To prevent back flow in your waterline caused by a frozen pressure vacuum breaker (PVB), the pressure vacuum insulator needs to be insulated. Insulating all of your aboveground valves and hose bibs in below-freezing temperatures can defend against the need for costly repairs. There are different products available for insulating outdoor valves, including insulation tape. Insulation tape is great for insulating pressure vacuum breakers because of the PVB's awkward shape. Most insulating cups will not fit securely on a PVB.
Open all valves. If your PVB has an upstream drain, open this as well. Let all of the water drain out of the PVB.
Close all valves, drains and test cocks to a 45-degree angle when there is no more water in the PVB. Take the towel and dry all water from the PVB. Make sure the PVB is clean and dry before applying insulation tape.
Unroll about six inches from the Insulation tape and stick it onto the end of the PVB. Wrap the PVB with the insulation tape until the PVB is covered, but do not cover the drains or vents. Cut the tape and attach the end to the PVB.