What? That’s way below freezing and totally unsafe. Why don’t you sleep in your car? (WARNING: Make sure your exhaust is uncovered and you’re not in your shed)
(Just to make it clear I didn't mean to downplay this. Obviously to survive in your house at -10c you do need preparations which I understand many people don't have not being used to these temperatures. It's easy to forget as someone who lives a bit further up north that it's not a given that people have thick air tight walls and heavy down filled blankets to help them through. So to answer my own question: It would be unsafe because they might not have the things they need to be safe.)
How do you think people safely camp in cold climates? You can sleep safely at far far below zero in a sleeping bag.