Lightweight Secondary Windows

Compared to window replacements, secondary windows are a cost-effective and efficient way to improve thermal performance and occupant comfort and reduce noise.

Researchers from the National Renewable Energy Laboratory found 15% average whole-building savings compared to single-pane windows. View full-size infographic. [PDF - 323 KB]

GPG Findings 049, January 2022, Lightweight Secondary Windows. Opportunity: How much window energy use could high-performance windows save? 75% OF THE ENERGY LOST THROUGH WINDOWS COULD BE REDUCED WITH HIGH-PERFORMANCE WINDOWS. How do lightweight secondary windows work? THIN GLASS IN INSULATED FIBERGLASS FRAME. PRE-MANUFACTURED LIKE STORM WINDOWS Single- & double-pane configurations 2 to 3 times lighter than inserts manufactured with standard glass. M&V. Where did Measurement and Verification occur? NATIONAL RENEWABLE ENERGY LABORATORY assessed the impact of lightweight secondary windows provided by Alpen High Performance Products in a two-story office building at the Denver Federal Center. Results: How did lightweight secondary windows perform in M&V? 15% AVERAGE WHOLEBUILDING ENERGY SAVINGS. EASY INSTALLATION < 10 MINUTES FOR 1 PERSON NO DRILLED HOLES OR PERMANENT DEVICES. COMFORT INCREASED 20° WARMER INTERIOR GLASS4 73% REDUCTION IN CONDENSATION 5 97% LESS AIR LEAKAGE. Cost-Effective Across Climate Zones Positive return on investment at average GSA utility rates, $0.11/kWh and $7.43/mmBtu. Deployment: Where does M&V recommend deploying lightweight secondary windows? RETROFIT SINGLE-PANE WINDOWS In cold climates, double-pane secondary windows will be more cost-effective. In warm climates, the single-pane configuration offers a better return on investment. This retrofit technology is particularly well suited for historic structures where changes to the facade are not allowed. [PDF - 323 KB]


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Last Reviewed: 2023-01-06