Now Available: Recording & Slides–Lightweight Secondary Windows

Green Proving Ground Webinar


Lightweight Secondary Windows


If you missed our last webinar, a Green Proving Ground (GPG) assessment of thin glass secondary windows, a recording of the webinar and presentation slides are now available.

Many thanks to all the presenters and to the participants for their thoughtful questions, some of which are answered below:


Q: Is the thin glass more prone to breakage from impacts?

A:  Actually, third-party testing has revealed thin glass to be less prone to breakage than standard glass because it is more flexible. In addition, all units are laminated with safety film.


Q: How are the secondary windows installed? Are they earthquake-resilient?

A: There are multiple installation options (from magnetically attached clips to C-channels) but the most common and least expensive option is an L-bracket installed at the top of the frame that the secondary window slips into. A restraint can be added at the bottom in earthquake-prone areas. At the testbed, one person was able to install an insert in under a minute.


Q: Are these secondary windows compatible with operable windows?

A: These thin-glass inserts are installed on top of existing windows and are not operable. Their manufacturer is developing a configuration that will be compatible with operable windows. GPG is also currently evaluating another window insulation panel that enables windows to remain operable.


Q: How does the system deal with window frames that are out of square?

A: We had this situation at our testbed, which was a concrete-and-brick structure built in the 1940s and had experienced settling so the windows weren’t square. The manufacturer was able to adjust the gasketing to create an airtight seal.


Q: What is the largest window size possible with this technology?

A: The largest thin-glass insert commercially installed to date is 50 square feet. Theoretically, thin-glass inserts can be as large as 70 square feet. For larger openings, units can be stacked together.


Q: Were there issues with condensation forming between the pre-existing window and the storm inserts?

A: There were no issues with condensation. The key to reducing/eliminating the condensation potential between the interior insert and primary window is to make the insert as airtight as possible. The insert evaluated has been rated by the Attachments Energy Rating Council (AERC) and is airtight. It has an air leakage rating of .06 cfm/sf. To receive an ENERGY STAR rating, interior inserts must have leakage < 0.5 cfm/sf.


Q: Are secondary windows only applicable for cold climates, or are they also appropriate for hotter climates?

A: Secondary windows can be cost-effective for both hot and cold climates. The single-pane insert with a low solar heat gain coefficient (SHGC) may be more cost-effective for hot climates. For cold climates, the double-pane insert offers better insulation and is more broadly recommended. 


Q: Is the technology appropriate for historic buildings?

A: Generally, a secondary window is an outstanding solution for historic buildings. It does not change the facade and it allows the existing single-pane historic window to remain in place while increasing performance to contemporary standards. For more information on secondary windows and historic buildings, attend DOE's Better Building webinar on April 6th:


Q: Do secondary windows help sound attenuation from exterior noise?

A:  We expect secondary windows to reduce exterior noise though this wasn’t evaluated in the testbed assessment.


Q: Are the secondary windows commercially available?

A: Yes. AERC-rated secondary windows can be found on the AERC website. The thin-glass inserts we evaluated were provided by Alpen High-Performance Products. Contact: Pierre Graas,


Reference 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.



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