Free-spirited daughter and her plans to live in a cabover

I agree with pianotuna and bb_94401. Do not use an unvented combustion heater. If she is as handy as you say make it a project to install a safe heating system. I know many people use unvented and feel safe, but it is your child and this is a very small step to take for the safety you get in return. All it would take is one night to forget to crack the window open. Sounds like fun good luck.