial condensation, the ability for warm moist air to travel from the interior to the exterior should be minimized by making the existing window as air tight as possible. Examine seals and weatherstripping around the window perimeter and sashes for deterioration, and replace or repair as needed. 

    Also when selecting low-E storm windows, choose configurations and operator types (vertical slider, horizontal slider, fixed) that match the original windows (e.g., fixed over fixed, vertical slider over vertical slider, etc.) if the ability to open the windows is desired or required for egress by code.

    Climate

    Cold Climates

    Historically, storm windows have been used in colder northern climates, but energy savings have also been demonstrated across most of the United States, including more moderate regions with a mix of heating and cooling needs. The ENERGY STAR® program for Exterior and Interior Storm Windows certifies low-E storm windows for both northern and southern regions (Figures 1 and 2), where the primary difference is the type of low-E glass. All low-E glass is more insulating than normal glass, but different options are available. Most low-E storm windows use clear low-E glass with a high solar transmittance that does not block sunlight. This is particularly beneficial in central and northern regions, when the sun’s warmth is allowed to help offset the heating needs of the home. For southern regions or other applications where cooling is of more concern than heating, the ENERGY STAR® program requires the low-E glass to have a lower solar transmittance. This solar control low-E is still insulating, but also blocks more of the sun to reduce the cooling needs in the home. It should be noted that both types of low-E glass save energy in all regions, but choosing the type in accordance with ENERGY STAR® guidelines for your region will optimize energy performance.

    The Attachments Energy Rating Council (AERC) also provides information and certified ratings on the energy performance of storm windows in both cool and warm climates (heating dominated and cooling dominated). This allows the consumer to select the product performance most beneficial to them, based on both geography and their specific circumstances (See Figure 3). 

     

    ENERGY STAR offers this label for certified low-E storm windows in cool climates
    Figure 1.  ENERGY STAR offers this label for certified low-E storm windows in cool climates (Source: ENERGY STAR).

     

    ENERGY STAR offers this label for certified low-E storm windows in warm climates
    Figure 2.  ENERGY STAR offers this label for certified low-E storm windows in warm climates (Source: ENERGY STAR).

     

    The Attachments Energy Rating Council (AERC) uses this label to indicate certified low-E storm window products.
    Figure 3.  The Attachments Energy Rating Council (AERC) uses this label to indicate certified low-E storm window products. (Source: AERC 2021).

     

    There are also climate differences regarding the potential for interstitial condensation to form between the storm window and the existing window. As noted on the Description tab, interstitial condensation is generally more of a temporary aesthetic concern rather than a durability or performance concern. It only forms under certain conditions when warm moist air from the interior of the home leaks through the existing window and reaches a cold storm window surface. Making the existing window as air tight a possible, as well as reducing any sources of excess moisture and humidity in the home will reduce the potential for formation of condensation. Condensation is unlikely to form in warmer regions and summer seasons because the temperature is warmer and above the dew point. In cold regions, condensation or ice is less likely to form during the deep winter season because humidity levels in winter air are typically very low, unless there are moisture issues in the home. Condensation formation is more often seen in the shoulder seasons of fall and spring when the outside temperature is cool but there is still sufficient moisture in the air. This condition is usually temporary and resolves on its own as the day warms, especially if there is sun exposure on the window. 

    Right and Wrong Images
    Image
    Exterior low-e storm windows were added to historic Halligan Hall on the U.S. Naval Academy campus in Annapolis, Maryland; frames were custom colored to match the existing seafoam green window trim
    Exterior low-e storm windows were added to historic Halligan Hall on the U.S. Naval Academy campus in Annapolis, Maryland; frames were custom colored to match the existing seafoam green window trim
    Exterior low-e storm windows were added to historic Halligan Hall on the U.S. Naval Academy campus in Annapolis, Maryland; frames were custom colored to match the existing seafoam green window trim
    Image
    ENERGY STAR offers this label for certified low-E storm windows in cool climates
    ENERGY STAR offers this label for certified low-E storm windows in cool climates
    ENERGY STAR offers this label for certified low-E storm windows in cool climates
    Image
    ENERGY STAR offers this label for certified low-E storm windows in warm climates
    ENERGY STAR offers this label for certified low-E storm windows in warm climates
    ENERGY STAR offers this label for certified low-E storm windows in warm climates
    Presentations
    Videos
    CAD
    CAD Files
    Window sill at wood frame wall
    Window sill at wood frame wall
    Window sill at wood frame wall
    Download:DWGPDF
    Window head at window frame wall
    Window head at window frame wall
    Window head at window frame wall
    Download:DWGPDF

    Compliance

    The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    ENERGY STAR Certified Homes

    The ENERGY STAR Reference Design Home is the set of efficiency features modeled to determine the ENERGY STAR ERI Target for each home pursuing certification. The requirements for the Reference Design Home, including the fenestration (windows) requirements, are listed in Exhibit 1 of the National Program Requirements document. They are not mandatory,  but can be traded off with other measures to achieve the ENERGY STAR ERI Target.

     

    DOE Zero Energy Ready Home (Revision 07)

    Exhibit 1 Mandatory Requirements.
    Exhibit 1, Item 2) Fenestration shall meet or exceed ENERGY STAR requirements.

     

    2009-2021 IECC and IRC Window U-Factor Requirements Table

    The maximum U-Factor and Solar Heat Gain Coefficient (SHGC) requirements for fenestration (windows) and skylights in new homes, as listed in the 2009, 2012, 2015, 2018, and 2021 IECC and IRC, can be found in this table.

     

    Retrofit: 2009201220152018, and 2021 International Energy Conservation Code (IECC)

    Section R101.4.3 (Section R501.1.1 in 2015, 2018, and 2021 IECC) additions, alterations, renovations, or repairs shall conform to the provisions of this code... Exception: The following need not comply provided the energy use of the building is not increased: 1. Storm windows installed over existing fenestration. (See code for additional requirements and exceptions.)

     

    Retrofit: 2009201220152018, and 2021 International Residential Code (IRC)

    Section N1101.3 (Section N1107.1.1 in 2015 and 2018, N1109.1 in 2021 IRC) additions, alterations, renovations, or repairs shall conform to the provisions of this code... Exception: The following need not comply provided the energy use of the building is not increased: 1. Storm windows installed over existing fenestration.  (See code for additional requirements and exceptions.)

    Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

     

    Existing Homes

    Storm windows are typically installed by the homeowner as a retrofit measure. The information provided in this guide on the Scope and Description tabs applies to existing as well as new homes. As noted elsewhere, inspect and repair or replace windows or broken panes, hardware, sill, trim, weather stripping etc., if needed, before installing the storm windows.

    More Info.

    Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    References and Resources*
    Author(s)
    Attachments Energy Rating Council
    Organization(s)
    AERC
    Publication Date
    Description
    Website developed by the Attachments Energy Rating Council to provide consumers with credible, relevant, and comparable information about window attachments and their performance.
    Author(s)
    LBNL
    Organization(s)
    Lawrence Berkeley National Laboratory,
    Building Green,
    DOE
    Description
    Website sponsored by DOE to provide consumers with intelligent and unbiased guidance on the best window covering for your climate, your needs, and your windows.
    Author(s)
    Knox,
    Widder
    Organization(s)
    Pacific Northwest National Laboratory,
    PNNL
    Publication Date
    Description
    Report describing a research study into the performance of Low-E storm windows installed in two test homes at Pacific Northwest National Laboratory in Washington state.
    Author(s)
    Baker
    Organization(s)
    Building Science Corporation,
    National Renewable Energy Laboratory
    Publication Date
    Description
    Document providing information and guidance about rehabilitating, retrofitting, and replacing wood window assemblies in residential construction.
    Author(s)
    Wilson
    Organization(s)
    National Research Council Canada
    Publication Date
    Description
    Document about condensation between the panes, on the inside surface of the outer glass of double pane windows.
    Author(s)
    Brown
    Organization(s)
    National Research Council Canada
    Publication Date
    Description
    Research study describing an evaluation of selected windows undertaken by IRC researchers at Ottawa’s Laurier House (now being used as a museum) to determine their effectiveness in controlling condensation.
    *For non-dated media, such as websites, the date listed is the date accessed.
    Contributors to this Guide

    The following authors and organizations contributed to the content in this Guide.

    Last Updated

    Mobile Field Kit

    The Building America Field Kit allows you to save items to your profile for review or use on-site.

    Sign Up  or  Log In

    Did you find this information helpful?