My house is very energy efficient, I put a lot of effort into E4 windows and multilayer insulation. So I want to make sure the door will be well insulated too. What's the best way to insulate it? Make a door core from closed cell foam insulation? If I put 1" foam between two 3/4" of wood this would make door 2.5" thick. Also how to make frame water and draft proof? Any references preferable with pictures and drawings would be greatly appreciated.