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What are some of the engineering considerations when evaluating fiberglass window frame profiles?

user-6115058 | Posted in General Questions on

My wife and I building a passive solar house in Eastern Ontario. Many of the articles on GBA have convinced me that fibreglass triple pane windows are the correct way to proceed.

What are some of the design considerations when choosing the best pulltruded frame profile? Some use a closed back or hollow frame and others are open back.

Though counter intuitive, the open back windows have better u factors. It would seem to me that the closed back frames are more rigid and stable, therefore allowing the locks and slides to perform better.

It’s terribly nuanced and would like a building scientist’s take is on some of the subtle differences and what impact it has on durability, performance and installation.

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  1. GBA Editor
    Martin Holladay | | #1

    Briefly, you are overthinking this aspect of your building project. The most important considerations when choosing windows are (a) the window specifications (U-factor, SHGC, and visible transmittance), and (b) the reputation of the manufacturer for customer service, and (c) the reputation of the local window distributor for customer service.

    When I first began researching fiberglass windows, I wondered why more manufacturers didn't fill the hollow parts of the fiberglass pultrusions with some type of foam insulation. The answer, I learned, is that (a) filling the voids with foam insulation is more complicated for manufacturers than it sounds, and (b) foam-fill frames don't change the window U-factor very much, and are therefore more of a marketing gimmick than a necessity. In other words, these details are complicated and sometimes counterintuitive. So trust the NFRC labels, and move on to other issues.

  2. user-6115058 | | #2


    Thanks for your quick reply, I believe you are correct on all counts - including the overthinking part :)

    The question was less about the foam insulation and more the pulltrusion profile. Some are formed with a rectangular profile whereas some are more like a U. I've welded metal structures together that used square tubing, others with angle and u-channel - all worked as intended. Does principle hold true for fiberglass window frame, where profile differences are not significant?

  3. Rob Myers | | #3

    I agree with Martin. Having just been through the process of choosing fiberglass windows I would recommend spending your time researching the reputation and product support of the manufacturer and not worrying as much about the details of how a particular specification is achieved. Having a manufacturer that stands behind their product is much more important than a small theoretical improvement in U value.

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