What is a rim joist?

LED lighting, energy-efficient appliances, a smart thermostat, and replacing draughty old windows and doors are just a few of the numerous methods to make your home more energy-efficient. Insulating your home is one of the best ways to save money on your utility bills, but there are many more ways to go green. It doesn’t stop at the external walls and attic; proper insulation extends to the basement or crawl area. Rim joists, a common source of heat loss in many houses, can be sealed and insulated in just a few hours. Rim joist insulation will help you save money by reducing your heating expenditures.

What is a rim joist?

What is a rim joist?  Rim joists if the snow melts quickly around the foundation of your house when the temperature is below freezing. If you’re unfamiliar with the major structural components of a floor frame, joists, you’ll want to read up on them first. Two more perpendicular supports, rim joists, are butted up against either end of each floor joist. The “band” or perimeter of the floor framework comprises these two rim joists and the two outermost floor joists.

Benefits of rim joist insulation:

Rim joist insulation helps seal air leaks and keeps moisture out of your home’s foundation; it also provides a thermal barrier at the bottom of the building envelope. It’s a common misconception among homeowners that stopping the chimney effect is as simple as fixing attic air leaks. However, the job isn’t complete until the leaks in the basement are sealed as well. Following are the benefits of Rim joist insulation:

Concerns about health and safety:

It’s possible to have severe issues with spray foam. Inflowing the manufacturer’s recommendations, such as mixing at the incorrect temperature or applying a layer that is too thick, can lead to incorrectly installed spray foam work. When dealing with the chemical off-gassing of a two-part spray foam system, all inhabitants must exit the building or wear adequate ventilation. It cannot be stressed strongly enough. Manufacturers claim that the components are hazardous when used.

Saving Money on Spray Foam:

Materials can be reduced in two ways. Spray foam insulation should be applied first, then a layer of rigid foam insulation. Apply a 2′′ layer of spray foam followed by fibreglass insulation for another alternative. While the spray foam acts as a moisture barrier, the fibreglass acts as an insulator by increasing the insulation value. DIY spray foam kits are available for purchase. Spray foam should be handled with extreme caution. A respirator, full-body suit, gloves, and eye protection are necessary.

Why is Vapor permeability not required for rim-joist insulation?

Rim-joist insulation does not need to be vapour-permeable “to allow a damp rim-joist to dry,” according to some homeowners. On the other hand, Rim joists are more susceptible to damage from moisture in the air than they are to benefit from inward drying. Choosing an air-impermeable type of insulation with low vapour permeance makes the most sense. Outward drying is possible in most circumstances since the sheathing, house wrap and siding are all located on the exterior of the rim joist.

How does it all work?

The rim joist area is best sealed with commercially prepared spray foam. The spray foam fills in and packs all the gaps and holes in the rim or band joist area. An experienced insulation contractor with the proper mixing equipment is typically employed to apply it. Spray foam polyurethane is a two-part material. Chemicals are heated and blended on the job site as the foam is sprayed.


Building professionals no longer suggest replacing fibreglass insulation for air sealing and does not comply with current building requirements in this region of the home. Fibreglass is poor for insulating the draughty rim joist area since it is easily permeable by air. A proper air seal is nearly impossible in this tiny environment. According to the advice, the rim joist fibreglass insulation should be removed and replaced with better insulation and a vapour barrier system.

Exterior rim joist insulation:

A continuous layer of stiff foam on the outer side of your wall sheathing can reach the mudsill level. You’re done if the stiff outside foam is thick enough to fulfil your R-value goal. Because the external rigid foam insulates against cold air, you can safely add some fibreglass insulation to the inside side of your rim joists to increase their R-value.

Insulating the rim joists in a cold basement:

When the seasons change, you’re more likely to notice a shift in your home’s temperature. Some sections in your home may be noticeably warmer or colder than others as Michigan’s cold weather and snow approach. If the temperature in your home is constantly fluctuating, it could be a sign that there are air leaks or insufficient insulation in your walls, windows, or attic. Your floors may be too cold because of inadequate insulation if you have to reach for an extra pair of socks or pull out your slippers to keep warm.

Insulation and air sealing:

Insulation and air sealing in your basement rim joists can also help keep pests and creatures out of your home. Intruders will have difficulty getting into your house in the winter if your rim joists are adequately insulated.

Rim joist insulation spray foam:

As an added benefit, rim joist insulation spray can do double duty as an air leak sealer and an insulator when used to insulate rim joists, a procedure known as the critical-seal method. Even though transparent fill material can be utilized for chilly climates, it is preferable to closed-cell spray foam for moderate temperatures. A two-component kit is likely to be required unless you employ a spray foam contractor.

Rim joist insulation fire code:

Foam can be as comprehensive as 3.25 inches “Nonetheless, See Section 314-11 of the IRC for more information. A total of two layers “Rim joist insulation fire code and a fire hazard would be thick foam board.

Rim joist insulation faced or unfaced:

Rim joist insulation faced or unfaced has a specific purpose in a home’s insulation and must be installed appropriately to keep out heat, sound, and vapour transfer. Mould and wood rot can occur if the vapour barrier is facing the wrong way when faced insulation is utilized, resulting in moisture being trapped. When it comes to reducing heat loss, vapour movement, and sound transmission through your home’s walls, both faced and unfaced insulation have distinct advantages that should be taken advantage of.

Rim joist insulation cost:

The cost of insulating a rim joist is heavily influenced by the region’s size that needs to be protected. Between $1,200 and $1,500 is the rim joist insulation cost with a spray foam insulation system. When it comes to insulating and sealing cracks, spray foam is an excellent alternative to traditional filler. Depending on how many rim joists need to be covered, the cost of this project can vary greatly. However, larger insulating projects, such as an attic or basement insulation, are less expensive.


The perimeter of a wood-framed house’s floor system comprises rim joists. A typical rim joist on a new house is a 2×10 or 2×12. Rim joists in older homes can be anything from 2x6s to 2x8s or even square timbers like 8x8s. When a place is balloon-framed, the bottom plates of the wall framing lay directly on the mudsill; therefore, there are no rim joists.


Is it better to put in insulation with the paper facing out or in?

Whether fibreglass insulation is installed in a home, the report facing should always be inside.

Rim joists require what kind of insulation?

Rim joists should be insulated with rigid foam. We went with “extruded polystyrene” that was 2 inches thick. Keep away from the less effective “expanded polystyrene,” a type of plastic that has been expanded.

How successful are basement rim joists in insulating the basement?

You’ll want to seal up the rim joists of your floor structure with this home improvement.

As far as basement insulation goes, why not use fibreglass?

Rim joist insulation should only be made of air-impermeable materials at this time. Rim joists have traditionally been insulated with fibreglass batts; however, this practice is no longer appropriate.

Why should basement rim joists be insulated with spray foam?

Air-impermeable insulation comes in two flavours: spray foam and stiff foam. This type of insulation is ideal for basement rim joists at Ecotelligent Homes since it can fill in all gaps.

Read also: Insulation for the interior of your home.