I lived in Northern Minnesota for a period of time and alot of folks have "block heaters" to aid in starting engines in extreme cold. 20 to 30+ below zero.
Up there it's the oil that decreases in viscosity and becoming like molasses in below zero temps making engines harder to start and wearing out batteries. Don't know if a block heater would prevent an engine block from freezing up and cracking or not. I wouldn't want to risk that chance. Instead I'd make sure that my anti-freeze is rated for the climate I live in. In fact I'd go ultra-conservative and buy the lowest rated anti-freeze just to make sure and if I am living in the frozen tundra, I'd probably install a block heater for extra peace of mind. And remember, there is raw air temperature and then there is that thing called wind chill!!
Another simple thing one can do to keep that engine block warm is to place a large wattage light bulb under the engine. Preferable in a garage and out of the wind. It can at least throw off some heat but you still need properly rated anti-freeze.
