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Closing Crawl Space Vents in the Winter - Good or Bad Idea?

Sealing the crawl space can prevent mold and mildew, make floors warmer, and reduce heating and cooling bills, according to building scientists. Here, learn how to protect your crawl space from cold temperatures.

Crawl space vent cover

Airtight crawl space vent covers can stop heat loss in the winter when combined with encapsulation and insulation.

Many homes with a crawl space have a vented crawl space. You can think of a crawl space vent, or foundation vent, like you would a window upstairs. Opening the crawl space vents allows outside air into the crawl space; letting in hot, humid air in the summer and cold air in the winter.

If you have a crawl space that’s not adequately sealed and insulated, you could be wasting money on heating and cooling (and creating an inviting area for mold, mildew, dust mites, insects, and rodents). And as the temperature drops and you turn up the heat, you might wonder about the benefit of those openings between the outside and your crawl space.

Closing crawl space vents in the winter is a good idea, especially if you have plumbing or air ducts in the crawl space. Unfortunately, just closing or covering crawl space vents for the winter isn’t the cure-all for cold floors, frozen pipes, and pests. Keep reading to learn what other protective measures are recommended to winterize your crawl space.

How to properly close crawl space vents for winter (and year-round)

Research by building scientists and real-world applications have shown that a properly closed crawl space (also called sealed, unvented, conditioned, or encapsulated crawl space) can provide better moisture control than conventional, vented crawl spaces. Encapsulated crawl spaces are isolated from the outdoors and upper living spaces. Homes with closed crawl spaces often experience fewer drafts, warmer floors, less humidity, and fewer pests. Homes with plumbing and HVAC systems located in a sealed, insulated crawl space typically benefit from greater energy savings and a lower risk of burst pipes.

Closing a crawl space, a process that’s referred to as crawl space encapsulation, consists of a few key steps:

  • Close off foundation vents with airtight vent covers that help keep out insects and rodents that often squeeze their way through poorly screened vent openings. Crawl space vent covers are installed over the outside of the vent opening; covers with built-in weatherstripping ensure a tight seal. CleanSpace crawl space vent covers are made of heavy-duty PVC that won’t rot like DIY plywood covers.
  • Seal and isolate the crawl space from the outdoors and inside of the home by air sealing and adding a vapor barrier over the foundation walls and structural supports inside the crawl space. All cracks, holes, and penetrations leading inside the home or crawl space should be sealed with caulk or foam. The CleanSpace vapor barrier is rugged and designed to stand up to foot traffic, making it a superior option if you plan to use your encapsulated crawl space for storage.

    If you do seal your crawl space, there are a couple of things to be aware of:

    At least a 3-inch gap should be left along the top of the masonry foundation wall to allow for future termite inspections. Check your pest control contract or contact your pest management professional if you’re considering crawl space encapsulation for your home.

    Homeowners and contractors should be aware of what’s inside the crawl space before you cover the vents and seal off the space. If you have combustion appliances in your crawl space, including a furnace or water heater, check the International Residential Codes and local codes for safety requirements. You should also test for radon and install a mitigation system if necessary.

  • Insulate the crawl space with insulation that’s resistant to damage from water and moisture. Crawl space experts typically use rigid foam insulation on foundation walls and inside vent openings. Existing fiberglass batts that are wet, compressed, falling, or have evidence of rodent activity should be removed.
  • Control humidity with a dehumidifier that’s specifically designed for high-humidity spaces, including a crawl space or basement. If you live in a flood-prone area, consider installing a sump pump to reduce your risk of a flooded crawl space.

Are crawl space vents ever necessary?

Open crawl space vents do more harm than good

Unfortunately, the answers to the questions of whether crawl spaces need vents and when the vents should be open aren’t simple. Crawl space vents don’t keep the crawl space dry. This was the original thought behind venting crawl spaces.

In states with humid climates, moist outside air coming in through the vents condenses on the cool surfaces inside the crawl space, contributing to mold, mildew, and rot. Musty odors and bouncy floors are common signs of a moisture problem in homes with open crawl space vents.

More recent research from building scientists explained that crawl spaces should be sealed, insulated, and outfitted with a vapor barrier to control moisture, improve comfort, and decrease energy use. The process of sealing the crawl space includes using airtight crawl space vent covers to block the vent openings.

Some building codes still require foundation vents

Building codes in some states still require foundation vents to be installed. Contractors who specialize in crawl space encapsulation can recommend the best solution to seal crawl space vents to stop heat loss in the winter as well as moisture buildup that can lead to mold growth.

Special flood vents are required in high-risk flood zones

Although normal crawl space vents aren’t necessary, special foundation vents are required in certain high-risk flood areas. The NFIP Regulations and Building Codes require the installation of flood vents in any residential building constructed in Flood Zone Type A with an enclosure, including a crawl space or garage. Flood vents prevent the buildup of hydrostatic pressure, caused by floodwater, which can damage the foundation or worse, cause the building to collapse. The mitigation technique, called wet floodproofing, allows floodwater to flow freely through the flood vents into the enclosure.

If you’re unsure of your property’s flood risk, check your community’s flood map by visiting the FEMA Flood Map Service Center and searching using your address.

Closing crawl space vents isn't just a good idea in the winter, it's a critical step toward creating a more healthy, comfortable home year-round. Even if you aren’t planning on using your crawl space for storage, consider installing crawl space vent covers along with sealing and insulating your crawl space to create warmer floors, prevent frozen pipes, and manage your heating costs. Your crawl space repair contractor can explain the local requirements for closing your crawl space and provide you with a free crawl space encapsulation estimate.

Updated on October 25, 2021

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