Its hard to know which the best solution would be to insulate an exsisting home. The performance of the system vs. the cost and return on investment. The words out-sulate are now being used in the high perfomance net-zero type homes. This out-sulate is a insulating material such as foam board wrapping the entire house on the exterior. With this system typically difficult areas to insulate like the band just above the foundation is addressed. To try to insulate and prevent heat transfer thru the floor joist is very difficult for the traditional methods. Here in Canada's harsh climate its proving to be quite effective. The foam boards are easy to install over the studs and sealed with tape. This makes the home very air-tight and the heat-transfer though the studs a non issue. You may consider yourself lucky in one sense having only siding to replace after the out-sulate as if you had brick siding would make it prohibitive. The materials are inexpensive and can be DIY. The siding may have to be replaced by a professional because of the extra details around the windows and doors.
I had been part of the construction of a huge home here in Ontario and the owner/builder went the extra mile with two layers (3" total) of foam board over his 2x6 stud wall (R20) His heating bill is stunningly in-expensive for a home that size.
Randen
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