What Is A Rim Joist On A House?

Home construction is fascinating. There are numerous terms that you've probably never heard of before, like cross bracing and backfilling. If you've never heard of rim joists before, you are not alone. This article will explain everything you need to know about the rim joists in your house.

South Central Services has insulated hundreds of new and existing homes with spray foam. The rim joist is a critical home area that should be insulated and air-sealed.

By the end of this article, you will understand:

• What a rim joist is
• What else rim joists are called
• How rim joists should be insulated
• If your rim joists need new insulation

What Is A Rim Joist?

A rim joist is the transition in your basement from a masonry or concrete wall to a wood floor joist.

Basements are below-grade spaces, meaning they should have masonry or concrete material for the walls and floors. Once we get beyond the basement, houses are generally above the ground. Above-grade spaces can be constructed with treated wood. Builders transition from masonry to wood framing.

"Rim joist" is the term for where that transition happens. You can think of this area as the "rim" of your home, similar to the "rim" of a basketball hoop. It's like an outline. When we combine the house's outline with the floor joists, we get the term "rim joist."

Are Rim Joists And Band Boards The Same Thing?

If you've never heard of a rim joist before, you may also have never heard of a band board. Another term for rim joists is band board or basement band board. Despite these terms sounding quite different, they refer to the same area of a home.

Our website uses these terms interchangeably, as some readers know one term better than the other.

How Are Rim Joists Insulated?

One of the reasons rim joists are so important, and why we're talking about them, is because they are an area of your home that should be insulated. As a part of that "rim" or outline of the house, your basement band boards make up part of your thermal envelope.

The thermal envelope is created by every surface in your home that forms a barrier between the outside world and your house. Band boards are a tiny sliver of that thermal envelope but are still contributors. The thermal envelope of a home needs premium insulation. Ideally, it should have an airtight seal.

When it comes to insulation, it is essential to put the right insulation in the right places to lower energy costs, improve air quality, and make homes more comfortable.

Traditionally, rim joists are insulated with fiberglass batt insulation. Since insulating the rim joist is part of the building code, home builders generally install some fiberglass in this area when installing fiberglass in the basement ceiling or walls. The rim joist is the joint where your basement walls and ceiling meet, so no matter which part of the basement gets insulated, the rim joist becomes part of that.

Even though rim joists are usually insulated with fiberglass batt, this insulation is ineffective for rim joists. As the transition between masonry and wood, band boards face air infiltration. Cold air from outside sneaks into the home uncontrolled through the rim joist. Fiberglass insulation lets air pass through, meaning cold air can get inside and make your floors cold.

Our recommendation for insulating basement rim joists is closed cell spray foam.

Spray foam insulation can create an airtight seal, stopping cold air from infiltrating the home. Spray foam also offers a better R-value than fiberglass. Closed cell specifically provides a vapor barrier, preventing any water in the air from leaking through the porous masonry walls.

Should I Do Something About My Rim Joist Insulation?

Now that you know what a rim joist is, you can go to your basement and assess the state of your insulation. Even without looking at your band boards, you may be able to determine from memory if your rim joist insulation needs an upgrade.

In our service area of southern Pennsylvania, cold winters and hot summers affect our indoor air temperatures. If your rim joist insulation is problematic, it will be most evident during the winter. Are your floors especially cold, so much so that you'll wear shoes or socks inside? Are your energy bills during the winter extra high because you have to crank your thermostat?

If you are flooded with memories of shivering under blankets and debating about increasing the thermostat by a single degree, you could benefit from an airtight seal in your rim joists.

Contact a qualified spray foam contractor for a professional opinion on the air leakage from your band boards.

The Bottom Line About Rim Joists In Home Construction

Rim joists are an interesting area of your basement that you may not have noticed before. For homes with basements, rim joists are necessary to transition from masonry to wood. Despite making up a sliver of a house's overall thermal envelope, band boards have a significant impact on energy efficiency.

While it is uncommon for rim joists to be insulated with closed cell spray foam, the product's airtight seal helps boost energy efficiency and comfort in a home.

Now that you know what a rim joist is and how they’re usually insulated, your next step is to:

• Estimate the cost of professional spray foam insulation in your rim joists
• Learn more about the benefits of an airtight seal in your basement band boards