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What Is a Rim Joist?

Warped joist in a crawl space.

You might not think much about the parts that make up your home’s structure, but knowing a bit about rim joists can help you understand and prevent some common issues. Rim joists help support the floors in your home, but they can allow cold air and moisture to sneak in and potentially cause problems.

By learning more about rim joists, you can take steps to protect your home and crawl space from damage. Let’s dive into what rim joists are, their purpose, and how to protect them.

What Are Rim Joists?

Rim joist images

Homes in the U.S. have different kinds of joists that form the floor frame. If you’ve ever seen a floor frame, it looks like a rib cage, with beams (joists) creating the structure.

These beams are supported by foundation or exterior walls. On top of this frame, a layer of plywood is placed to create the floor.

The ends of these joists are attached to a perpendicular beam called the rim joist. Rim joists keep everything in place and handle a lot of weight, so they’re always made of solid, straight wood.

What Is the Purpose of Rim Joists?

Rim joists hold the floor frame together and provide crucial support. They stop the other beams from bending under the weight above.

But that’s not all—they also cover the ends of the beams to prevent moisture from entering and damaging your foundation. Made of strong wood, rim joists also help secure exterior wall sheathing, trim boards, and siding during installation.

How Rim Joists Are Built

Rim joists run parallel to the longer side of your house and connect with other joists at a 90-degree angle. If your home is rectangular, the rim joists go from front to back.

During construction, contractors place these joists on the top levels of the exterior and foundation walls, then nail them into the framing. To make sure they stay put, even in strong winds or earthquakes, professionals add metal strapping for extra reinforcement.

Each floor frame joist is nailed to the rim joist with three nails driven through the rim joist into the end of the joist.

Keeping Rim Joists Safe With Insulation

Man installing foam boars insulation.

Insulating your rim joists is crucial for protecting your home. Rim joists are exposed to the outside, allowing cold air and moisture to enter.

Without insulation, this can lead to higher energy bills, cold air drafts, mold growth, and wood rot. Proper insulation creates a barrier, keeping your home warm, dry, and energy-efficient, while also preventing moisture-related issues.

Insulating Rim Joists: Your Options

Insulating rim joists keep them safe from water damage, but which type of insulation is best? Let’s look at the three most common types to compare:

Fiberglass Insulation

Falling fiberglass insulation.
  • Pros: Affordable, commonly used in various parts of the home.
  • Cons: Can become damaged if exposed to moisture. Not recommended for floor joists.

Spray Foam Insulation

Spray foam insulation surrounding a wooden beam.
  • Pros: Seals hard-to-reach places, reduces drafts, eco-friendly, hard for pests to break through.
  • Cons: Can block utility lines if not installed properly, so always hire professionals.

Foam Board Insulation

Foam board insulation close up.
  • Pros: Affordable, versatile, comes in easy-to-use sheets, best at temperature regulation.
  • Cons: May not seal as tightly as spray foam.

Ultimately, the type of insulation you choose is a matter of personal preference. If you’re looking for a professional team to install insulation, consider working with AFS.

We install ExTremeBloc™, a thermal foam board insulation that keeps your home warm and dry by blocking both heat and moisture. With its reflective foil surface, our insulation has world-class insulating power.

Protect Your Crawl Space With AFS!

AFS crewman inspecting the door of a crawl space.

If you’re facing moisture issues in your crawl space, insulating rim joists might be the right step. Don’t try to do it yourself—reach out to professional contractors at AFS for a free inspection.

We serve areas in Mississippi, Tennessee, Florida, Georgia, and Alabama. Our team will check your crawl space, identify any problems, and recommend the best solutions.

Rim Joist FAQs


Yes, insulating rim joists may significantly reduce heat loss and improve your home’s energy efficiency, leading to lower energy bills.

While some types of insulation might be DIY-friendly, it’s often best to hire a professional to ensure proper installation and avoid potential issues.

Rim joists are exposed to the outside elements and can let in cold air and moisture, which makes extra insulation necessary to prevent drafts and moisture damage.

Related Resources

Ted Dryce

Ted Dryce

Content Writer

Ted is an SEO Content Writer who has been with Groundworks since 2021. He’s covered home repair topics ranging from crawl space encapsulation to regional soil conditions. When he’s not working, Ted is performing improv comedy and working on his own creative projects.

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