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What Are Rim Joists in Home Construction?

insulation in crawl space rim joists

Home construction involves many intricacies. You are not obligated to know all the fine details, but it’s helpful to have a basic understanding of your home’s structure so you are equipped to protect it.

This includes rim joists in the crawl space. These seemingly small but important areas help enhance stability but can cause problems if they are not properly protected.

This article explains rim joists, their purpose, how to safeguard them, and how local crawl space repair experts can help.

What Are Rim Joists? 

To properly explain rim joists, we have to first talk about joists in general.

Floor joists are beams that are the primary structural elements of your home’s floor frame. They are ribs that create a frame, which is then supported by the exterior walls or the foundation walls. When a layer of plywood subflooring is added to the top of the frame, you have a floor platform.

The end of these joists that rest on the exterior or foundation walls are attached to a perpendicular joist called the rim joist or band board. Those bands are made up of the exposed faces of the rim joists.

Rim joists are designed to withstand a lot of weight, so they are made from solid wood without knots. 

What Is the Purpose of The Rim Joists? 

The main task of a rim joist is to support all the other joists. It keeps them from bending due to the weight they need to withstand. Another purpose of a rim joist is to cover the ends of other joists so there aren’t any exposed cavities. Otherwise, moisture could easily enter your home and damage your foundation. Since rim joists have such an important role, builders must not cut any corners on them. 

How to Install Rim Joists 

As mentioned, rim joists and regular joists form a 90-degree angle. Rim joists are parallel to the longer side of the house. So, for example, in a rectangular house, rim joints are placed in a front-to-back direction.

Here’s a look at the installation process:

  • Place rim joists on the upper levels of the exterior walls or the foundation walls
  • Nail the rim joists into the framing
  • Reinforce the connection with metal strapping
  • Nail rim joists to each floor joist with three nails, penetrating the outside face of the rim joist and stopping at the end of the joist

Protecting Rim Joists With Insulation 

If your crawl space is not properly waterproofed, it can easily become moist. Unfortunately, moisture and cold air can easily enter the crawl space through cracks, gaps, rim joists, and vents. When this area of your home becomes humid, various problems can occur, from wood rot and energy loss to mold infestations.

Keeping your crawl space in top shape means maintaining your home’s health. By insulating rim joists, you can prevent moisture from entering the crawl space and create a healthier environment.

Here are the three most common insulation methods:

1. Spray Foam Insulation 

rim joists insulated with spray foam

This insulation method is popular because it easily seals all the areas that are difficult to access. Spray foam prevents outside air from getting in and reduces drafts near the floorboard. It is safe for the environment and won’t attract pests. However, if not installed properly, it can obstruct utilities, wiring, and piping, so it should be handled by a professional. 

2. Foam Board Insulation 

insulation panels installed in crawl space rim joists

Foam board insulation is a more affordable solution if you wish to keep moisture out of your below-grade areas. It is made from polyurethane or polystyrene and comes in panels or sheets. This product is not just good for joist rim insulation; you can insulate almost any part of your home with it. 

3. Fiberglass Insulation 

fiberglass insulation in rim joists

Another affordable solution is fiberglass. It is made from plastic reinforced with fine glass fibers, and contractors use it to insulate not just rim joints but also basement walls, crawl spaces, and attics. However, fiberglass batts are air-permeable, so this type of insulation in the rim joist is prone to mold issues

Need to Address Crawl Space Rim Joists with Insulation? Contact Tar Heel Basement Systems Today!

The condition of your crawl space rim joists can make or break your home’s safety and health. Concerned with your rim joists and their insulation? Don’t opt for a DIY project. Instead, call local trusted professionals and gain peace of mind knowing that the job has been properly done.

Contact Tar Heel Basement Systems today to schedule a free inspection. One of our experts will visit your home, assess the situation, and recommend the best solutions for your crawl space. 


Reusing wet fiberglass insulation is not advisable as it loses insulation efficiency and may harbor mold. Tar Heel Basement Systems installs durable, water and mold-resistant rigid insulation panels. Contact us for a no-cost inspection and repair estimate.

Well-insulated and sealed rim joists can significantly improve a home’s energy efficiency by reducing heat loss and preventing drafts. It’s important to use the right type of insulation to ensure the best outcomes and avoid damage like mold growth and pest infestations.

While minor fixes might be DIY-friendly, significant repairs including replacement or insulation often require professional expertise to ensure structural integrity. Our team at Tar Heel Basement Systems has more than 20 years of experience in addressing crawl space issues. Contact us today to schedule a free inspection and estimate.

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Holly Richards-Purpura

Holly Richards-Purpura

Content Writer

Holly is a Content Writer for Groundworks who has written and edited web content for the foundation services industry for almost 10 years. With a background in journalism, her passion for the written word runs deep. Holly lives in Columbus, OH, with her husband. Along with educating homeowners, she also has a big heart for the Big Apple.

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