Crawl Space Ventilation for the Seattle, WA Area

Why are crawl spaces vented?

All dirt surfaces under your home, in the crawl spaces, are required by local building codes to have “ventilation” to vent out moisture and gases (like Radon) that emanate from the soils. Ventilation is achieved in the form of openings (air vents) in the side of the building or in the concrete foundation walls. In theory the outside and crawl space air exchange with one another naturally.  To aid this process and contain some of the moisture, the dirt is covered with a code compliant plastic vapor barrier. But in many cases this type of construction unknowingly fails many home owners.

Are air vents bad?

They probably are about as good as it will get, but there are a few details to consider. Air vents are “passive”, so there must be a force, like a change in the barometric pressure (the weather) or wind to move the air. There is also the known condition in a home called the stack effect, which is the natural flow of air moving from under the house up to the attic or roof, so air from the crawl space carries throughout the house from bottom to top. (How do you think a fire place chimney vents out the top?). In the winter months, open air vents let cold air in while at the same time causing heat loss from furnace and ducting equipment located under the home. If you live in a humid climate, summer humidity enters the crawl space potentially causing water and rot damage to wood and structural components.

Why Mechanical Ventilation?

Mechanical ventilation is one way to actively move the air out of your crawl space and effectively have a cleaner healthier home in the process. A few products to consider are listed below.

Unit Pro Con
Low Power Fan Runs continually, low power draw, pulls in outside air, vents out crawl space air, inexpensive, best at exchanging air flows. Can pull in more air in the winter than is normal causing house to have to produce more heat. Needs proper crawl space insulation.
More air vents Easy, common, increases natural air flow. Increases heat loss, requires more insulation, greater chance for rodents to take up sanctuary in the house.
Dehumidifiers Dries out air (must have closed vents and encapsulated moisture barrier), helps to limit mold spore reproduction. Expensive to run continuously, may mask the source of moisture, does not “exchange” any fresh air, recycles the same air, not needed in certain climates (Pacific NW).

Encapsulate the Crawl Space and Close the vents!

Encapsulation of the crawl space is the process of sealing off the soils and foundation walls, the largest sources of moisture entering the space in the first place. The heavy and thick vapor barrier permanently separates the house and the occupants from the moisture and gases. Limiting the moisture and gases is the main concern, then ventilation becomes a secondary concern.   A healthy crawl space always starts with insuring there is proper drainage on the exterior of the house, or drainage inside the crawl space to prevent the build up of seasonal ground water. This may be all that someone needs.

Once the soils are sealed, a home owner (or contractor) can close air vents and install any form of mechanical ventilation to promote positive air flow and fresh air entering the space. Energy code and insulation requirements must be met at every step of the process, before sealing off air vents.

Newton’s Third Law of Motion

For every action, there is an opposite and equal reaction. So only performing one component of this process may not be sufficient and could result in harm to your home or health. You should never just install ventilation or just close your air vents. To achieve the most beneficial results is a combination of drainage, encapsulation with concrete or a plastic liner, closing air vents, and installing mechanical ventilation.

Contact Us

Want to know more about your Bellevue, Seattle, or Tacoma, WA, crawl space and the benefits of encapsulation? Call today for an in home inspection and repair estimate. 206-233-9003

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PermaDry Waterproofing
608 SW 12th Street
Renton, WA 98057


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