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Community and Q&A

Inline Fiberglass Window Experience

Efficient Builder | Posted in Green Products and Materials on

I am interested to learn about other people’s experiences with inline fiberglass windows. I have seen a few comments here and there from Jonny_H, Reid Baldwin, Forrest Twombly, Sofiane Azzi, Alan Afsari and I would like to hear if they have any updates because I have had a less than great experience. 


I learned about inline fiberglass windows from this website and went ahead and ordered 12 windows for a high-performance build in climate zone 6. (This site was an invaluable resource for sorting out all of the building science details). It was completed in summer 2019. There were 4 fixed windows, 6 casement, 1 inswing, and 1 tilt n turn door. To date I have had three window seals fail (two fixed and one casement) and one casement window locking mechanism fail. All of the casement windows are difficult to close and we are reluctant to open them because we don’t know if they will close again.


Watching the casement windows operate, they simply do not close all the way. On the locking side, the handle stops turning when the IGU still has ¼-1/2” to go. Then it’s up to the somewhat flimsy locking mechanism to pull the window in the rest of the way. There are clips on the non-locking side that match up to clips on the IGU- I am wondering if these are misaligned and not allowing the window to close all the way. When they were first installed, they were somewhat tough to lock, but nothing like now. I ran strings from opposite corners of the frame and those indicate the frame is planar. One very involved solution would be to shim out the nailing flange on the locking side of the window by ¼-1/2”. This presents some complexities with the top and bottom nailing flange as well as flashing, window returns, and trim (rabbet the trim?). 


There is also a fiberglass ramp for supporting the IGU when closed, but it gets hung up on this- loud creaks and squeaks when opening and closing. I imagine this is stressing the IGU a bit each time and it’s another thing the locking mechanism has to fight. There are decent wear marks on the IGUs from the ramp. I did add some lithium grease to the ramp which did help.


Other issues that came up included telling me they couldn’t be shipped to my site after I had placed the order and not including NFRC labels which delayed the inspections. I am pushing nine months now trying to get this resolved with Inline, but I have nothing to show for it. Customer service leaves a bit to be desired. So, I am quite disappointed and frustrated and would like to hear how other people’s inline windows have been holding up and what their general experience has been. I am curious to know if this is an anomaly or not. 


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  1. Steve Knapp CZ 3A Georgia | | #1

    I don’t have any experience with Inline, but I wonder about the quality of your install. With your casements, it sounds like the window are not square and plumb in the openings. Not to discount possible manufacturing defects, but have you checked? Also… Sometimes this type of window needs to be adjusted after installation. I had to do this with one of my Andersen casements to correct a bit of “sag.”

  2. Jonny_H | | #2

    My Inline order should be shipping within the next week or two -- I'll update my impressions once I have them installed! So far, I've been happy with my dealings with them.

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