Having your lower level or basement finished is one of the most economical ways to almost double your living space. Around the exterior walls at the ceiling (called the rim joist) we spray foam to an R40. Yes, this is above the Building Code requirements, but it is an area of house design where there is so much heat loss and this is the time to do it right. You may want larger windows installed or a garden/terrace door to the rear yard. One of the most important things in design is to not have your basement still feel like a basement. Another item, warm air vents should be at floor level not coming out of the ceilings. If a contractor informs you that a Building Permit is not required, because you are not making anything structural changes, for that part he is correct; however, it is required for insulating the exterior walls. If this is not done correctly you will probably have mold issues in a few years. Get the permit, it is only about $150. Have the assurance that the work is done correctly and inspected by an independent party.