Secondary windows—also known as internal storm windows, insulating panels, or secondary glazing systems—are a cost-effective, high-performance alternative to full window replacement for commercial buildings with old, inefficient windows. Secondary windows simply attach to the interior or exterior of an existing (i.e., primary) window for quick installation, resulting in improved occupant comfort, health, and wellness—while reducing heating and cooling energy use by up to 20 percent. Secondary windows achieve about the same performance as replacing windows with new high-performance models, but for as little as half the cost (depending on the product and application).

Secondary windows create an insulating pocket of air between the existing window and the secondary window to significantly reduce air leakage, outdoor air infiltration, and heat transfer. This results in fewer drafts, reduced outside noise, and significant energy savings. The addition of a low-e coating provides increased benefits by allowing daylight through, while reducing unwanted exterior heat gain and interior heat loss—which reduces glare and the fabric damage caused by UV light.

With little-to-no occupant disruption, secondary windows can be installed during or after normal operating hours in as little as 20 minutes per window. Installation takes place inside, eliminating the need for intrusive scaffolding or large equipment during installation. Fully customizable, secondary window products make it easy to match the existing window aesthetic and retain the building’s current or desired look.

Secondary window products work best in existing buildings with single- or double-paned windows without low-e coating or tint. In addition to helping bring these older buildings up to more modern levels of energy performance and comfort, secondary windows do not impact the building’s aesthetic—a big benefit for historic buildings. Secondary windows enhance tenant comfort and reduce noise infiltration from outside, making them especially effective for settings that place a premium on tenant comfort and noise reduction, including  offices, hotels, and hospitals.

Combining the installation of secondary windows with other upgrades is a smart approach to maximizing energy savings and reducing project costs. It is best to sequence upgrades such that the quality and condition of the building’s envelope is the first consideration. By improving window performance (and air-leakage control), you can reduce the building’s HVAC load, which may present an opportunity to downsize the building’s heating and cooling equipment upgrade or replacement. Secondary window products with low-e coating can also impact lighting needs by reducing glare, thereby increasing the use of daylighting over blinds.

The Attachments Energy Rating Council (AERC), an independent, non-profit organization, offers a list of certified secondary windows.

AERC compiles this list by testing and certifying the energy performance of secondary window products. Their rigorous certification process includes product simulations, plant inspections, air-leakage testing, and individual product verification.