I was checking for your website to recommend your product to a friend.  We bought two of these for outdoor cat shelters, and have used them for the past 5 winters.  Our feral cats love them, and we've always put straw in the shelter on top of them for their comfort.  The pads only warm to the cat's body temp, so even though I guess the manufacturer can't say it's okay to use straw, we do without any problem.