Pex won't freeze and burst but it most certainly will freeze and push apart at fittings.
The only failsafe is not letting it freeze.
That is right, but not right too... It CAN happen, but mostly don't. The expansion in the lines takes up enough generally... Although, if a fitting does become a problem, the fitting usually breaks before the PEX will push off. The fittings, being hard plastic, don't have the same give.
My washer lines are on an outside wall, and pass close to a vent - Crawl spaces here have operable vents that you open and close with the season. But even closed, the vent itself remains uninsulated, and the cold can pass through there.
Those lines froze hard several times at 10 below until I got a heat tape on em to resolve the problem.
A couple things to consider that northern homes are outfitted with:
Foremost a secondary heat source is paramount. I have both GFA and wood. Wood is my primary but the gas furnace is there if my stove becomes problematic for some reason (which happens more that you might think). I CANNOT be without heat. Hence the emphasis... Y'all are getting a taste of that right now.
Foundations or skirting need operable vents. That way you can close them in cold weather. It is an annual ritual here. I close em on the first freeze, and open them with planting.
Skirting particularly needs to be insulated. But if you can close off under the house, the house itself will usually retain enough heat radiated downward into the crawl to keep things thawed.
Add to that a thermometer showing ambient temp in the crawl, and a string of 100w access lights providing light to the crawl... Those 100w bulbs add significant heat if needful, which tends to stay in there if it is sealed up fairly... If I am worried about cold, (if the temp down there is under 40's) those lights are turned on.
An air pump system to exchange the interior air into the crawlspace.If it is getting cold enough to freeze down there with every other option exhausted, I flip open a duct into the crawlspace and a return duct on the other end. I have been here going on ten years, and only used that option once and I was dang glad to have that option in place.
A propane jet heater will do that too, or space heaters if you have the electricity.
All of the options above are better than broken plumbing.
Additionally, my whole house is plumbed off of a common curb-stop valve, and the lines can be drained. In the case of a major outage, with no heat either, I can go shut off the curb, open the drain, put a hose on the hot water tank and drain that, and pour antifreeze into the pea traps and toilet, and the house is impervious to frost damage.
