dry cracked plastic glass bead??..sounds like it. most improvement stores will carry that, as for the glass?..i usually just use a bead of regular latex caulk (the cheap stuff..lol)put the bead on allsides off frame..press in new glass...install new bead(btw...it cuts easiest with scissors)

sorry - here they are. the first one shows what most of the windows in my house look like (aluminum frame only, storm windows and screen removed), followed by a close up of the gap & dirt that collects. final shot shows one of a few windows that still has storm windows (ugly blinds & my husband's antenna don't help matters). thanks.

Find the manufacturer's installation directions for whatever the replacement window will be. Here's Andersen's: https://images.homedepot-static.com/catalog/pdfImages/27/273888e4-efab-4463-a51f-20b85cb44957.pdf best way to get water tight seal?? Remove the siding so the window can be properly flashed. $1400 more now but a fraction of what it will cost to replace it (again) later. When this rotted window comes out, be sure you're there to see it and get the forensics of why the installation failed. I'm betting that knowing that will make you feel a lot better about the addt'l expense for removing some siding. Oh, and the upside is that you have cedar so trying to match new siding to old siding shouldn't be a problem.

20% would be pretty extreme IMO. Glass loss may be a little more than a "non-issue" in some cases, but 20% is high, and furthermore there are many times where it can be truly negligible. Think about an old double hung window with an aluminum storm that sits on the blind stops and measures about 2.5"+ from frame to viewable glass. This is extremely common (at least in my area), and that is just about where a reasonably slim lined replacement window can measure for visible glass as well, within a 1/4" or so. I think we can agree that the measurement of viewable glass on the prime window sash of that original wood window is irrelevant if it is impeded by the view of a storm window frame. Is 20% loss possible? Sure. Depending on the window coming out and the window going in, but I tend to agree that the glass loss is indeed overstated, and that the number would generally be far less than 20%.