Best Insulation for a Crawl Space

If your home has a crawl space, you probably don’t pay it much attention. The crawl space, as the name would imply, is uncomfortable to access, and usually hot, sometimes damp, and almost always dirty. Though it might be tempting to deal with your crawl space at a later date, ignoring issues in your crawl space insulation can result in significant problems down the road. 

The air in crawl spaces has to go somewhere, and in a vented crawl space that air usually goes upward through the flooring. The problem lies not in the air that passes through crawl spaces, but the moisture it carries with it. Recognizing and minimizing that moisture is essential in order to avoid the worst outcomes associated with a dirty and unkempt crawl space: lots of mold growth, a damaged crawl space floor, poor air quality in your home, and ineffective insulation.

What is a Crawl space?

Homes may be built on different types of foundations. Typically in the United States, homes are either built on a concrete slab-on-grade, with a basement, or on a crawl space. Crawl spaces create a gap between the ground underneath your home and the floor of your house. Crawl spaces may be as little as a foot off of the ground, or as high as three feet.

While crawl spaces may not be common in every home, they tend to be more common in areas where elevated levels of moisture in the soil and wreak havoc on slab foundations or basements. 

There are two basic types of crawl spaces:

  • Vented – A vented crawl space is one that is not conditioned and where air circulates from outside. This is the most common type of crawl space, but it can also be very problematic. This type of space may need crawl space repair as time goes on and moisture builds.
  • Unvented – Unvented crawl spaces are a conditioned part of your home and don’t circulate air with the outside. These types of crawl spaces are best thought of as a mini-basement. Though there are advantages with an unvented crawl space, they are less common and require more initial labor and materials to construct.

Let’s take a moment to dig into how a vented crawl space is set up since that is the most common type. A ventilated crawl space will typically consist of the ground at the lowest level, which may be covered with gravel, a moisture barrier, or a dirt floor. Above the ground will be a 1-3 foot gap. Above that gap is the subfloor below the first floor of your home, followed by whatever type of flooring you put in. 

Most vented crawl spaces passively circulate air with the outside, meaning a simple opening to the outside is present at various points around the crawl space and no equipment is used to facilitate air circulation. Some crawl spaces are completely vented and open to the outside, but most have only a few venting points around the home. 

Crawl spaces can have some advantages over slab-on-grade foundations or basements. These include:

  • A lower price point. Crawl spaces are cheaper than an unfinished basement and may be cheaper than a slab-on-grade foundation depending on your excavation needs.
  • Crawl spaces give you access to functional systems of your house, including ducting, electrical, plumbing, and insulation. This can result in cheaper major repairs if you have a problem with one of these systems or components of your home.
  • Crawl spaces are used in areas with large amounts of moisture or certain soil types rather than other foundation types that lack venting.
  • Crawl spaces can be installed in colder regions where the freeze-thaw cycle can damage a traditional slab-on-grade foundation. In this instance, a basement would not be ideal and instead, a ventilated crawl space would be the most ideal. 

Moisture and Crawl spaces

One of the most common reasons a crawl space is used rather than a basement or concrete slab foundation is due to the moisture level in the surrounding environment and soil. In very wet areas or coastal areas with sandy soil, a vented crawl space may be necessary to help avoid moisture leaching into the basement or slab. 

While crawl spaces may be used to solve moisture issues, if they aren’t properly set up they can introduce their own moisture problems into your home. 

The problem lies in the temperature difference between the air inside your crawl space and the outside air. During the warmer months of the year, the ground in the crawl space area is colder than the air outside. As outside air enters the crawl space area, it brings with it moisture. That warm, moist air cools when it enters the crawl space until it hits the dew point and forms condensation. 

Modern houses built on crawl spaces place insulation, usually fiberglass batt insulation, in between the floor joists. Due to the fact that the ground in crawl spaces is usually a few degrees cooler than the underside of your floor insulation, condensation will begin to form on the underside of your floor joists and insulation. Over time that condensation will lead to mold growth. Eventually, the moisture in your subfloor may also impact your crawl space floor, particularly if your flooring is something impermeable to moisture like vinyl.

What is the Best Insulation for a Crawl Space?

If you’re looking for the best insulation for crawl space, it’s important to consult with the experts. Here at Attic Construction, we’ve used fiberglass batt insulation successfully in crawl spaces for many years. That being said, it is possible to use other types of insulation material, such as foam, in a crawl space. 

However, crawl spaces have unique considerations that make it a good idea to consult with professionals before any installation takes place. Specifically, in order to eliminate any build-up of moisture in your crawl space, it’s important to effectively manage the moisture levels in that space. Unmanaged moisture in the crawl space will lead to mold growth on your floor joist and insulation material, and that moisture will seep up into the subfloor. This situation can potentially lead to serious damage requiring extensive and costly repairs.

How Should I Insulate My Crawl Space?

If moisture is such a problem, you might be wondering about the best way to insulation crawl space. According to the Building Science Corporation, a building science consulting firm that promotes building best practices, the ideal way to insulate your crawl space is to make it a conditioned part of your home. The best practice is to treat the crawl space as a mini-basement, which involves eliminating venting to the outside, ensuring ground vapor is sealed, insulating and air sealing the walls of the crawl space and treating that space with conditioned air. 

Of course, your home may have a vented crawl space, in which case it may be impractical to switch to an unvented crawl space. In unvented crawl spaces, effective moisture mitigation can be accomplished through the use of a vapor barrier in addition to the fiberglass batt insulation. 

An effectively installed vapor barrier can help eliminate the buildup of condensation on your floor joists or insulation, eliminating the mechanism through which mold growth and damage to your subfloor and flooring occurs. Poor crawl space insulation can be a disaster for you and will ultimately lead to a need for crawl space repair down the road. Find out what type of crawl space your house has and take the necessary steps to complete proper insulation before a problem arises. 

The Importance of Professional Installation

To avoid any costly repairs down the road, it is incredibly important to have your crawl space insulation installed by professionals. Poorly installed crawl space insulation may contain air gaps that allow moisture to penetrate upwards into your home. That moisture will condense, and eventually lead to mold growth in the subfloor, joists, or existing insulation. It may also impact your flooring, particularly if you have flooring such as vinyl which doesn’t allow moisture to pass through. 

It is also important to recognize existing damage and put in place remediation measures before you install new insulation. If you have existing problems with moisture buildup in your crawl space, there may be more work to do before you can begin installing proper insulation. Keep in mind that working in a crawl space is difficult. Crawl spaces are usually cramped, tight spaces that may contain exposed plumbing, electrical, and ducting you’ll have to work around. 

Finally, it’s important to start with a new, clean foundation before starting your new installation process. Check out a crawl space cleaning service to help aid you in the new insulation process.

Closing Thoughts

Crawl spaces are a common type of housing foundation that consists of a gap between the ground and the subfloor of a home. Crawl spaces are typically used in areas where weather and soil conditions make a concrete slab-on-grade foundation or basement impractical. The most common types of crawl spaces are passively vented, which allows air to circulate in from the outside. In the summer months that warm air mixes with the colder air in the crawl space and forms condensation as it cools. 

While the insulation you use in a crawl space is important, it’s even more important that you have your insulation, and vapor barrier if required, installed by a team of professionals. When crawl space insulation isn’t installed correctly it can allow air and moisture to pass through to the subfloor of your home. Over time that moisture can result in the buildup of mold, impacting the air quality throughout your home. That moisture may also result in extensive damage to your subfloor and flooring.

Insulating your crawl space correctly isn’t easy, but it is possible. Leave it to our professional team at Attic Construction. With decades of experience working in attics and crawl spaces across Southern California, our team can help you get your crawl space cleaned up and insulated in no time. To learn more about our San Diego and Orange County crawl space cleaning service, please contact us today.


  1. “Crawlspaces – Either in or Out” spaces-either-or-out
  2. “What is a Crawl Space?”
  3. “Crawlspace Insulation” space-insulation
  4. “Conditioned Crawlspace Construction, Performance and Codes” space-construction-performance-and-codes/view

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