I am helping this home owner fix a problem left for them by the previous owners of a newly purchased home. In our mixed humid climate, crawl spaces should not be vented, but it has been done for years by builders that do not understand building science. This home had the added feature of a fan pulling in warm moist air into a cool crawl space. What you can see in this picture below is water pulling the insulation apart as it actually is raining in the crawl space from moisture condensing on the cool surfaces. While not every vented crawl space is wet, almost all have enough moisture to allow for mold growth. A vented crawl space in a home with a forced air HVAC system usually pulls the make up ‘fresh’ air from that crawl space into your living space. Do you know if your indoor air quality is healthy? Based on my experience, if your attic or crawl space is vented, you probably could improve your living environment through some simple steps.