Gap in sash for weatherstripping

A landlord from Framingham, MA says:
April 26, 2021
I have some ~20yr old vinyl replacement windows that need new pile stripping. They are 3/8" wide, but the problem is that the frame seems to have warped so that there is a gap around the middle of the sash, but it is otherwise reasonably tight with the existing pile. When I measure the gap it seems to be about 1/4". But I'm concerned if I get 1/4" pile, then it will be too tight on the upper and lower portions of the sash and the window will be difficult (or impossible?) to raise/lower. Any thoughts on how to handle this?
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Bob from SWISCO responded:
April 28, 2021

Knowing the exact size of the base on your old wool pile, as well as the height of the pile, is the only reliable way to say what you need. From there I can try and narrow down your best options. See the video below.
A landlord from Framingham, MA says:
April 28, 2021
Hi, as I said the base is 3/8" wide. Hard to tell what the height was originally since they are 20 years old and completely flat, but my guess is that they were 1/8" originally. The problem is that the gap in the middle of the sash is 1/4" now.
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Tom from SWISCO responded:
April 29, 2021

One thing you could try is to go with the 1/4" high pile, and then if it ends up being too thick you can give it a "haircut" by running a razor right along the top. You can do a little at a time and try it bit by bit until you get it to the size you need. It's time consuming and you need a steady hand, but I've done it myself when I wasn't sure about the gap and I think it works. 

Otherwise, if the old pile is too damaged to accurately measure, then you're looking at a gamble no matter how you approach it. There's no way to know what size you need, exactly, without using the original as a reference.
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