Crawl Space Ventilation Systems

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 as an example) 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. The quantity of venting is based on the air exchange rate code applicable to code at the time of construction. Today we try and achieve an air exchange rate of 4 exchanges per hour. 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 due to many variables such as soil composition, natural springs, sub-standard drainage systems, and many other reasons.

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. They are set-up to allow for cross ventilation which assumes no other structures or landscaping impeding air flow. There is also the condition in every structure called the stack effect, which is the natural flow of air moving from under the structure up through the attic or roof. So, air from the crawl space or sub-structure carries throughout the house or building from bottom to top. (EXAMPLE: How does fireplace smoke travel? UP…out the chimney). In the winter months, open air vents let cold air in while at the same time causing heat loss from furnace and furnace ducting 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. In both seasonal extremes there is a potential for mold growth as well.

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.Runs continually, low power draw, pulls in outside air, vents out crawl space air, inexpensive, best at exchanging air flows.

Encapsulate the Crawl Space and Close the vents!

Encapsulation of the crawl space is the process of sealing off the soils and insulating the foundation walls, the largest sources of moisture entering the space in the first place. The heavy and thick vapor barrier permanently separates the soils from the house and the occupants from the moisture and gases. Limiting the bulk of the moisture and gases is the primary focus, then ventilation becomes an enhancement to ensure a stable environment. A healthy crawl space always starts with ensuring there is proper drainage on the exterior of the house and drainage inside the crawl space if needed to prevent the build up of seasonal groundwater or allowing a conducive condition that will bring unwanted conditions to the home.

Once the soils are sealed, a homeowner (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. In our region there are 3 standard methods of maintaining code compliance. Ask your field rep for details.

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 without discussing with someone familiar with building code and more importantly someone who deals with these complicated issues on a regular basis. To achieve the most beneficial results is a combination of drainage, encapsulation with concrete or a liner system, closing air vents, and maintaining code compliance within the parameters of air quality requirements please reach out to our office.

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