Do you guys have ideas or dyis of keeping out the cold air
Make a fabric bag into a "snake" shape the width of the door and stuff it with old rags, or whatever to block cold air that comes under the door. . . . For windows, I use the plastic sheets that come in Window kits from the hardware stores as they aren't expensive and are fairly easy to put up. I do find, though, that the adhesive tape the kits include can be hard on the paint or varnish - just a caution to consider.
You could put up weather stripping around the door, place it where the door meets the frame. It is a sticky length of cush like foam, this will block air leaks, available at many stores...walmart, diy's...
I have a steel door, and even with good weather stripping, I can feel the cold sometimes. What I do is hang a curtain on a tension rod and fit it in my door frame. Works great.
Check the gasket around the door and the threshold, making sure you can't see light through and around the door.