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Interior Curtain Retrofit Saves 16%

William Opalka

ThermoliteAnnual energy savings by an average of 16 percent have been achieved in a recommissioned building using a RetroWAL interior curtain retrofit system, Thermolite says.

Thermolite installed supplemental windows onto the west-facing offices on the 20th floor of an office building in Chicago, which had already undergone retro-commissioning that addressed heating, lighting, and building envelope improvements.

The following Thermolite interior window retrofit systems were added and monitored for three weeks to measure their energy performance:

  1. A ¼-inch laminated glass with low-e hard coating (interior side)
  2. A 1-inch insulated glazing unit with low-e soft coating
  3. A ¼-inch laminated glass with low-e hard coating (interior side) and – inch blinds in the air cavity.

A 2014 case study of the building immediately showed apparent temperature control improvements after three interior window retrofit systems were added, even though the Chicago building was already retrofitted for superior energy efficiency compared to its counterparts.

Temperature fluctuation was reduced to approximate set points of 68 degrees and 76 degrees compared to the previous range of between 63 degrees and 92 degrees. The installation of supplemental windows in the building affects the heat transfer processes through the building’s envelope and their contribution to heat gains and losses in perimeter thermal zones.

Thermolite’s interior curtain wall retrofit window system reduced thermal conduction through glazing (due to higher R-value), through glazing into interior space (lower SHGC), and by marginally reducing air infiltration.

Other key findings from the case study include:

  • 13-16 percent decrease in peak energy usage
  • 33-37 percent decrease in annual gas consumption
  • 5 Db reduction in sound level intensity


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