I usually use one layer of cotton batting and one layer of insul-brite. My DD tried to use two layers of insul-brite with a layer of cotton batting and my dear Bernina 1530 started breaking needles. We broke four before I decided it was the thickness of the project that might be creating a problem. I have since dismantled her crazy potholder and pulled one of the layers of insul-brite out. I've tested the machine on regular fabric and she sews fine now. I think she will be okay. No more experimenting for us.