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Secondary Windows

High-Performance Windows at a Fraction of the Cost

Secondary windows—also known as interior 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 reduce heating and cooling energy use by up to 20 percent and cost 30 to 50 percent less than window replacement (depending on the product and application).


How It Works

Secondary windows attach to the interior or exterior of an existing (i.e., primary) window, resulting in improved occupant comfort, health and wellness.

Secondary windows work by creating an insulating pocket of air between the existing window and new secondary window that significantly reduces 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 results in reduced glare and fabric deterioration from UV light. 

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The following studies provide additional information on secondary window performance: 1) The Minnesota Department of Commerce's Division of Energy Resources white paper on the potential for energy savings in residential and commercial buildings, and 2) The Ernest Orlando Lawrence Berkeley National Laboratory's study analyzing and validating the thermal, moisture and solar performance of secondary windows. 

The Right Application

Secondary windows work best when paired with the following existing window conditions: clear, single- or double-paned windows; windows without low-e coating or tint; and existing frames in reasonable condition. While secondary windows improve performance, they do not impact the building’s aesthetic—an important consideration for many historic buildings. There are several product options available with most being highly customizable, including for color, size and window type. They have proven especially effective for offices, hotels, schools and hospitals, for which occupant comfort and noise reduction are essential. 

Combining the installation of secondary windows with other upgrades can maximize energy savings and reduce project costs. For sequenced building upgrades, the quality and condition of the building’s envelope should always be the first consideration. For instance, improved window performance can reduce a building’s HVAC load, which may present an opportunity to downsize the building’s heating and cooling equipment when replacement is needed. Secondary windows with low-e coating can also impact lighting needs by reducing glare, thereby increasing the use of daylighting over blinds.

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Secondary Window Features & Benefits

Saves energy and money

  • 30-50 percent less than the cost of full window replacement
  • Achieves the same performance as replacing windows with new high-performance models
  • Reduces a building’s peak demand, saving an average of 20% on combined heating and cooling energy use
  • Updates an older building to current code performance standard


Improves tenant comfort and wellness

  • Improves tenant comfort by reducing air leakage, heat transfer, and outside noise
  • Improves occupant health by decreasing outdoor-air infiltration
  • Improves tenant wellness by reducing glare and increasing access to natural daylighting


Quick and easy installation

  • Little to no occupant disruption, secondary windows can be installed during or after normal operating hours
  • No demolition or abatement needed
  • Installs in as little as 20 minutes per window
  • Installs from the interior eliminating the need for intrusive scaffolding or large equipment
  • Creates a tight seal which eliminates the need for cleaning (for inoperable products); easy to open or remove for cleaning (for operable products)


Seamlessly matches existing window aesthetic

  • Matches the existing window aesthetic and retains the building’s current or desired look
  • Matches current window functionality with operable and fixed options


Nationally Tested & Certified

The Attachments Energy Rating Council (AERC) is an independent, non-profit organization whose mission is to test and certify the energy performance of secondary window products to help homeowners, architects, and builders make more informed decisions. This rigorous certification process includes product simulations, plant inspections, air leakage testing and individual product verification.

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Secondary Window Manufacturers Now Servicing the Northwest

Click a manufacturer logo below to learn more about available products: