Residential basements offer additional functional space but pose complications in maintaining an energy-efficient building envelope. Prone to increased humidity levels, colder temperatures, and air leaks, basements cause adjacent living spaces to feel cool. They also contribute to increased HVAC use and higher energy bills. One of the most cost-effective solutions is insulating rim joists in your basement, creating an effective thermal barrier to keep your home cozy year-round.

Learn more about the benefits of insulating rim joints, the best insulation materials for the job, and other details to make an informed decision before your next project.

What Are Rim Joists?

The rim joist is a horizontal board that sits on top of a home’s foundation, capping off the floor joists that form the perimeter of the floor frame. Every floor of the house has (or should have) a rim joist, but basement rim joists are the most important elements to insulate because they form a crucial boundary between the exterior of the home and your conditioned spaces. Basement rim joists are among the most significant sources of air leaks and energy loss in any home.

What Is Rim Joist Insulation?

This insulation sits on top of the rim joist and extends up to the subfloor of the next story. It doesn’t look like much, but insulating rim joists is the best way to prevent air leaks and energy loss. The goal is to introduce a complete air barrier between the exterior environment and indoor spaces.

With the right material and professional installation, homeowners enjoy the significant benefits of rim joist insulation, including:

Comfort: Insulated rim joists eliminate cold drafts emanating, making floors and adjacent rooms warmer in the winter.

Lower energy use: Preventing air leaks and energy loss maintains a consistent indoor temperature while lowering demand for heating and cooling use. You’ll use your heater less in the winter and your AC less in the summer!

Moisture protection: Insulated rim joists also control indoor humidity and moisture levels, which lowers the risk of moisture damage to wood.

The Best Insulation for Rim Joists, by Material

Choosing the right insulation to meet your home’s needs improves performance and helps you stick to your budget. The first step is to choose the right rim joist insulation R-value for your climate. In most cases, the insulation you select for your rim joists should be the same R-value as the rest of the above-grade exterior wall insulation for your region. We have a handy guide to R-values that’s worth checking out.

Spray Foam Rim Joist Insulation

Using closed-cell spray foam is the best way to insulate rim joists due to the material’s high R-value, rigidity, and density. It expands to seal every gap and adds a degree of structural support, strengthening your home. Spray foam insulation should always be professionally installed to maximize the benefits of your investment and to ensure safe, long-lasting performance.

Note that open-cell spray foam isn’t recommended for basement applications because it is more prone to moisture absorption.

Read More: Open-Cell vs. Closed-Cell Spray Foam: What’s the Difference?

Foam Board Rim Joist Insulation

Rigid foam board is made from extruded polystyrene (XPS) or polyisocyanurate (Polysio), two low-cost materials available at many home improvement stores. Insulating rim joists with foam board is less expensive than spray foam and offers adequate moisture resistance, but it’s not a great DIY option. That’s because installation requires careful cutting and sealing to achieve a truly airtight barrier.

Fiberglass Rim Joist Installation

Fiberglass, also known as batt rim joist insulation, isn’t a great option for basements or any area with high moisture levels. All fiberglass batt materials are prone to condensation and moisture damage, especially during the winter months. Over time, this leads to wood rot and structural damage to the home. Fiberglass doesn’t provide an effective air seal to prevent leaks, contributing to cold drafts even if the material has a high R-value.

Can a Rim Joist Be Insulated from the Outside?

Insulating rim joists from the outside during an extensive renovation or remodel is a great option, but it’s impractical if you’re not planning significant work in the near future. Installing exterior rim joist insulation requires the complete or partial removal of siding and other cladding before applying rigid foam board. Interior installation is much more common because it’s less disruptive and less expensive.

Do I Need a Permit to Re-Insulate My Rim Joists?

In most cases, insulation is considered an “ordinary repair” that doesn’t require a permit. However, requirements vary based on local ordinances. We strongly recommend checking your local city or county building code office or asking your local Koala Insulation contractor about rim joist insulation near you.

The Bottom Line: Is Rim Joist Insulation Worth It?

Depending on the age of the home, existing insulation materials, and local climate, homeowners see varying energy and comfort improvements after insulating their basement rim joists. Households in colder climates realize more significant savings from this home improvement. You can expect to recoup rim joist insulation costs in a few years, especially with the help of federal, state, or utility energy efficiency rebates.

Insulate Rim Joists and Save With Koala Insulation

From a friendly, free estimate to selecting the right rigid board or spray foam insulation for rim joists, you can always count on Koala Insulation. We bring national resources and local expertise to every project, promising convenient installation and lasting results. We’re proud to help energy-conscious homeowners meet their environmental goals and lower energy costs with comprehensive insulation services. Find a Koala Insulation location near you to schedule a free estimate today!