Low-E Exterior Storm Windows

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. Coatings can also reduce the amount of solar heat gain, reducing cooling loads in summer. 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). 

For these reasons, the U.S. Environmental Protection Agency (EPA) now recognizes low-E storm windows in its ENERGY STAR® Program (Figure 2). Information on ENERGY STAR certified storm window products may be found on the ENERGY STAR Certified Storm Windows webpage.  

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.

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). 
• Installed storm windows are removable, an important consideration for historic preservation projects (Figure 6).
• Low-E storms can significantly improve the energy performance of the home by reducing air infiltration, thermal conductance, and solar heat gain 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.

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 an 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 storm window 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. 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.

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.
• Dry 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.

How to Install Exterior Low-E Storm Windows

1. Determine the ENERGY STAR climate zone of the house.
2. Select a storm window that is ENERGY STAR rated for your climate zone. The ENERGY STAR Product Finder can filter rated products by climate zone and other criteria.
3. Inspect and repair or replace original window, sills, and/or weather stripping, if damaged or missing.
4. 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.

5. 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."