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]


Reference above to any specific commercial product, process or service does not constitute or imply its endorsement, recommendation or favoring by the United States Government or any agency thereof.

Last Reviewed: 2022-09-25