Skip to Main Content
Girl with umbrella

Rim Joists and Band Boards

Unsealed rim joists are the easiest entry point for outside air, water, and even pests escaping the extreme conditions outside your home.

Get a Free Estimate

Your subfloor has several joists that help support your walls. One of these is the rim joist, which can let in moisture and air into your house. 

When you take an out-of-sight and out-of-mind approach to rim joists, you can easily neglect them like some homeowners in Colorado do. 

Let’s delve into the world of rim joists, what purpose they serve, how contractors install them, and how to insulate yours. 

What Are Rim Joists? 

A rim joist or a band board is a component of your subfloor that helps bear the weight of your walls. This joint works in conjunction with two outer joists, forming a band for your floor’s framework. 

In a two-story home, the rim joists bear the weight of the wall, the upper floor, and its wall, and the roof.  We hope you now have an idea of how much of a load a single rim joist can bear. 

The conventional material for creating joists is solid wood that’s free from knots. Engineered wood products and laminate wood are other common materials. Often, the material is of the same size as other joists ensuring that rims are flush with joists on top of them. 

What Do Rim Joists Do? 

A rim joist or band board provides lateral support for the other joists in your subfloor preventing them from collapsing under your structure’s weight. On top of this, it caps off the terminal areas of other joists. This in turn prevents cavities that might expose your home to outside air or moisture. Another thing that the rim joist does is provide a mass of wood for nailing during the installation of your sheathing, siding, or trim boards. 

Rim Joist Installation 

As a homeowner, it’s important to know how your contractor will install the rim joists as they frame your floor system. 

Rim joists run perpendicular to standard joists and are usually parallel to the long sides of the house. In a rectangular house, the standard joists span from the front wall to the back wall. This orientation ensures that your joists get lateral support while still capping off cavities. 

During installation, your contractor will set the joists on the upper levels of your basement or crawl space walls and then nail them to the framing using three nails that go through to the end of the joists. Once the rim joists are in place, they’re fastened with a tie-down metal strapping that prevents them from splitting whenever there’s a tremor or high winds. 

Rim Joist/Band Board Insulation Options 

Leaving your crawl space open is an open invitation for moisture and toxic air from the outside to get into your home. These can come in through vents, rim joists, wall cracks, and gaps and drive up humidity levels, leading to energy loss and high utility costs. 

Here are your insulation options: 

Spray Foam Insulation 

Your first option is spray foam, a fast-expanding material that insulates rim joists and seals potential air leaks in hard-to-access areas in your crawl space. 

Foam Board Insulation 

If you’re looking for cheaper and easy-to-use joist insulation, consider foam board, which is made from polyurethane or polystyrene. Standard size sheets measure four by eight inches and range from a quarter-inch to two inches in thickness. Foam board is multi-purpose and can insulate just about any part of your house including the basement or crawl space walls. 

Fiberglass Insulation 

Another low-cost option is fiberglass, which is made from reinforced plastic. Contractors often use this to insulate exposed areas and entry points including attics, crawl spaces, unfinished basements, and rim joists. While fiberglass is easy to install, it doesn’t seal walls completely. This might expose your crawl space to outside air and moisture. This type of insulation also can easily fall victim to moisture and harbor mold and pests. 

Rim Joist Repairs 

Unfortunately, the wood that makes up the rim joists can rot due to moisture. This scenario is likely to play out if the sill plate is installed close to the floor or wasn’t pressure-treated before installation. Either way, moisture will infiltrate the flooring and make its way to your rim joist. This can lead to structural damage. To fix this problem, your contractor will have to raise the floor to replace the rotted rim joist. 

Protect Your Joists 

Left unsealed, your rim joists and band board can let in unconditioned air and moisture into the crawl space. You can stop energy loss, control moisture, and create a comfortable indoor environment by sealing your rim joists, insulating your crawl space, and encapsulate it to help prevent moisture infiltration. 

If you’re struggling to tame moisture or outdoor air, the best thing to do is to have your crawl space insulated by the experts at Complete Basement Systems. We use the best insulation materials to seal up your crawl space. To get started, request a free inspection and quote

Serving Greater Denver area, Colorado Springs, Eastern Colorado, and parts of Nebraska and Wyoming

Complete Basement Systems, Colorado Springs, CO

5695 Parachute Cir
Colorado Springs, CO 80916

Complete Basement Systems, Denver, CO

11795 E. 45th Avenue
Denver, CO 80239