According to The Warm Company, Insul-Bright IS NOT RECOMMENDED FOR USE IN MICROWAVE OVENS.

It contains 6.25% metalized polyester.

https://warmcompany.com/products/warm-home/insul-bright

(Bold face and capitals are mine. bear)

I may be wrong; but, I don't think you can use Insulbright. It has an aluminum type layer on it. This could spark in the microwave. The batting made specifically for what goes into the microwave is what you should use.

Insul Bright is NOT 100% cotton! It should never be used in the microwave. Please check with the manufacturers first. Warm and Natural responded saying their 100% cotton batting is 87.7% cotton. Be safe.

I do not heat mine with the bowl in the microwave, I put the bowl in the cover after it has been heated. Mine have held up nicely, maybe has something to do with being heated time after time. I am cautious because of all the stories I have heard about them catching fire.

Microwave bowl buddies
 

I always use 2 layers of cotton batting and quilt each one separately. When they come out of the dryer just shape them, they will be a bit more limp than the new ones but still look good.

I'm not sure if this is correct. Insult Bright has some metal threads in it which is dangerous in microwaves, I use the microwave batting in mine, Joannes sells it

I do like Onebyone. I put a piece of batting in the microwave and see what happens. I have had 100% cotton burn in less then 30 seconds! I use Warm and Natural for the bowls.

Warm and Natural is advertised as 100% cotton, but it's not. It has a very thin polyester scrim that may cause problems in the microwave. Warm and Plush does not have a scrim and is 100% cotton. Because it doesn't have a scrim it needs to be quilted very closely.

I know you have already made a number of the bowls. Something a lady in our sewing group discovered is that even though she used cotton material/batting/thread that one of her bowls got crispy. She could only think it was the fabric sizing used by the manufacturer and now she washes all the fabric before making the bowls. Insulbrite should not be used.

So that is why my friend's potato pouch burned up. Boy, was she surprised. Another quilter had made it for her and swore she used all cotton fabric and Warm and Natural. The plastic scrim makes sense in the Warm and Natural. I didn't know that they made batting for the microwave; I'll let them know so it won't happen again.

With all of the different type of sizing out there, it seems like a good idea that the fabric is washed first. Better safe than sorry.