Our living room is attached to a jetted out roomy thingy turned into our kitchen. The room was used for people to push their beds out on in the summer months for the breezes b/f air conditioning. The room never got insulated. I rent and my landlord doesn’t get that this is a problem. Needless to say it is freezing downstairs. Our kitties all cuddle up against the radiator during the winter. Last year we started using a space heater b/t the kitchen and living room entry way. We also have curtains over the doorways, insulated window shades, and well I still have to put the plastic up on the windows. Our space heater is a safety ceramic heater, that turns off if it gets knocked into and maintains at a set temperature. I keep it turned on over nights and sometimes on excessively cold days. I also set up their bunny bed w/ extra rag rugs and keep it covered w/ a willow tent and the willow tent covered w/ another rag rug. They seem to be doing just fine w/ that.
When I used to foster kittens, I used to use a pet safe heat pad. It was for kittens and senior animals. It didn’t get the hot spots like heating pads. It didn’t get too hot and used radiant style heat. It was covered w/ a fleecy bed pad. It was very safe. I gave it away to someone fostering kittens about a year ago.