Exploring a band joist in the field of construction

Joists — rim joists and band joists — comprise the primary structural elements of your home’s floor frame. Think of them as ribs that make up the frame of the floor. They are then covered with plywood subflooring to create a floor platform. 

Joists are supported by the house’s foundation walls or exterior walls through their ends. If you see a house under construction, you’ll find a band of solid lumber running along the bottom of each floor level. That’s your band or rim joist. 

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What’s a band joist in construction?

If you’re constructing a frame over your basement or crawlspace, framing a deck or a floor system, you’ll be using framing members called joists. These rest atop the foundation sill and support the floor above. 

The ends of these joists, and the space between them, are covered with a band joist or a header, or a rim board, that’s usually the same size as the floor joists. It runs around the entire perimeter of the building structure.

So basically, the end joists and the rim joists are called band joists collectively. These are boards that cap off the floor joists and joist cavities — forming the outer box of the floor assembly. It’s important to air seal and insulate this area to prevent the wastage of energy and moisture problems.

Difference between a rim joist and a band joist

Rim joists are set perpendicular to the joists so as to provide lateral support for the joist ends. Keep in mind that rim joists are not the end joists which primarily are the first and last joists that make up a floor or deck frame. The rim joists and the end joints together form the band joist. 

And that’s why, the rim joists are made of the same material, depth, and thickness as that of the joists. They are nailed into the top plate, the upper-level walls, or the sill plate on the foundation walls with framing nails. 

Is it worth insulating the rim joist?

For a home of any age, rim joist insulation is essential. That’s because it’s important to seal all cracks against air leakage. Keep in mind that an uninsulated band joist can result in a lot of heat loss. In such a case, the only thing that would separate the inside from the outside would be about two inches of wood and the siding material. Moreover, the heat loss will be even greater if the basement has a heating system or contains water heating equipment.

The good news is that you can seal and insulate your rim joists in just a couple of hours. Good joist insulation will help you lower your heating costs and save money. It will make your home more energy efficient. Another plus point is that the cost of insulating a band joist is minimal as compared to the potential energy savings. 

How do you insulate a band joist?

Insulating a band joist is easy enough as it’s usually readily accessible from the basement. Since there’s a small area involved, you do not need too many insulating materials. Moreover, you don’t need expensive or specialized tools. If you have the basic skills, that’s enough to insulate the joists.

You can use either fiberglass batts — preferably of six-inch thickness (R-19) — or rigid foam insulation boards — preferably three to five inches of thickness depending on the R-value per inch.

If the fiberglass insulation has a facing or vapor barrier, ensure that the vapor barrier is towards the heated space during the insulation process. 

Apart from the insulation material, you’ll need fasteners to hold it in place. Fiberglass insulation can be stapled or fastened with wire fasteners. Foam board insulation can be friction fit in place by correct measuring and cutting.

Read more: Crawl space insulation

Last thoughts

The main role of a rim joist or a band joist is to provide lateral support to the joists. That way, they do not lean under the weight of the load-bearing walls resting on them. Basically, these joists sit on top of the foundation and structurally support your floor sheathing. 

Whether you’re constructing a new house, building a deck, shed, or anything else with a wood-framed floor — you’ll need a rim or band joist.

Written by Ramona Sinha. November 11, 2022

Ramona is a content writer for Kukun. This experienced blogger uses simple and succinct words to decipher the complex phenomenon called life. An avid traveler, she’s a digital nomad at heart and an animal lover from the depths of her soul.