Exposed water pipes in the crawlspace or other unheated areas of a home are at risk anytime the temperature drops below freezing. They can easily freeze and rupture if the water isn’t flowing through them.
Leaving the faucets slightly open keeps the water moving, so they’re less likely to freeze, but that isn’t a long term solution and it wastes water.
Foam pipe insulation is the right answer. It comes in three-foot lengths that are already split down the side, so that all you have to do is slip it over the pipe and a self adhesive strip holds it together.
You’ll want to measure the pipe diameter first, so that you can get the right size material. You may also want to invest in a disposable crawl suit, if your pipes are under the house.