It's important to monitor the humidity levels in your crawl space. To do that a number of us have placed sensors in our crawl spaces. Making accurate measurements can be difficult because thermometer/hygrometer readings can vary considerably. It also makes a difference where the sensors are located. You probably have a sensor on your crawl space fan and on your dehumidifier, plus your home temperature/humidity sensor. Warm air holds more moisture than cold air, so relative humidity will decrease as temperature increases.

For remote measurements, many of us use Acurite 00611 or LaCrosse 308-148 indoor-outdoor thermometers which are easy to find. The LaCrosse seems to have a short sensor battery lifetime. If you have a dehumidifier in your crawl space, place the sensor in your crawl space away from the dehumidifier air outlet. These indoor-outdoor devices have quite a range, so make sure you are reading your own sensor and not your neighbor's.

• To measure the relative humidity in the vicinity of the door fan, turn the fan on for 15 minutes. Then turn the humidity control on the fan back and forth until the fan just turns on.

• The indoor-outdoor thermometer probably gives the best temperature and humidity readings.

• The readings from your dehumidifier console depend in part on the model but will probably give values in the physical vicinity of the air intake.