Here is the scene this evening:
Oliver is the one on the roof. I'm assuming the two inside are the kittens. The outside temperature is 33.8°F (1°C) while inside Oliver's house the air temperature is 49.2°F (9.6°C). I'm sure the temperature on the heating pad is quite warmer than that.
As you can see we have ourselves a situation here. I hope Oliver squeezes into the warm kitty house for a family snuggle rather than spending the night on the roof. I'm not sure what the maximum kitty capacity is for Oliver's house, I built it with just Oliver in mind.
The weather is forecast to get colder over the weekend with a chance of snow. Now that the kittens have discovered the warm heating pad in Oliver's house, I'm guessing they may be here to stay. In an attempt to ease the overcrowding situation, I have ordered a second heating pad that should arrive by Saturday. It will be installed in the smaller insulated kitty house, which I believe is currently unoccupied.