Don't mix it with cotton fabrics. Ask me how I know.
Carol B

My MIL made many quilts in the '70s out of double knit and tied them with yarn. When my kids were young and still at home, they used them all the time. I always joke that in 200 years when they are unearthed by archeologists, they will be in as good condition as the day they were made.
Yes, they are heavy, but wonderful to snuggle under in cold weather.

Same here. My MIL made several many years ago and they look today just as good as they did when new. She tied them all because it's so hard to quilt through them, and they weigh a ton. I think BarbaraTX is right about them still looking fine in 200 years.

Those old poly double knits will never die!!! I seem to remember there was a group that was using them to make quilts for the homeless.

My first quilt was 4 or 5 in squares from double knit, at least 30+ years ago. I made it for my daughter, who is now 38. She still sleeps with it every night and takes it with her if she is gone for more than a few days. We have re-backed it 3 times now, using sheet blankets usually. Patched it with receiving blankets. Originally I used a poly batting. I think that has all decinigrated though. --- My second quilt was also made from double knit. I have a sheet pinned to it now for re-backing. --- My last double knit quilt is still squares in a box waiting. One of those mia's. It has been in the same spot for about 25 years. I learned to appreciate patchwork. I am now retired and plan to drag it out and finish. --- That is what patchwork quilts was all about. - left over fabric. My mother made all our clothes and her own. Double knit was high fashion in the late 60's - 70's. I had a large selection of colors and textures of double knit. I would add a picture but I am out of town.