Mine have never gone under in those temps. The only time I've had one go under was when he was sick, he died within a day. I always had trouble getting him to eat and keep on weight, in hindsight, wish I had run a fecal test on him earlier on. How is your humidity? When I was having trouble with that boy, I read that they'll go under in low humidity. Instead of warming the whole house, you could heat a little with just a heating pad or even just an incandescent light during the day to help with digestion. I have used a 15 watt on top of my grey tree frog tank in the past to warm things up when the house got really cold.