Skip to Content

When to Close Crawl Space Vents? A Brief Guide

When to Close Crawl Space Vents? A Brief Guide

Share this post:

This post may contain affiliate links. If you click an affiliate link and make a purchase, I may earn a commission. Also, as an Amazon Associate I earn from qualifying purchases.

If you live in an area with high humidity, it’s extremely necessary to have a crawl space in your house structure, which acts as a buffer zone to protect your house from mold and mildew that thrives in high humidity, and that is where crawl space vents are for.

Crawl space vents will help in circulating the air to prevent the formation of mold that can damage the house. Although keeping them open at certain times is quite essential, many homeowners may wonder “when to close crawl space vents?

The weather is the main factor to consider when deciding to open or close a crawl space. Ideally, you need to keep the vents open in hot weather, as humidity levels are high.

But in winter, the humidity levels are pretty low. Since the risk of mold is much lower, you may close the crawl space vents as the temperature falls below the freezing point.

Read on if you want to find out more about crawl space vents, when to close them, and how to do it properly.

How Do Crawl Space Vents Work?

Before answering the main question today, you should first understand how crawl space vents do their job and why they’re considered necessary.

The main point of leaving a crawl space under the house is to separate the house foundations from the damp earth below. It’s also utilized as a buffer zone where you can put utility cables all over the house.

In warm temperatures, the moisture carried by the hot air will rise up and come in contact with your house foundation.

Since moisture build-up is one of the necessary aspects for spores to thrive, mold and mildew might develop, causing a lot of damage as well as health issues, and that’s when crawl space vents come to the rescue.

These vents allow the air in crawl spaces to circulate, which prevents moisture from building up easily, and therefore, reduces the risk of mold and rot in the house.

When to Close Crawl Space Vents?

If you already have crawl space vents in your house that are open, the ideal time to close them would be in winter as temperatures start to dip.

As previously mentioned, there’s a significant association between warm climates and humidity levels.

Ideally, warmer air has a higher capacity to bear moisture. As a result, humidity levels will reach their maximum levels during the summer, especially if you live in a coastal area.

However, during the winter months, the air becomes relatively dry because of its low capacity to bear moisture. Also, cold air will not rise up, so the moisture won’t rise up as well.

Not only that, but in some areas, temperatures will fall well below freezing levels in winter. In that case, leaving the crawl space vent open will cause the pipes running within the crawl space to freeze.

What Temperature to Close Crawl Space Vents at?

As you now know, you should consider closing your open crawl space vent when it’s extremely cold outside.

In other words, you should try to keep them open for air circulation for as long as possible and close them only when there’s a true risk of freezing.

The exact temperature at which you should close the crawl space is quite debated. On one hand, some people associate it with the freezing point of water, which is 32 degrees F or 0 degrees C.

On the other hand, some believe that the isolated nature of crawl space vents makes them colder, and therefore, they should be closed shut when the ambient temperature is around 40 degrees F or 4.4 degrees C.

Since the difference between the two is quite small, it should be okay if you go with either of them, depending on the conditions in your house and crawl space.

When to Open Crawl Space Vents?

The best practice when it comes to using crawl space vents is to keep them open for as long as possible if your pipes aren’t at risk of getting frozen by cold weather.

As a result, many experts believe that you should open back the crawl space vents as temperatures, as well as humidity levels, start to rise back up.

However, there are some misconceptions when it comes to maintaining a healthy crawl space environment, as preventing mold isn’t only about opening the vents in summer and closing them in winter. But more about that in the following section.

Do I Need to Close My Crawl Space Vents in Winter?

A lot of experts believe that the concept of closing and opening crawl space vents is outdated. This is because crawl space vents can only improve the circulation of air inside the crawl space, but it won’t be as effective if the outside air is already moist.

Despite that, crawl space vents are still commonly used in many homes and are still among building codes in many regions. However, building codes require their presence, but you don’t necessarily have to use them.

In fact, there’s a field study that debates this concept, claiming that unvented crawl spaces can be drier in some conditions than vented ones.

Instead, it’s widely accepted nowadays that the best way to keep your crawl space dry and protected from mold is by encapsulating the crawl space area.

How to Properly Close/Seal Crawl Spaces

As previously mentioned, the best way to prevent moisture from finding its way to your house structure is by insulating your crawl space altogether (crawl space encapsulation), which is much better than simply plugging the vents with foam blocks from the outside.

In this section, we’ll walk you through a brief guide that shows you how to properly seal your crawl space, which can actually help you keep the moisture at bay all year round:

Step 1: Inspect the Crawl Space

Before sealing a crawl space, you should always do a quick inspection to make sure that you’re not trapping something that shouldn’t be there.

This includes checking for mold, rodents, termite infestations, etc. You’ll need to fog the crawl space to remove any infestations before continuing.

Additionally, you need to make sure that the house has proper drainage or seek a professional to check the house grading for you before heading on with the encapsulation.

Make sure to also repair or replace any damaged structural elements in the house foundation, especially rotten wood.

Step 2: Seal the Vents and Gaps

Now that you’ve made sure that the crawl space is healthy and ready for insulation, you should start sealing all gaps and openings that lead to the crawl space.

Since you’re sealing them permanently, it’s better to use vent covers to close them than using regular foam blocks.

Next, you should seal the inside walls of the crawl space. For this, you can use insulating materials like taped foam boards for the walls and the ceiling.

Step 3: Install a Proper Moisture Barrier

Whether you already have a moisture barrier on the ground or not, it’s highly recommended that you install a new one for maximum efficiency and odor control. Here’s a quick guide that shows you how to do that:

  1. Apply a thick plastic moisture barrier to the floor and wrap the foundation piers with it to make sure that it curved up the sides of the foundation piers and walls.
  2. Make sure that all the seams and edges are sealed properly with insulating tape
  3. Use a caulk gun to seal any cracks or gaps in the crawl space as well as around pipe joints and holes that lead up to the outside.

Step 4: Install a Humidity Controlling Method

Now that you’ve made sure that the crawl space is properly sealed, you should install a system that controls the moisture level inside.

The easiest way to do that is by connecting the air supply to the HVAC system or by installing a dehumidifier that works all year round to keep the crawl space dry and control humidity within the sealed enclosure.

Step 5: Monitor the System Regularly

Lastly, before calling a job well done, make sure that you install a system that is capable of alerting you if the sealing fails and humidity levels in the crawl space start to build up.

Luckily, there are plenty of humidity monitoring systems out there that you can use to keep the crawl space encapsulation in check.

Final Thoughts

There you have it! A brief guide that shows you when to close crawl space vents and the necessary aspects to check if you want to do it properly.

As you can see, keeping crawl space vents open in summer is quite essential in states where humidity levels are extremely high. Yet, if temperatures fall below freezing point in winter where you live, you should consider closing them during that time.

With that said, some researchers debate that sealing off crawl spaces altogether is a better solution than using vents. However, they’re still a part of building codes in many regions that you have to stick by.


If you want more backyard tips including recipes, how-tos and more, make sure you subscribe to my youtube channel

Share this post: