They become warm rather than over-hot (I use them outside for my little semi feral cat in her kennel, in winter); although they have covers, I tend to put an extra towel over the top anyway, more for her comfort to lie on than protection from the heat. And I most certainly would use them indoors for your piggies, because a room without heating could be quite chilly in the winter. (And last year when I had a really bad back I "borrowed" one to sit against, as they retain their heat such a long time)