Dehumidification and Active Ventilation in Closed Crawl Spaces – A Closer Look

Dehumidifiers and ventilation fans are certainly a good idea in finished living areas of the home. Living spaces with carpet, bedding, drywall and traditional insulation contained within wood framing are good examples. Keeping air moisture to a minimum (50% RH and lower) will minimize mold growth and dust mite colonization – leading to improved indoor air quality for your clients.

However, encapsulated crawl spaces are a separate issue. Properly sealing walls, floors, pipe penetrations, foundation vents and eliminating drafts will turn any crawl space into the driest room in the home.

Dehumidifiers are costly to purchase, expensive to run and present a fire hazard. If installed in a “closed crawl space,” they will certainly remove moisture from the air; however, this moisture is actually “drawn down” from the living areas above! Laws of physics tell us that moisture always moves from areas of high pressure to areas of low pressure (always wet to dry; never dry to wet). When viewed in this light, it clearly makes more sense to install dehumidifiers in living areas of the home, especially in units containing filtration systems that filter the air.

Active ventilation systems (power fans) that pull already conditioned air (heated or cooled) from living spaces above the crawl space and expel them outside are also a poor choice for encapsulated crawl spaces.

Fact: “Heavy, moisture laden air” does not settle to the crawl space. It is impossible. The weight of the air has nothing to do with moisture movement. Mother Nature’s hydrological cycle proves that moisture evaporates upward, moving from wet to dry. I contend that these systems are a disservice to the customer. They are expensive to install and very costly to operate, considering the fact that the air expelled has already been heated or cooled.

Note: Dehumidifiers and ventilation systems can be beneficial in low clearance crawl spaces that cannot be properly encapsulated.