I have a roll of thinsulate that I bought about fifteen years ago, and I was thinking of using it to make an insulated jacket. But most people seem to use primaloft or something similar these days. I am wondering why thinsulate fell out of favor. Does it have less warmth for its weight or something?

There are many varieties of Thinsulate and so this is a generic answer. It is composed exclusively of fine fibers (65% Olefin/35% Polyester). It does a superior job of reflecting thermal radiation and not compressing, compared to high loft insulations like Polarguard or Primaloft. As a result, it is about 1.7 times warmer for thin applications under pressure, like gloves, boots, diving suits, and tight fitting fashion-wear.

It doesn’t compress well to allow stuffing into a small pack. It is much heavier and doesn’t provide any warmth advantage for applications (jackets and sleeping bags) requiring thicknesses above about 3/4”.

E,
What about killer insulated socks? In holiday colors. Thinsulate is really good in socks. That could take care of your holiday gift problem.

the socks are good idea. Do you know of a pattern for sewn socks or slippers? I make knitted socks all the time, but I don't know how to make sewn ones.

Once I made a frostline kit of down booties, but that was thirty years ago.

I can see that it wouldn't compress very well, so that it wouldn't be ideal for a jacket stuffed into a pack.

Elizabeth,
You might get an idea for your socks on this
thread

I made a pattern for a sock of sorts and then used Cuben material for them. It made a VB sock. I made a couple of patterns and used scrap material till I got them to fit. I think you could do something like it with your Thinsulate.