I currently live in a 1 1/2 story cape cod house with 4 inch rafters and a "hot roof". We have some prety bad ice dams during the winter and in the summers the upstairs is a furnace. I need to reshingle the roof and was thinking of having some closed cell insulation sprayed. With the current setup, I believe the best I can achieve is a R level of 20. I was wondering if I were to have the decking removed, add 2" furring strips, and spraying the closed cell insulation 5" deep, I'll get the R level up to 35, but will it be worth the extra cost? The added expenses will be removing the decking, adding the furring strips, redecking, and extra insulation. How noticeable will the living conditions be when comparing the two options?
It's impossible to give you a cost-benefit analysis without a lot of information including your location, current energy costs and fuel type. But I can tell you a cheap an inexpensive way to fix your problem.
The fact that you have ice dams on a Cape Cod style home leads me to believe you have a tremendous amount of warm air leaking up through the framing near the exterior walls. As the air you've paid to heat rises up through the walls, it melts the snow which then refreezes when the temp. drops at night. This results in an ice dam.
Air sealing your attic floor is a cheap and easy way for you to correct some major heat loss and comfort issues. You can do it yourself with a few cans of Great Stuff and tubes of silicone caulk. Or you can have a pro do it for $700 - $1500 depending on the extent of the air sealing.
Either way, it's a lot cheaper than spraying 4" of closed cell spray foam which won't stop the air from leaking into the attic space. And at $4+ per square foot, it's your most expensive option by far. I'd expect it will take 20 years for you to recoup your investment at current energy prices.