Ways to Get Rid of a Musty Smell in a Crawlspace
Run a Dehumidifier in the Crawlspace
To help get rid of a musty smell in a crawlspace when high humidity is a problem, run a dehumidifier in the crawlspace. Set it up to automatically drain into the sump pump pit. Sure, it will pull some of the moisture that comes from the pit, but it will serve double duty. The sump pump will drain when the water that is pulled out of the air reaches a certain level. It will help keep the water in the pit from becoming stagnant and smelly because it will automatically drain more often.
Add Water to the Sump Pump Pit to Drain It
If you do not plan to run a sump pump directly into the pit, occasionally add clean water. During the driest months of the year, a sump pump will not run nearly as often. The water left inside will become stagnant, and it will smell musty. Pouring fresh water into the pit will force it to drain. If smelly water seems to be the source of the problem, add enough to force it to drain every three days. With the foundation vents open and the pit clean and clear, the crawlspace should no longer be the source of a musty smell.
Precautionary Statement and Additional Suggestions
When in doubt, have the crawlspace professionally checked for mold, sewage and sump pump problems. Also make sure that foundation vents are open during the warmer months of the year. They will help keep the air beneath the house from becoming stale and musty. If opening and closing the vents is a problem, opt for automatic varieties that will open and close when temperatures reach certain levels. They are surprisingly inexpensive.
Source: Personal Experience with a Musty Crawlspace and a Working Sump Pump