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Rim Joist

A rim joist or a band board offers lateral support to end joists that carry the weight of the walls to the roof.

Any well-built house consists of several joists, one of them being the rim joist. It’s where the foundation walls meet the floor joists of a building. It works in tandem with two outermost joists, which help form a ‘band’ for the floor’s framework. However, rim joists can also be an entry point for moisture and air, hence, the need for insulation. 

Let’s explore what a rim joist is, look at its role and how it’s installed, then finish off with rim joist insulation options. 

What Is a Rim Joist? 

A rim joist offers lateral support to the end of all other joists and is attached perpendicularly to them. You can compare it to ribs that hold up the entire frame. All these joists get their support from the exterior and foundation walls. 

Because they’re supposed to withstand a lot of weight, rim joists should be made from solid wood that’s free from twists or bows. Such joists will strengthen your subfloor. Perfect materials for rim joists are usually engineered wood products or laminate wood. 

What Do Rim Joists Do? 

A rim joist or band board works as lateral support for the joists. It prevents the joist from leaning over the weight of the load above it. In addition, this joist covers the end of other joists to prevent cavities, which can open up your home to moisture. The wood used in rim joists also provides nailing support when installing exterior wall sheathing, trim boards, and siding. Because of its critical role in a building, a rim joist is an aspect of framing that builders should never overlook. 

How to Install Rim Joists 

Rim joists run perpendicular to standard joists and need to be parallel to the long sides of the house. In a rectangular house, the standard joists span from front to back. 

When framing a floor system, you’ll install the rim joist perpendicular to the other joists. This ensures that the end of the joists gets lateral support while still capping off the end of the floor. Outermost joists span the short side of your house. 

Rim joists are usually set on the upper levels of the walls or the foundation walls then nailed into the framing. Contractors use three nails driven through the outside joists to the end of the joists. Because rim joists are susceptible to wind and other seismic activities, which can cause them to separate from the floor, they have to be reinforced with metal strapping. 

Rim Joist Insulation 

An open crawl space is usually a hotbed for air and moisture. Typical entry points include cracks, gaps, rim joists, and vents. These spaces not only attract moisture and unconditioned air, but they also allow air to escape from your home. This can lead to energy loss and ultimately high utility costs. 

You can improve energy efficiency and create a comfortable interior by insulating your rim joists. All types of homes, including old and modern ones, stand to benefit from rim joists insulation. Let’s look at some of the common insulation methods. 

Spray Foam Insulation 

Spray foam insulation is fantastic insulation for rim joists. It insulates joists and seals air leaks. Not only is it easy to use, but it can seal all the hard-to-access points that allow air into the 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 from the basement walls to the roof. Our insulation of choice is ExTremeBloc™, rigid panels made of expanded polystyrene that also is waterproof and treated to resist termites. 

Fiberglass Insulation 

Fiberglass is another common material for rim joist insulation. It’s made from plastic that’s been reinforced with fine glass fibers. You can use it to cover your basement walls, crawl space, attics, and rim joists. Fiberglass is easy to install and affordable. On the flip side, this material doesn’t seal walls, meaning there is the possibility air and moisture could infiltrate the crawl space. It also can be easily saturated by moisture and harbor pests and mold growth. 

Would you like to insulate your crawl space and rim joists? Contact JES Foundation Repair for a free crawl space insulation inspection and quote. We’ll advise you on the choice of insulation materials and cover the walls and joists so your crawl space stays warm and dry. 

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