Should Crawl Space Vents Be Open or Closed?
Making the decision to open or close crawl space vents isn’t such a breeze
Highlights
Closing your vents prevents airflow, which can retain heat.
Open vents move air but lack security from outside effects.
Secure your vents with vent covers to prevent pests.
Fully encapsulating your crawl space is the most expensive but best option.
Wondering if you should leave your crawl space vents open or closed? You’re not alone. Even many long-time homeowners and DIYers are curious about the effects an open vent has on their crawl space. Will it invite unwanted guests? What about rain? Will a closed space save money on utility costs due to temperature retention? Read on to see why there are different guidelines for different situations.
Is It Better to Leave Crawl Space Vents Open or Closed?
It’s better to keep your crawl space vents closed rather than open. Keeping them closed seals them off from the outside elements while leaving them open allows moisture to enter your crawl space. This can increase the risk of mold growth and mildew in your crawl space, which can lead to further issues down the road.
When to Leave Your Crawl Space Vents Open
Conventional wisdom says to open your vents when winter ends and warm weather moves in. This gives your house the ability to circulate air through the base of your home, which—we are assuming if you’re reading this—is a crawl space. This is one of a few tips you should follow as part of a summer home improvement routine. Other reasons for opening your crawl space when the weather warms are listed below.
Gas Transfer
This one may seem a bit strange, but houses produce a lot of different gases. Opening your crawl space vents helps to clear the air below your house. Opening vents also prevents the buildup of natural gas if a pipe is leaking. As the weather warms, hot air rises, and with it the potential of passing dangerous gases through the floorboards. Bringing in fresh air helps to dissolve any dangerous concentration of gases.
Pest Management
To address the saying “nests mean pests,” it’s good to have fresh air flowing through your crawl space. Animals are similar to people in that they will seek out shelter from the elements, which a crawl space can give, especially in summer months when a crawl space can be significantly cooler than outside temperatures.
Ventilation Control
Ventilating your crawl space can be expensive, or you can simply open the vents. New air moving through your foundation prevents mold in your crawl space, which will eventually spread to your home or require a new foundation entirely. Air quality in your home should improve as a result of opening the vents.
Heat Transfer
This one is less about the physical structure of your crawl space but about the comfort of your home and your monthly bills. Crawl spaces typically aren’t insulated as well as livable spaces in a home and can quickly become a heat sink. Hot air rises, and a sweltering crawl space means your air conditioning has to work extra hard.
When to Close Your Crawl Space Vents
Closing your crawl space vents when the weather turns south has benefits as well, often the complete opposite of those listed above. Although the reasons are different, the purposes are the same: Keep the air the temperature you want and keep your crawl space dry.
Heat Retention
Just as opening vents in the summer can cool your house, closing them in the winter can help keep it warm. Cold air sinks and crawl spaces are often built slightly below ground level, so open vents become pits of chilly air.
By closing your vents, you are sealing in any warm air that leaks down through your floor or any heat generated by appliances like water heaters or furnaces. Also, closing your vents can keep your pipes from freezing or bursting in cold climates.
Periods of Heavy Rain or High Humidity
Serious rain can cause water to sit on the ground for days, producing humidity. Depending on the ventilation in your crawl space, it can act as a suction, pulling the humidity under your house. Excessive and prolonged humidity can cause all sorts of problems, so you’ll want to keep your vents shut during heavy downpours. If the budget allows, you may want to consider a full crawl space encapsulation.
Energy Efficiency
If you can seal off the area beneath your home when it’s bone-cold outside, that’s what you’ll want to do. Your house itself will produce heat, and the worst-case scenario is any wasted heat being sucked into an unlivable space, such as your crawl space. You may need to add vent covers to keep as much heat trapped as possible when the thermometer drops.
Severe Weather
Fire creates smoke, and you don’t want that smoke under your house rising through your crawl space into your home. Although this is a rare instance, consider closing your crawl space vents when anything strange is happening. This could be smoke from a fire, severe rain or flooding, or any other random weather event.
Benefits of Opening or Closing Crawl Space Vents
Open your vents to aid in air circulation, pest control, and gas mitigation. In contrast, you should close your vents when you want to keep your house slightly warmer in the winter, block humidity from entering, and prevent mold from building up under your house.
If you haven’t been in your crawl space in some time, you may find some surprises—not the good kind—and should absolutely talk to a foundation repair specialist near you.
| Benefits of Opening | Benefits of Closing |
|---|---|
| Promotes air circulation | Keeps your house warmer in cold seasons |
| Provides pest control | Blocks humidity |
| Gas mitigation | Prevents mold growth |
| Prevents mildew growth in hotter months | Promotes energy efficiency |
| Can prevent costly repairs |
How Do You Open or Close Your Crawl Space Vents?
To close your crawl space vents—especially for colder seasons—you can use foam blocks to plug them up from the outside. It's important, though, that you use the type of foam blocks designed specifically for this purpose. Opening your crawl space vents depends on their exact design, but there is often a lever or dial on the side for opening.
Frequently Asked Questions
Most houses will have vents in the crawl space. This allows for air passage and energy retention, which in many ways can help control the temperature inside the home. If you are able to consider the cost of a crawl space encapsulation, you may not need your crawl space vents at all.
The best ventilation for a crawl space is a fully encapsulated crawl space with its own separate ventilation system. If you go this route, ensure you have the proper crawl space code requirements done and dusted before encapsulating your crawl space with a moisture barrier and installing appliances that may not be up to the proper specs.
It is better to have a crawl space that is tailored to the outside temperature, your needs, and the season. For example, it is much better to have a vented space when it’s very hot outside and your AC bills are sky-high. Conversely, when the temperature drops and humidity rises, it is almost always better to have your vents shut and covered if possible.
If a crawl space isn’t vented, there can be moisture buildup, which can lead to mold damage, easily destroying the foundation of your home. A closed-off crawl space can trap heat, which can be useful in the winter, but can also trap dangerous gases and prevent clean air from entering the home through the floorboards.