I don't like the insul-brite type of batting because it's "noisy" from the silver lining. Also, insul-brite is polyester, so would theoretically melt.
I use 2-3-4 layers of cotton batting, cheapest batting I can find that is 100% cotton (or close), or else scrap batting from projects. Thick pot holder from the store use just cotton, without other fancy materials.