This question comes up pretty frequently.
“Why did my irrigation stuff freeze this year?!”
Sometimes it is a repeat incident – 3rd year in a row. Sometimes a valve or backflow preventer has been good for a decade (and then it busts due to weather). We see many above ground backflow preventers (like the picture) and in many municipalities it is required – lakeway. One thing is certain…it is far less expensive to prevent freeze damage than to replace a backflow preventer.
The preventer in the picture looks fairly well protected, and yet the number of these that we have replaced is well into the hundreds. The idea with freeze protection is to completely isolate the unit from the elements, allowing the ambient temperature of the water in the pipes to remain above freezing. This is why a whole industry of purpose built, freeze protection pouches exists.
They completely surround the device and block the air from circulating inside it. The pouch is filled with a high R-factor insulation for a winning combination that allows the temps outside to fall well below freezing for a number of hours without damaging the sensitive valve inside. The bag is expensive, but when you take into consideration the savings in a backflow preventer it is often worth it. Voila! Saved from an expensive repair.