When Should You Open the Vents Under Your House?

The vents at the base of your house are known as foundation or crawl space vents. Many houses come with custom foam blocks that fit into these vents so that they can be blocked or opened. Opening and blocking your vents at the correct times throughout the year can save energy and keep mildew from building up in your foundation.

These foundation vents are very small, and you may be wondering what purpose they serve. In fact, they do not help so much with air flow as with moisture displacement. Soil, even dry soil, can contain a lot of moisture. This moisture can seep into the crawl space from underneath, driven there by pressure and temperature differences. Moisture inside a crawl space is negative: it can rot wood, crumbling cement supports, and creating a breeding ground for mold. Foundation vents are designed to open passageways between the crawl space and the outdoors so that the moisture can escape back into free air, and the crawl space and air outside can be balanced. Thus, it's important to know when to open and close these vents.

If the vents allow humidity to equalize with outside air and prevent the build-up of moisture, along with mold and rot, then why are foam blocks used to close off these foundation vents? The problem is that you must heat and cool your house as the climate changes throughout the seasons. In colder months, when you heat the air in your house, the heat eventually travels to your crawl space, where it escapes through the foundation vents. This creates an energy drain on your home that costs money over the long term.

To avoid this problem, block up your vents in the winter, when you begin using your heat pump more frequently. Unblock the vents during spring, when the days start to get warmer and you use heat less, so that the moisture buildup can escape back into the air.