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Community and Q&A

Sourcing High Performance Fiberglass Windows

neumanno | Posted in Energy Efficiency and Durability on

Hi everyone, I am in Duluth, MN climate zone 7 and am in the process of quoting/sourcing windows. The goal is a net zero house that is relatively small in size at less than 1500 sq. ft. I have one window option for fiberglass from a local dealer (that I know of) and that is Duxton windows.

The main problem I have is that Duxton windows are not listed in the NFRC directory, they give you performance values on their website. I am most likely going with Dual pane as triple is not looking cost effective (according to PHPP software). Duxton states that their Series 325-Casement Dual Pane with loe-180 glazing has a u-value of .27 and a SHGC of .68, but they state in a note at the bottom of the page that the SHGC is based off the glazing and not the whole unit. Does anyone know if this is how SHGC is typically measured?

I also cannot find any performance values for their doors…

What would you do? Keep looking for other window manufacturers or trust that the performance values given by Duxton are accurate?

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    In the U.S., the standard method for rating a window's U-factor and SHGC (according to the NFRC) is a whole-window rating, not a glass-only rating. Glass-only ratings often exaggerate the performance specs of a window compared to the standard U.S. method shown on NFRC labels.

    For more information, see All About Glazing Options.

  2. charlie_sullivan | | #2

    I'd get some quote from others near you, and decide whether it's worth it to you to get a true NFRC rating for similar construction with similar glass. Another I'd try is Comfortline Fiberframe.

  3. PrairieBurner | | #3

    Alpen windows out of Colorado make some of the best performing windows and are fiberglass.

    We just got a 925 picture window (quadruple pane equivalent) and 525-S horizontal sliders. Absolutely amazing!

  4. Adam Emter - Zone 7a | | #4

    Oliver, give Ben a call or visit his website at I ordered my triple glazed windows and doors from him and the prices were surprisingly affordable. Plus, Ben has helped me every step of the way. The units are assembled in Poland, using German machines and hardware.

  5. brp_nh | | #5

    We used Alpen 525 series in our house (zone 6, <1500 sq. ft., net positive) and have been happy with them. if building again, i would go for their higher performance line, budget allowed.

    Regardless of brand, I would suggest paying less attention to cost effectiveness numbers and get the best performing windows you can afford. Certainly triple glazed or equivalent performance in a zone 7 climate.

  6. Reid Baldwin | | #6

    We used Inline fiberglass windows from Canada. The exchange rate made the price favorable compared to local brands. We ended up with triple pane U 0.17-0.19 (depending on type) for several thousand dollars less than Andersen U 0.32. I believe that the frames of Inline and Alpen are very similar but Inline uses three panes of glass whereas Alpen uses films for the intermediate layers.

  7. welivehonestly | | #7

    @Oliver, what did you end up using for your project? We are near Duluth and working with the same distributor which sells Duxton locally. Thanks!


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