GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Fiberglass window frame connections

RMaglad | Posted in General Questions on

I attended a local fall home show this past weekend. I spoke to several local window companies, and when i asked if they supplied fibreglass windows, they all turned up their noses, and essentially denounced fibreglass.

So I pressed them a little, “why do you hate fibreglass”. The common response was, “oh, you can’t weld/fuse fibreglass, like you can with vinyl!” “The mechanical joints used in fibreglass windows are junk, they leak, nothing beats a welded joint!” to which i responded, “Houses are full of ‘mechanical joints’, such as the wall to window connections, or wall to floor/roof connections, which rely on quality detailing and installation of said connections”. To which they respond “welded pvc connections are stronger and can handle more load”. I had to walk away, i simply wasn’t armed to respond accordingly.

I was under the impression that fiberglass windows were stronger, allowed for thinner frame profiles, were the least susceptible to expansion and contraction.

While a welded joint surely is an attractive feature, i don’t know why a mechanical joint can’t be just as good? Fibertec, for example, claims to use nylon keys and high quality sealant for their frame joints.

Fibreglass windows score just as good, if not better on the NRCan product listing for all categories, air leakage, water resistance testing, etc etc. Is there any valid reason, or testing which shows problems, or lack thereof?

I had been fairly set on fibreglass windows, mostly in large part from the recommendations found on this site.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    I've never heard of any fiberglass sash failing at the joint (although any sash that is very large and heavy -- whether vinyl, fiberglass, or wood -- may distort over time, a problem that used to happen with casement windows especially).

    The corner joints of my fiberglass windows all look very tight.

  2. Expert Member
    ARMANDO COBO | | #2

    I've designed and specified Pella Impervia fiberglass windows with nail fins for many years, and to my knowledge, no window has failed at any joint.

  3. jackofalltrades777 | | #3

    The reason WHY you got the answers you did is because PVC/Vinyl manufacturers FEAR fiberglass framed and windows and they will make false misleading statements against them to keep vinyl sales going.

    Vinyl expands and contracts like crazy in heat/cold, that is a fact. European manufacturers install steel U channels into the frames and then screw the steel frames into the PVC to help stabilize and minimize distortion in the PVC. This design works but 95% of all US made PVC windows do NOT use such a design assembly. It's typical extruded PVC with no steel reinforcement.

    Fiberglass windows like those made by Alpen in Colorado achieve NFRC U-Values (both frame AND glazing combined) in the U-15 or even better range. The fiberglass windows do NOT distort or move much in extreme sun or cold. The frames are very strong and the windows perform very well.

    What the vinyl window sales guys are doing is selling fear and misinformation.

  4. lance_p | | #4

    Ryan, I'm interested in fiberglass windows for many of the same reasons. We're building in Ottawa as well, starting in the spring.

    I recently took a trip to Toronto to visit two fiberglass window companies: Fibertec and Inline. Both product lines look very nice. I have yet to decide who to go with for our windows.

    The sales guy at Inline took me for a tour of their facility to see the pulltruding process and their assembly line. He claims they sell raw extrusions to other fiberglass window companies, though he wouldn't say who (for obvious reasons).

    One of the biggest reasons I want to use fiberglass is because I'm not impressed with the vinyl windows we have in our relatively new (2004) house right now. One of the larger casements appears to be sagging slightly, so it doesn't close well. Knowing that a triple pane glass package in a vinyl window would add stress to an already marginal design (often the same extrusions are used regardless of the glass package), this would simply exacerbate the issue.

    To be fair, it's entirely possible that the window in question is installed out of square or something, which could be the cause of the issue for all I know. But...

    On that same window I realized the middle sliding vertical latch wasn't engaging the catch on the window, so it wasn't closing properly. After an hour of messing around trying to shim the guides I came to the realization that the window frame was simply not straight! A firm push on the frame and it was straight again, the sound of the rubber seals sliding on the glass confirming that their friction was enough to keep the frame bowed inwards in the middle of the window, likely the result of the force applied while screwing the catch to the frame... 13 years ago. And it has never closed properly since then.

    The pulltruded fiberglass frames I handled at Inline seemed MUCH stiffer and stronger than what I experienced with my vinyl window here. Add to that all the previously mentioned advantages, and I can't imagine using vinyl windows in a house I plan to live in for the next forty years.

  5. RMaglad | | #5

    all good responses so far. While a welded joint surely is an attractive feature, i just can see why a "mechanical" joint wouldnt as good for what it needs to do.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |