With extreme cold temperatures in the forecast, that is a ripe wind chill coming which will be severe enough to make your outdoor pipes freeze and cause your pipes, even in an unheated crawl space, to freeze. It is amazing what one burst pipe with street pressure can do. The best way to prevent this catastrophe from happening is to shut off all of the water going to the outside and keep a slow drip of water in any areas above an unheated crawl space so that the water always keeps moving.
If you have a hose bib on the outside of your house (the spigot where you hook up your hose - and make sure you disconnect your hose), then there should be a shutoff valve in your basement right in line with the exterior hose bib. Most of the time, the shutoff valves are usually in the ceiling of the basement. The valve itself will be one of two types:
Quarter Turn Ball Valve
The quarter turn ball valve looks like a lever handle for a door. If this is the case in your home, you will turn the lever a quarter turn so that it is perpendicular to the pipe. Make sure to never use a wrench on the lever. Using a wrench can put too much leverage on it and break the stem valve which will make a bad situation even worse. If the lever will not close by hand, be sure to call a plumber to replace it entirely. Once the valve has been turned, this will shut off the line so that no water will be able to go out of the bib if the exterior bib freezes over.
Twist Knob Valve
The valve could also be just a valve with a twist knob which will look like the one on your exterior hose bib spigot. In this case, turn the valve tightly to the right, clockwise, until it stops and the water will be shut off.
Unheated Crawl Space
In many cases, first floor kitchens or bathrooms are built over unheated basement crawl spaces. If you have this type of crawl space, the pipes beneath it can freeze. To prevent this situation from happening, you will need to run one of your sinks in the room over the crawl space. Allow slowly dripping water to flow through the faucet so that it will make it much harder for the water to freeze. The water flow is similar to a flowing river taking longer than a lake to freeze in the same temperatures. Just the smallest movement of water will make the chances of the pipe freezing much lower.
If the room above the crawl space is a bedroom or office, the crawl space itself should not have any pipes in it. Double check, however, because there is always the chance that pipes are there which go to a second floor bathroom, and in that case be sure to slowly drip water from that second floor sink.
In general, all of this should also be done if you are going away for a winter vacation so that you don't come home to a water emergency and a huge and expensive mess.
Brad Gosselink has worked in residential carpentry for almost 20 years.