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Rim Joists

A rim joist is an important element of the home’s floor frame, but if not insulated, it can present a huge problem for homeowners.

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Although you are not obligated to be highly informed about all structural elements of your home, it doesn’t hurt to get familiar with rim joists. Why is that? Because rim joists are the place where cold air and moisture can enter your foundation and cause problems.  

While a lot of homeowners are aware that open crawl space vents let the unconditioned air in, most are not aware of the similar problems rim joists cause. Here, we will address this structural element, explain its purpose, and take a closer look at rim joist insulation. 

rim joist

What Are Rim Joists? 

Homes across the U.S. have all kinds of joists. These beams are used to create the floor frame for your home. If you have ever had the chance to see a floor frame, you know that it looks like a rib cage. The ribs/beams create a frame that is supported by the foundation walls, or in some cases, exterior walls.  

To create a floor platform, a layer of plywood is placed on top of this frame. The ends of these joists that look like ribs are attached to a perpendicular joist called the rim joist. Rim joists hold this rib cage in place and are designed to withstand a lot of weight. This is why rim joists are always made of solid wood that is free of any twists. 

What Is the Purpose of Rim Joists? 

As mentioned, the main purpose of rim joists is to hold the other joists in place and support them. Thanks to the rim joist, the other beams will not bend under the pressure caused by the weight above. However, this is not the only purpose of these joists. They are used to cover the end of other beams, so there aren’t any exposed cavities.  

Without rim joists, moisture would easily enter the cavities, and eventually your home, damaging your foundation along the way. Rim joists are made of wood that provides nailing support during the installation of exterior wall sheathing, trim boards, and siding. These joists are an essential part of the floor frame so they should always be made of high-quality material. 

Rim Joist Installation 

Rim joists are parallel to the longer side of the house and connect with other joists at a 90-degree angle. If you have a rectangular home, your rim joists run from the front to the back of your house. During the construction of the floor platform, contractors place rim joists on the upper levels of the exterior walls and foundation walls and nail them into the framing. Since nails on their own are not enough to secure this connection, it is reinforced with pieces of metal strapping. This way, everything will stay in place even during strong winds or earthquakes. Every joist that forms a floor frame is nailed to the rim joist. To connect them, contractors will take three nails and drive them through the outside face of the rim joist to the end of the joist. 

Rim Joist Insulation 

Crawl spaces have many advantages, but also one big problem – they are prone to moisture. This is why homeowners who live in more humid areas avoid this type of foundation. However, this problem is not unsolvable. Your contractor might suggest insulating rim joists in addition to installing a vapor barrier or sealing off the vents. If your home’s rim joists are not waterproofed, moisture will still enter your crawl space and cause a lot of problems, from rot and energy loss to mold infestation.  

With insulated rim joists, you can prevent moisture from wreaking havoc underneath your home and create a healthier environment for your family. Regardless of whether your home was built in the previous century or three years ago, it can greatly benefit from joist insulation. There are several ways contractors can complete this task. Let’s take a closer look at each of them. 

Spray Foam Insulation 

The best thing about spray foam insulation is that it can seal all those places that cannot be easily accessed. With this insulation, the outside air won’t be able to come in and drafts near the floorboard will be greatly reduced. It won’t harm the environment in any way, nor attract pests. Unfortunately, it can obstruct utility lines, wiring, and piping if it is not installed correctly. This is why only professionals should handle this type of work. 

Foam Board Insulation 

If you are looking for an affordable solution, foam board insulation is a good option. Made of polyurethane or polystyrene, this foam comes in four-by-eight-inch sheets. A lot of contractors use this type of insulation for every part of the structure, not just the joists. 

Fiberglass Insulation 

Fiberglass insulation is another affordable option. It is made from plastic reinforced with fiberglass, and just like foam board insulation, it can be used to insulate basement walls, crawl spaces, attics, and other parts of the home. Unfortunately, fiberglass insulation is air permeable, which means it can become a suitable environment for mold growth. 

If you wish to solve moisture problems in your crawl space and you think that insulating rim joists is a step in the right direction, do not try to do it on your own. Instead, contact professional contractors at AFS to schedule a free inspection. We serve areas in Mississippi, Tennessee, Georgia, and Alabama, so call us today. One of our inspectors will examine your crawl space, determine if there are any potential problems, and recommend the best ways to solve them. 

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