Here in upstate NY we can easily see 20 below for a few days at a time. You want the quilted carhartt not the blanket lined stuff. I also like the gloves from napa. If you're in snowmobile climate; look for glove liners that wick away moisture. Same thing with socks and your 'base layer' (long underwear). They also make insulated jeans and sweatshirts (have a long underwear style lining)
I use my snowmobiling jacket from a few years ago as my shop coat when I buy a new snowmobile jacket and pants. However these have a nylon outer with waterproofing....not exactly fire friendly so if theres heat involved, I reach for the carhartt.
Also, a foam mat, like for Yoga; helps because you get an airspace between you and the floor.
BE CAREFUL with kerosene or propane heaters...in small spaces, lack of good oxygen may be a problem. As a kid on the farm, I found working infront of the salamander heater; I got tired quickly...lack of oxygen