Should You Permanently Close The Vents In Your Crawl Space?

Should you permanently close the vents in your crawl space? This is a question often debated by North Carolina homeowners, since most houses in this region are built with crawl spaces rather than basements. Homeowners most often seal their vents due to excess moisture in the crawl space, which can lead to a multitude of expensive problems (including bowing floor joists, mold and insect infestation). There are a number of important considerations here, and remediating the issue may require other steps aside from closing your vents.

During the winter months, leaving the vents open reduces the energy efficiency of your home by allowing cold air to leak in from the outside. Also, if temperatures routinely drop below freezing, pipes in the crawl space are more likely to freeze if the vents are open. One school of thought has always been to close vents during the winter and open them during the summer to allow moisture to escape. However, it’s not quite that simple.

During the summer, warm and moist air can enter your crawl space from the outside, leading to condensation and water pooling. Since insulation, floor joists and ducts are porous and readily wick water vapor from the air, your crawl space can quickly trap large amounts of moisture. This makes your crawl space an attractive nesting place for vermin, including termites and other wood-destroying insects. If you keep the interior of your home cool in the summer, this can increase the amount of water pooling in the crawl space due to condensation. While leaving the vents open theoretically creates an escape route for moisture, a cool crawl space may in fact be drawing outside moisture in through the vents. If you live in a region where the relative humidity is high for most of the year, chances are you would benifit by completely sealing your crawl space. {Note: See Vapor Bariier below}

Drainage Issues

Another factor to look at is the slope of your property. If water runs downhill toward the foundation, this will compound the moisture issue and it may require a foundation specialist. Ideally, the land should slant downhill away from the foundation with at least a 5% grade. Another common issue: crawlspace vents are sometimes too close to the ground, which permits water to enter the crawlspace. This is easily addressed by adding a window well.

Finally, make sure that your gutters are kept clear of leaves and debris, as clogged gutters can lead to additional water buildup around the foundation of your home. I don’t recommend splash blocks for down spouts; in my experience, they don’t carry water far enough away from the house. The best solution is to install flexible plastic piping, which should extend at least 4-6 feet from the house. You can bury piping underground, but the discharge end should be exposed to sunlight. Damaged foundation drains are generally best handled by a professional.

Eliminating Moisture from Your Crawl Space

As you can start to see, there are a number of different factors that contribute to moisture buildup. The first thing you need to do is find the moisture source in order to determine a solution. Should you decide to fully seal your crawl space, I recommend heating and air conditioning your crawl space all year round. This might sound crazy, but it will actually save you money. Failing to condition a sealed crawl space will have significant adverse effects over time. You may hear that some people use dehumidifiers to keep a crawl space dry, but this makes no sense unless your crawl space is sealed. If it’s open, you are essentially trying to dehumidify the atmosphere.

Here are a couple of other tools that can help:

• Foam blocks. These can be effective in the winter as a means of keeping cold air out of your crawlspace or if you are trying to seal it in order to use a dehumidifier.
• A vapor barrier or vapor retarder. This is hands-down the single most important improvement a homeowner can make to their crawlspace and should be installed regardless of whether you are sealing your crawlspace or not.  It should cover 100% of the ground and seams should be sealed with tape.

As far as the Triangle region is concerned, I am a believer in closing off your crawlspace vents in the winter and unblocking them in the spring. If the 10-15 minutes this takes annually is simply too much work, you’re better off leaving them open year round than closing them off. As always, you should consult a professional to determine the best course of action for your home.