A recent spray foam job we did for a client shows that the owner saved over 42% on their heating bill by just air sealing the bottom sill area within the home. We had no access to the attic, but had we did get into that savings would have been close to 60% just on the heat alone!
Insulation was installed prior to this project on the sills, but we removed it and replaced it with four inches of closed cell with a R value of close to R 28.
Not only did the owner save money their comfort also increased which allows them to keep the thermostat lower saving even more money.
So before you decide to insulate, air seal all gaps, you can use spray foam in a can, caulking or simply call a company that does this type of work. It is more expensive as professionally installed spray foam runs around $1.50 per board foot and up, but the savings more then pay for the cost in the near future.
Great stuff Woodbridge. It's nice to see real-world examples and numbers for the insulation stuff I keep jabbering about.