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With the recent cold weather, I've been trying to best insulate my house and keep it warm while not using too much energy. While applying weather stripping to my windows, I noticed that some of the windows have these tiny plastic inserts between the sliding window sash and the frame. See photo. I can move them a bit if I poke at them but they appear to move fairly smoothly with the window when opening an closing.

Close up of white plastic insert Zoomed out photo of plastic insert

I see them at all four corners of the windows that have them and my BEST GUESS is that they are used to ensure the rubber weather stripping is pressing as tightly to the frame as possible and making a proper seal.

My question is what are these tiny inserts called, where can I get them? I have several of my windows that don't have them (presumably fallen out/gone missing) and I have noticed those windows don't sit as tightly into the frame as the others. I've looked up every variation of "plastic" + "insert" + "sliding" + "window" that I can think of and haven't gotten anywhere. Poked around my local big box store's window section and didn't find anything.

Appreciate any help!

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By looking at the space between the window frame and the sash (approximately 1/4 inch ?) I believe those shims are installed to keep the sash straight and from rattling around when it is opened and closed.

Or they could be part of the spring assisted lift system of the window. The piece that is in view may be attached to a mechanism on the edge of the window which compromises the spring lift mechanism.

I'd say some sort of shim, though.

It may be easier to find the manufacturer of your window and get the shims from their site.

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    Thanks for the reply, I should clarify as the picture may have distorted it. Gap is APPROXIMATELY 1/8th of an inch. I put a decent amount of effort into removing the plastic insert but can only slide it up and down. Believe it's possibly wrapping around the side of the sash. Will attempt to remove the sash (when the weather is above freezing). Sash lock shows branding from [Champion ](championwindow.com). I'll contact their desk to see if they know more.
    – Will
    Jan 12 at 15:18
  • If they are just shims to keep the window tight or to prevent rattling when the wind blows, then almost anything plastic of the right thickness will work. Buy a sheet of phenolic (?) and cut pieces off of it to fit. Or buy a package of plastic shims from a hardware or big box store and break them apart at the proper thickness. They already have a taper to them, which would help wedging them in place.
    – SteveSh
    Oct 3 at 11:18

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