I think the dogs like the windows too. I have often seen them staying in the house where its warm and watch me through the window as I go out to the shop, only venturing out if they know I am doing something in the yard.
I put a 'Hound Heater' dog heater in it along with a Wireless (Bluetooth) Temperature/Humidity gauge. I had no idea how affective the insulation was going to be and how much the dogs own body heat would contribute to the temperature. Turns out that I have not had to turn the heater on at all to keep a comfortable temp. I expect it would need to get down into the low teens before I would need to turn it on. Unfortunately my attempts to protect the Wireless gauge fell short and one of the dogs chewed it up ($25.00 down the drain). At least is served its purpose before it got mauled.