Should Crawl Space Vents Be Open Or Sealed?

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The vents in a crawl space let in fresh air from outside to circulate, thus reducing moisture build-up. You may be wondering if the vents in a crawl space should be opened or sealed. We talked to the experts, and this is what we discovered.

Proper ventilation prevents moisture build up in a crawl space. Therefore, it is advisable to open crawl spaces when the air outside is cooler than inside and close the vents when the air outside is warmer. Cool air carries less moisture; hence, it lowers the moisture content in a crawl space as it circulates. On the contrary, warm air has high humidity content, which would have adverse consequences if allowed to flow into a crawl space.

You now know whether to seal the crawl space vents or leave them open. Keep reading as we elaborate on when to open and close the vents. We will also discuss the ventilation requirements for a crawl space, how to ventilate the crawl space, and exemptions for having ventilation openings in your crawl space.

Wooden framing and floor joist under a crawlspace of a small house, Should Crawl Space Vents Be Open Or Sealed?

When should crawl space vents be opened and closed?

Proper air circulation is critical to keeping a crawl space free of moisture. It is advisable to keep the crawl space dry since moisture can cause wood to rot, resulting in compromised structural integrity.

High humidity levels can also cause mold and mildew to grow, resulting in severe respiratory complications and allergic sensitivities as people inhale the contaminated air.

Although crawl space foundations are common in homes in warm, moist climates, they may trigger more problems than they are intended to solve if not properly maintained. Therefore, it is crucial that you open and close the vents in a crawl space at appropriate times to keep the moisture out and the space well aerated.

Home inspector checking for termites under the crawlspace of a house

Daytime Ventilation

Since warm air carries a higher moisture content, it is advisable to shut the vents during the day in summer. If the vents are left open, the warm and humid outdoor air will flow in and condense on the walls of your crawl space.

Further, because warm air rises, this air will also condense on the ceiling of your crawl space, which also doubles up as a floor for your home. Thus, your floors will be very cold and may also buckle as they absorb moisture if they are wooden.

Nightime Ventilation

You may open the vents at night when the outdoor air is cooler than the air indoors. The warmth in the crawl space will cause cold air to expand. Because the cold air holds less moisture, it dries up and siphons moisture as it circulates the crawl space. As a result, humidity decreases.

Winter Ventilation

You may also open the crawl space vents during the day in winter since the outdoor air is cooler than the indoor air. Just like allowing the cool summer air into the crawl space at night, opening the vents during winter will lower the humidity level.

However, it is important to control how long the vents are left open during winter to keep energy bills from sky-rocketing. If the vents are left open for long, it will be too expensive to heat the home.

It is not advisable to open the vents if the temperatures are too low, as this may cause pipes in the crawl space to freeze. Pipes may burst when they freeze, causing high repair and maintenance costs. It will be ideal to shut the vents if the temperature drops below 40 °F.

What are the crawl space ventilation requirements?

A tight crawlspace under a house

The condition of a crawl space has a significant effect on the comfort of your home and energy consumption. When left unattended, the crawl space can negatively affect the safety and value of your home.

The International Residential Code outlines the requirements for crawl space ventilation, which are intended to safeguard the health and safety of all inhabitants in a home. To comply with the outlined regulations:

  • It is advisable to ensure that the crawl space has one square foot of screened ventilation opening for every 150 square feet of space.
  • Cover the floor with an approved vapor barrier material to minimize moisture levels in the crawl space.
  • Provide one ventilated opening in each corner of the crawl space to encourage cross-flow of air. The opening should be within three feet of each corner. The openings allow air to flow in from multiple directions and evenly aerate the crawl space. The openings should be positioned to catch the cool air breeze and eject the warm, humid air from the crawl space in the opposite direction.
  • Use materials that allow a restricted airflow, sufficient to get rid of moisture and maintain moderate humidity levels. Therefore, expanded sheet metal plates with a thickness of at least 0.047 inches, cast iron grill, corrosion-resistant wire mesh that is at least 0.125 inches thick or sheet metal plates with holes that are at least 0.07 inches thick may be used.

You may also be interested in finding out whether cellars need ventilation. If so, read this article: "Do Cellars Need Ventilation? [Including Wine & Root Cellars]"

How do you ventilate a crawl space?

Fully suited worker going under the crawlspace of the house

You can ventilate a crawl space either by allowing air to circulate in the crawl space or by completely sealing the crawl space and regulating aeration and moisture content inwardly.

Natural ventilation is unpredictable and unregulated, and consequently unreliable. This is because it depends on many factors that are not in your control, such as outdoor temperatures and the direction of the wind, among other factors.

Moreover, you would need to install the ventilation openings strategically to facilitate cross ventilation. It may also result in very high energy bills in a bid to control the indoor temperatures in your home.

It is more efficient to ventilate a crawl space by sealing it off completely and mechanically regulating the internal atmospheric conditions. To do this, you may opt to:

  • Move air from the crawl space to the outside using a sizeable exhaust fan strategically positioned in the crawl space.
  • Direct some of the heated air from your home's HVAC system to the crawl space. This will aid in keeping the crawl space dry. However, this will only be effective if the crawl space is encapsulated. Encapsulating the crawl space is whereby the floor of your crawl space is covered with a moisture barrier, the foundation vents are sealed completely, and the crawl space walls are properly insulated.
  • Install an energy-efficient dehumidifier, ideal for the size of your crawl space. Calculating the area of your crawl space will help you determine what capacity of a dehumidifier you should invest in.

This energy star certified dehumidifier effectively eliminates moisture from your crawl space without inflating the energy bills, thus saving money. Check it out on Amazon.

Read this article to find out "When To Run A Dehumidifier In The Basement? [And How Often]."

Exemptions To Ventilating A Crawl Space

The International Residential Code provides exemptions for installing ventilation openings in crawl spaces in the following circumstances:

  • Where you substitute natural ventilation with mechanical ventilation, circulating air artificially between the crawl space and the conditioned area of the home.
  • Where you use an air circulating device that moves at least one cubic foot of air for every 50 square feet of crawl space.
  • Where the floor of the crawl space is completely sealed with a moisture-resistant material, which overlaps at least six inches of the inner foundation wall.
  • Where all seams are sealed such that moisture does not seep in.
  • The crawl space is properly insulated to prevent any heat loss.

Final Thoughts

Wooden framing and floor joist under a crawlspace of a small house

Crawl space vents are important in controlling moisture build-up and, consequently, eliminating mold and mildew growth and rotting of wood that may crop up due to high humidity levels. It is advisable to open the crawl space vents in mild climatic conditions when the outdoor air is cooler than the air indoors.

Ensure that you comply with the International Residential Codes when determining whether to open or seal the foundation vents. Further, if your area's local building codes are different from the International Residential Code, ensure that you abide by the stipulated codes.

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