In CO spray foam would be very useful and by far the best way to go. It provides a higher per inch R-Value and a much tighter air seal then batt insulation. Open cell will is air permeable so is a great solution for insulating roof rafters in your area. No cold air space from roof vents will be needed with this method. Closed-cell will be better for new construction wall cavities since it yields a higher per inch R-Value but at a greater costs. As to costs, you need to refer to local vendors.