Frozen sprinkler pipes are the worst! One year I had to replace all the valves in one of my valve boxes because they had froze. This was a harsh awakening to home ownership when I had just purchased my first home nearly 12 years ago. One of the best ways to avoid the need to fix broken sprinkler pipes in the spring is to winterize your sprinkler system by turning off the water to the sprinkler system and then blow out any remaining air with an air compressor. However, sometimes you might forget to winterize or you might not winterize it soon enough and your pipes could freeze. The dirt that the pipes are buried in will provide some insulation to keep them from freezing but in the valve box the plastic lid is not adequate enough to prevent freezing. It would be nice if the lids came pre-insulated but like some garage doors they are cheap and not everyone needs insulation if you live in a warm climate. For those of us who get freezing temperatures it's a good idea to provide some insurance against a freeze by insulating the sprinkler valve box cover. My father in law gave me this idea because he had used spray foam to insulate the cover of his sprinkler boxes.
