Where should a freeze sensor go in a crawlspace?
On or right next to the coldest water supply line, not at the access hatch. That is usually near a foundation vent, along the rim joist on the windward side, or where the supply line comes through the foundation. A waterproof probe taped to the pipe measures what the pipe actually feels.
Find the coldest pipe
Cold air enters through vents, gaps at the rim joist, and around the pipe penetration. The supply line nearest those spots freezes first.
If you are not sure, put two probes out for a week and compare them on a cold night; the one that dips first is where the sensor belongs.
How to mount it
Use a waterproof probe such as a DS18B20 on a cable. Tape it to the pipe and wrap the pipe insulation back over it so it reads the pipe, not the air.
Keep the board and power supply off the ground in a dry spot; crawlspaces get damp. A second probe in the open air is useful context.
Add a leak contact while you are down there
A wet/dry contact on the floor under the main line catches a slow leak or a thaw after a freeze, which is often when the damage shows up.
Frequently asked questions
Is a crawlspace humidity sensor enough to catch freezing?
No. Humidity helps spot moisture problems, but freeze risk is the temperature at the pipe. Use a temperature probe on the pipe.
Should crawlspace vents be closed in winter?
In freezing climates, many homeowners close or cover foundation vents for winter to keep cold air off the pipes. A stuck-open vent is a common cause of crawlspace freezes.
Related
Get a freeze alert for your space
ProbeHarbor watches probes in garages, crawlspaces, and cabins and alerts you before pipes freeze. The free plan includes email alerts; bring an ESP32 probe or connect Home Assistant.