Rim Joist: Purpose, Insulation & Common Mistakes

Rim joists run around the outer edge of wood-framed houses, providing critical structural support to a home’s floor frame. 

Properly insulated rim joists can increase your building’s energy efficiency. However, improperly insulated rim joists will not only impede energy efficiency but can also lead to structural damage. 

Data collected by the U.S. Census indicates that over 50% of houses in America sit on basements or crawlspaces in cold climates. This data suggests a significant opportunity for contractors and homeowners to examine and improve existing rim joists insulation. 

But before you break out the toolbox, let’s look at the purpose of rim joists and the benefits of proper insulation. Later we’ll get into the best materials to use when insulating rim joists.

What Is a Rim Joist?

Any structure with a framed floor requires a rim joist. A rim joist is a board that runs perpendicular to the floor joists and end joists. Rim joists close off the open cavities of the floor joints, creating the outer edge of the floor framing.

The rim joist is a critical part of the building enclosure. Also known as rim boards, band joists, or band boards, rim joists serve several purposes: 

  • Stabilize floor joists
  • Keep floor joists level 
  • Prevent floor joists from twisting
  • Maintain floor joist spacing

Rim joists help transfer vertical and lateral loads, ultimately providing critical structural support to your entire building. 

Builders typically position rim joists around the entire perimeter of a house, set vertically on top of the foundation wall sill plate (also known as the mudsill).

Properly constructed rim joists are load-bearing and can support exterior walls. 

Rim joists also provide a solid wood surface into which builders can nail sheathing. This sheathing connects the wall and floor framing, creating a more structurally sound building. 

Note: Although rim joists help create the exterior perimeter of the frame, not all outer sides are rim joists. The outermost edges running parallel with the floor joists are called end joists. Therefore, rim joists are only those boards that run perpendicular to floor joists.

Materials 

Builders typically construct rim joists from the same material as the other joists. This material is most often traditional or engineered lumber, but builders can also use steel or a combination of steel and lumber.

The architect and engineer will determine the materials based on how strong they need the rim joist to be. 

Wood-framed houses usually use various sizes of 2x or engineered lumber. In new homes, rim joists typically are made from 2x10s or 2x12s.

There is more variation in older houses, with rim joists found using 2x6s, 2x8s, and even square timber such as an 8×8.

In some cases, older balloon-framed homes don’t even have rim joists. In these cases, the bottom plates of the wall framing will rest on the mudsill directly.

Traditionally, builders have used three galvanized 3½” nails to connect the rim joist to the floor joists. Although strong, these nails do not hold up against twisting forces. Structural screws have greater holding power and are thus replacing nails.

In cases where the rim joist is made of traditional lumber, and the floor joists are made of engineered wood, builders typically use hangers to secure each floor joist to the rim joist.

You can also use hangers when connecting the rim joist from the outside is impossible. Otherwise, builders avoid using joist hangers as they add cost to a project’s overall budget. 

Curious about cost? Here’s a quick overview:

  • 2×10 Framing Lumber: $24 for 8-ft
  • 2×12 Framing Lumber: $22 for 8-ft
  • 3 ½” Galvanized Nails: $5 for 60 pieces
  • 3 ½” Structural Screws: $15 for 50 pieces
  • Face Mount Joist Hanger: $2 per piece

These prices are approximate but will give you a rough idea of your cost range when considering which materials to use for your project. 

Installation 

floor trusses ready for rim joist
Floor trusses ready for rim joist

Rim joists run around the entire perimeter of a house. Engineers typically run floor joists across the shortest possible length to prevent flexing. Since rim joists run perpendicular to floor joists, rim joists usually run parallel to the long side of a house. 

On foundation walls, rim joists sit on the sill plate. On upper-level walls, they sit on the top plate.

Rim joists are connected to both plates using framing nails and sometimes metal strapping called tie-downs. Strapping provides an extra level of reinforcements against joist separation that might otherwise occur due to high winds or an earthquake. 

Want to install a rim joist? Consider the following steps.

Use a string line to double-check check the floor joist ends are the same length. Trim the ends if they’re too long. Replace the joists if they are too short.

While working with the string line, make sure that all joists are within 1/16” in height. Shim if necessary to correct height differences. 

Once you have set your rim joist in place, use a square to mark where the floor joists meet the rim joist. 

Find the middle floor joist. Line it up with the mark on the rim joist, ensure it is flush with the rim joist, and drive a structural screw into the middle of the rim joist. This connection is crucial to maintaining floor joist stability. 

Moving outward, alternating in both directions, fasten one screw into the middle of each rim joist. After connecting each floor joist to the rim joist with one screw, add a top and bottom screw. Make sure each floor joist is aligned correctly by using a speed square.

Air Sealing

Before you insulate your rim joist, be sure to address air leakage. Air leakage often occurs around the area where the foundation wall, mudsill, subfloor, and rim joist meet. This area is structurally critical, so you want to do everything possible to protect it from rot.

First, form an air barrier by applying air-impermeable sealant such as caulk or spray urethane foam to the framing components. 

Creating building airtightness is not easy. It is especially tricky to address air leakage where the rim joist meets the sill plate and subfloor above.

If you use spray foam to create an air barrier, the foam needs to connect from the foundation wall to the rim joist and then up to the floor sheathing. 

Fiberglass batts do not address air leaks near the rim joist. However, applying a sealant to this area can help reduce the risk of interstitial condensation between fiberglass insulation and rim joist. 

When installing rigid foam leave around a 1/2″ gap around the perimeter to install one-part spray foam from a can for air-sealing. This is best done using an adjustable foam gun.

Can Spray Foam Gun Makes Air-Sealing Rigid Foam Easier

Rim Joist Insulation

Insulation increases a building’s energy efficiency. It will help regulate a consistent temperature, keeping a house cooler in the summer and warmer in the winter.

This temperature regulation can help keep energy costs down, but it can also maintain the strength of your building’s structure by preventing the buildup of rot. 

Insulating rim joists serves two purposes: 

  • Preventing condensation 
  • Maximizing drying potential

If your homes rim joist is uninsulated, the only thing separating the rim joist from the outdoor elements is a thin layer of sheathing, housewrap, and siding.

You wouldn’t leave above-grade walls uninsulated, so don’t leave above-grade rim joists uninsulated either. (Above-grade refers to the part of a building above the ground. Below-grade refers to the portion of the building below the ground.)

For reference, the Department of Energy recommends a minimum R-value of R-13 for exterior wall insulation (DOE is increasing R-value recommendations – PDF).

Rim joist insulation is essential, especially on the first framed floor, where rim joists are often exposed to an unfinished basement or crawlspace.

When constructing a building, even in a dry region, you have to assume that the building enclosure will become wet – and then take steps to reduce moisture accumulation.

Hopefully, you won’t experience flooding. But even without floods, moisture can accumulate through within-wall condensation and bulk moisture intrusion (usually occurring at house seams and penetrations). 

Read on to learn how insulating rim joists can prevent rotting and mold growth. 

Preventing Condensation

Condensation promotes mold growth, which can negatively impact the health of people occupying the building. More worryingly, condensation can degrade the rim joist material over time; this rotting can weaken the building’s structure.

Rim joists are especially susceptible to condensation.

If the first-floor framing is exposed to crawlspaces or unfinished basements, humid air can contact the interior surface of rim joists. During the winter, the surface of the rim joist will get colder than the interior environment’s dew point. When warmer interior water vapor comes into contact with the surface of the rim joist, condensation will occur.

Proper insulation will help prevent condensation in one of two ways: 

  • 1.) The insulation will act as a physical barrier that prevents warmer interior air from contacting the cooler rim joist.
  • 2.) Insulation will warm the rim joist’s surface to a temperature higher than the interior air’s dew point. 

Maximizing Drying Potential 

Vapor diffusion is the primary method of drying wet walls, with the layers on both sides of the rim joist affecting its ability to dry. 

Wood and engineered wood (the materials typically used for rim joists and exterior sheathing) have low vapor permeability. Therefore, when considering how to maximize rim joist drying potential, consider the vapor permeability of the insulation rather than that of the rim joist itself. 

When adding insulation to rim joists to maximize drying potential, you want to pay special attention to the insulating material’s vapor permeability properties. The higher the vapor permeability, the faster diffusion will occur.

Lower vapor permeability allows diffusion at a slower rate.  

Note: When researching materials for your construction project, you might encounter the term throttle. The throttle refers to the material with the lowest vapor permeability in your building’s assembly. For example, if the throttle is located on the exterior side of the rim joist, the structure will have a more significant drying potential on the inside. 

Best Insulation Materials for Rim Joists 

Adding insulation to rim joists can help reduce your building’s energy costs. However, when selecting insulation to add to a rim joist, the most important thing to consider is how the insulation might interact with moisture. 

Insulate rim joists at every level, including the upper floors. While it is possible to insulate rim joists after a house is already built, it is much easier to complete this insulation before installing the drywall. 

Note: If you live in an older house, check that your rim joists are insulated, as this is a somewhat recent standard. The type of insulation material you apply to rim joists greatly impacts moisture prevention. Consider the following most commonly used materials before beginning your installation project.

Fiberglass Batt

rim joist with fiberglass insulation

Up until recently, builders have used fiberglass batts to insulate the interior side of rim joists. Fiberglass insulation is air permeable, which allows the rim joist to dry on the interior and exterior sides. 

However, this also means that, during the winter, warm indoor air can travel through the fiberglass insulation and contact the cooler rim joist through convection. So although fiberglass provides the rim joist with thermal resistance, it does not prevent condensation on the rim joist’s interior surface. 

Do you live in a cold-climate house? Are your rim joists insulated with fiberglass? Head to the north side of the house and pull any insulation away from the rim joist. What’s the condition of the lumber? You may discover dampness or rot. 

Ultimately, the risk of condensation outweighs the drying potential of fiberglass. 

Interior Foam Insulation

Today, most builders use a type of foam to insulate the interior of a rim joist. Foam has low air permeability. This means that insulating rim joists with foam focuses on lowering the risk of condensation rather than maximizing drying potential. In addition, foam reduces the chance for warmer air to contact a cool surface, instead of providing a surface on the interior side of the rim joist at a temperature higher than the dew point. 

Whether you use polyurethane foam, spray foam, EPS foam, or XPS foam board insulation will depend on your project’s budget, desired environmental grade, and level of difficulty. 

Rigid foam is the cheapest foam insulation option. However, it’s also the most difficult to manipulate and can be hard to install in awkward areas.

There are three types of rigid foam: polyisocyanurate, expanded polystyrene (EPS) and extruded polystyrene (XPS). Of these types, polyisocyanurate is considered the least environmentally destructive. Conversely, XPS is manufactured with a blowing agent that can cause global warming. 

EPS is most commonly used due to low cost, and no blowing agents used.

In a mild climate, 2 inches of rigid foam should prevent condensation. However, if you live in a colder climate, consider installing 3-4 inches of rigid foam. (You can use multiple layers if needed.) 

Unlike rigid foam, spray foam insulates rim joists and seals air leaks simultaneously. You can hire a spray-foam contractor to spray your rim joists or purchase a two-component spray-foam kit. Available at most lumber yards, these kits cost between $300 and $400 for 200-bd.-ft. (For reference, 400bd. Ft. of spray foam will provide 3 inches of insulation to an area measuring 1ft high x 130 ft. long.) 

If you’re in climate zone 6 or colder, closed-cell spray foam is often suggested. Otherwise, you can use either open-cell or closed-cell spray foam. (Two-component spray-foam kits will typically be closed-cell foam, but it’s always best to double-check.) 

Applying foam to a rim joist reduces condensation risk. However, it will not help the rim joist stay dry in events of bulk moisture intrusion. 

Interior and Exterior Foam 

In some situations, you can install foam to both the interior and exterior of the rim joist to increase energy efficiency. However, this method will reduce the rim joist’s drying potential. 

Insulating your rim joist with interior and exterior foam will increase performance (ensure vapor barrier and dew point location is properly designed). However, if moisture contacts the rim joist another way, the rim joist will not dry quickly enough to prevent damage. 

Exterior Foam and Interior Fiberglass Batts

The best way to insulate your rim joist is to apply foam to the rim joist’s exterior side and fiberglass to the interior side (in most climates an exterior vapor barrier on wall sheathing is suggested with 70% of r-value on exterior).

If you’re already planning to install rigid foam to the exterior side of your wall sheathing, give your rim joist extra protection by extending this foam down to the mud-sill, or lower. In some cases, you meet your R-value simply by applying a thick enough rigid foam to your rim joist’s exterior. 

Increase your R-value, or add another layer of protection by adding fiberglass insulation to the interior of your rim joist. The exterior layer of rigid foam should keep the rim joist warm enough to prevent condensation, while the fiberglass will allow drying through the interior side.  

Building Codes & Safety

Rim joists affect the structural integrity of your building, so it’s important to meet all safety recommendations and requirements when constructing or insulating rim joists.

Keep reading to learn about the potential risks associated with fire, older houses, spray foam, replacement, and venting. 

Fire Safety 

Certain types of insulation are very flammable. 

The International Residential Code requires builders to cover the rigid foam with a thermal barrier, such as a layer of ½ -in drywall. Spray foam does not require a thermal barrier so long as the cured spray is no thicker than 3¼ inches. (Find more information about these requirements in section R316.5.11 of the IRC.) 

Most building inspectors don’t require Dow Thermax to be covered with a layer of drywall as it has passed fire safety tests. When selecting a brand of polyisocyanurate, Dow Thermax is a good option. 

You can also use mineral wool as a insulation and thermal barrier.

rim joist with mineral wool insulation

Older Houses

Older houses often lack a capillary break between the foundation and the mudsill, putting the rim joist at a higher risk of dampness and subsequent rot. However, even if your house has stayed dry for many years, adding interior insulation might impede inward drying – increasing the likelihood of rot. 

Spray Foam

Wear proper safety equipment when applying spray foam: respirator, gloves, goggles, and clothes that cover skin. You will also want to remove moveable objects from the area and cover any items you can’t move. Spray foam is nearly impossible to remove. 

Tip: Keep your spray foam in a warm location for 24 hours before using. This will make the spray foam easier to apply, reducing mess and exposure. 

Replacing Rim Joists

Although challenging, it is possible to replace rim joists if they become damaged due to moisture accumulation, insects, or other unforeseen hazards. 

You will replace rim joists from the outside of the house. (Because this involves removing sides of the house, complete this project in sections.) 

Remove any siding and sheathing to expose the rim joist. You will need to remove any strapping, bolts, or other hardware attached to the board. Finally, remove any wires, vents, or pipes installed through the rim joist; you will have to reinstall these things after putting in the new rim joist. 

Depending on the construction of your house, you might need to support the floor joists before removing any sections of your rim joist. Check the building’s structural integrity before demolishing any part of the rim joists. 

Once you’ve completed these preparations, cut and remove the damaged sections of the rim joist. Then, replace these pieces with new material, securing the new pieces into the existing joists. 

Reinstall any hardware you had removed, as well as any piping, vents, or wires. Reinsulate as necessary. 

Two-Story Houses

If you live in a house with more than one floor, the first-floor ceiling will have another set of rim joists. Unless you are especially skilled at DIY and construction, consider hiring a cellulose-insulation contractor to insulate these rim joists. 

Venting

If you need to run a pipe, vent, or wires to the outside from a basement, you can drill through the rim joist. However, make sure the hole isn’t so big that it will weaken the framing.

Air-seal and insulate around the hole after installing the pipe, vent, or wire. 

Conclusion

Rim joists are a main structural component in a house’s floor system. They close the cavities between floor joists, preventing twisting or other movements. 

Properly insulated rim joists will prevent moisture accumulation and subsequent structural damage from rot. Effective insulation can also help make your building more energy-efficient.

Remember that rim joists run around the perimeter of your house and provide critical structural support. Therefore, do not modify your rim joists without proper research and safety checks. When in doubt, ask a professional.  

References

U.S. Census Housing Data: Link

Vapor Throttle Presentation: Link