While it may be somewhat common, personally, I find it a very bad practice to have something like a WH in an area that can freeze. While it may or may not freeze, the piping to and from it certainly can and while there may be some conduction through the water, most are installed with heat traps to minimize that. By having it there, your standby losses are significantly increased (except during your summer where the WH may end up getting hot without running!). Insulation does not create heat, but slows the transfer of heat. If it gets cold enough and there's no hot water use, things will freeze. If it's an electric one, hope you don't get a prolonged power outage, either. I'd purchase the best (thickest) pipe insulation and consider maybe a roll of the foil faced bubble wrap, and wrap the tank as best I could with a bunch. Since there usually isn't much water exchange in the tank, even using hot water may not warm it enough, although the hot running by its inlet will warm it a little. An attic typically is ventilated, and any moving, cold air is your worst enemy when it comes to freezing pipes. Hopefully, someone with more local knowledge will have some more definitive suggestions.