I would not pay to have block walls filled with injected foam. First of all, the most heat transfer occurs through the "thermal bridge" created by the wall areas that have continuous concrete through the block wall. Secondly, the block cavities cannot be adequately filled with foam because many cells are either completely filled with concrete for structural reasons, or the cells are blocked by the excess mortar that protruded into the cells when the mason laid the blocks. There also may be rebar and other items in the cells that prevents the foam from filling the cells. About two thirds of energy leakage in a house occurs through the roof and windows, not through the walls. So attic insulation or new windows will provide the best return on investment. I would ask the foam salesperson to install the foam for free and pay for it based on a percentage of the reduction in electricity cost that you actually experience. See how that goes over.
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