I am thinking about mounting a 1000W 120V wall heater on an exterior wall, in my bathroom. Assuming I have room to do so, would it be possible to put rigid insulation behind the heater's mounting box? Could this be a fire hazard?
Yes, it will be a fire hazard. The smoke when it burns is toxic, the reason fire-fighters first started using oxygen tanks. IRC requires it be covered for fire safety in an attic or crawlspace under most conditions....
If any ads are present below my answer or words underlined/colored, I do not condone/support/use the product or services listed/linked to, they are there without my consent.
WELCOME to the forums, if I forgot to say it!