Heating question

[zuk99]

Hello - I'm the owner of a cabin that has roughly 300 square feet of kitchen/bedrooms/bathrooms and then it has a section that was an 'addition' which has about 500/600 sq ft of great room and master bed.  I'm looking for a solution to not have to heat the entire cabin and am thinking of heating only the part with plumbing (pipes run underneath kitchen and well tank is in a 4 foot crawlspace under bathroom.  My question is this, there's a hallway that I'm thinking of sealing off with a new door for preventing having to heat the entire place (the air is leaking through this opening and I really don't care what temps get in this area as it doesn't have plumbing.  What sort of door would you recommend installing?  Should I have something with insulation in it?  The original cabin was old (built in 40's/50's) - and I'm sure my windows that are pretty old are not the most efficient.  Any ideas will be greatly appreciated!  BTW - I keep it at 50 degrees and intend to continue to do so.  My main goal is to save money and not have any impact on structure/plumbing.

[flyingvan]

Well, you figure exterior doors are already designed for that purpose---insulated to keep heat in (or out)...