or is it more conventional with studs & drywall inside? If it's conventional, you could blow insulation between the studs from the inside. If it's solid brick it will be a little more complex. I'd think about adding closed cell foam insulation on the exterior, but you'd also have to cover that with siding or plywood of some type. It can get expensive. Another possibility would be to use furring strips on your interior walls, fill between the furring strips with foam insulation. I've used a foam that's got aluminum on one side and black plastic on the other, can't remember the brand, but it was easy to work with, comes in several thick nesses (the thicker the better). Foil side goes toward the interior. Finish up with drywall over the furring strips and foam. Whateve you do, make sure to insulate the roof when you make your improvement. Most heat loss goes straight up thru the roof.