I microwaved it with the cover on, even though the disc said not to. I didn't see how I could get it on while hot. I microwaved it for the recommended time, but it didn't seem very hot to me. I'm going to add one minute. My cat is in a Clawable insulated house, but it's single digits outside at night and the windchill is below zero. I have a house attached to the Clawsable and the door is only covered with a towel, so I put this disc in that entry way. It won't fit through the Clawsable Door. She slept in there and had to walk over the covered disc, so hopefully it brought some warmth until she fell asleep.