Okay, so why do we need to close the vents in the crawl space? First of all, please understand that the building codes in most areas still say that leaving the crawl space vents to your crawl space open is a best practice.
They are ultimately giving the builder or homeowner the choice of whether they want to create an encapsulated (or closed) system versus an open system. If you decide to open it, they have precautions you need to take. The same goes for if you decide to close it, however, we recommend closing the crawl space. One of the reasons why is because there is no way to control external factors like humidity.
Imagine spending the afternoon in your house as usual. It’s 90° outside and you live in a humid south climate. You have the AC on and it has brought the temperature down to 72 degrees inside the house. You decide to go and open all the windows around your house. Soon enough, all of that hot and humid air will overtake your house and it will no longer be at 72 degrees!
As a matter of fact, your vents have become cold from the cold air blowing through them, and now they are building condensation. This is because the hot, humid air is passing over the cold metal. This being said, we always recommend closing the crawl space and controlling the ventilation of the crawl space. You can control the humidity with a dehumidification system. It is one of the only real ways to control humidity while ventilating a crawl space.
Builders are still building open crawl spaces in areas that the codes have not yet been changed. These codes once reflected that crawl space vents were necessary to keep homes cool. This was true back in the times before families had air conditioning, but has since changed.
Whenever you have a heating and air unit downstairs, you have hot, humid air coming into the crawlspace hitting a cold duct. This is when the condensation causes a problem in the crawl space, essentially leading to mold and high humidity.
Fortunately, there is an option of keeping your crawl space either open or closed in most places. Of course, it’s recommended to close the crawl space for all of the reasons mentioned above. Hopefully this article has helped you to understand the reason for closing the crawl space vents during an encapsulation job.