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When someone asks whether sealing crawl space vents is a good solution, he understands the stack effect and how mold and moisture are a deadly combination in a crawl space. However, if sealing the crawl space vents is his only solution, he will only make the problem worse.
In the summer, warm humid air enters crawl space vents and creates condensation on colder surfaces—the dirt floor gets wet, cold air ducts get wet, wood surfaces and insulation get wet and begin to mold or rot. In the winter, a vented crawl space lets in cold air under your feet and creates high energy bills, cold floors, and uncomfortable drafts. Venting a crawl space doesn't solve the musty crawl space problem, because it doesn't get rid of the moisture and humidity that causes the problem in the first place.
The same goes for sealing the vents—if all you do is seal off your crawl space vents, you effectively seal the condensation, humidity, and mold inside your crawl space. That's because you've done nothing to dry out your crawl space. There are two things you must do before you ever seal the crawl space vents: fix water leakage and isolate your house from the earth.
Because you are trying to completely dry the crawl space out, you'll need to start by eliminating groundwater seepage. An efficient drainage system and high quality sump pump system with an airtight lid is essential. You want the water to get out, but you don't want an open hole with water sitting in it that can evaporate back into the crawl space. The pump will need to be powerful enough to keep up during heavy rain and pump water away from your home. When you have a plumbing leak, you'll want to know about it; a built-in floor drain and alarm system will allow the sump pump to accept and drain water while alerting you to call your favorite plumber.
To eliminate vapor transmission through foundation walls and the dirt floor, install a strong and durable vapor barrier. A thin plastic liner is not enough. For example, 6 mil polyethylene rips easily and gets holes poked in it when you crawl on it. It doesn't seal off the dirt floor, and your plumber's or HVAC service technician's belt buckle will likely tear such a thin liner. Instead, install a heavy and flexible vapor barrier to stop water vapor and allow for drainage to the sump.
Only after you've fixed water leakage and encapsulated the crawl space should you seal the vents and other air leaks. You'll want to cover the vents, of course, but look for other sources of air leaks around the rim joist, sill plate, and pipe or cable penetrations. A certified building analyst can help you locate and air-seal leaks around your foundation and throughout your home.
Once you've completed all of the above, you can effectively control humidity and eliminate condensation in the crawl space with a powerful dehumidifier. A word of caution, however, as a household dehumidifier won't do the job. It's just too small—the unit is too small, the cold coil is too small, the fan is too small, the bucket is too small (and has to be manually drained), and the unit can only wring out a small amount of water. Install a powerful and energy-efficient dehumidifier that can dehumidify a naturally-damp environment, filters the air, and drains automatically.
So, what you're really asking when you inquire "Can I seal off my crawl space vents?" is "Will sealing my crawl space vents solve my musty crawl space problem for good?" The short answer is "No." Fix the water problem first, and then seal the crawl space and the vents. You'll be much happier—and healthier—with the results.