Resources & Articles

Most Effective Way to Insulate at the Rim Joist

The rim joist is the space between your basement walls and the subfloor of your home.

The rim joist is above-grade, so it makes sense to insulate the rim joist to the same degree as the above grade walls. Stuffing fiberglass insulation between floor joists is a standard method of insulating the rim joist in many homes. Because fiberglass insulation doesn't fit snugly, it is air permeable. Rim joists (Ban Joist) expand and contract with the change of seasons allowing air to enter and exit at will.

Unlike fiberglass, which is only a thermal barrier, spray insulation creates both an air and vapor barrier at the rim joist. Moist air can filter through fiberglass insulation creating condensation at the rim joist, which can lead to mold and mildew problems.

Spray insulation sticks to the rim joist eliminating the condensing surface. Improper insulation at the rim joist can result in high energy bills, air quality issues, and moisture damage. The spray insulation is highly adhesive, so it sticks and stays in place as it expands to fill gaps. Once cured, the spray insulation provides an effective air seal as well as insulation. Because the spray insulation doesn't allow air to move through it (which diminishes R-value) it always performs at its full rated R-value.

Benefits of Rim Joist Spray Insulation

• Reduces heating and cooling costs
• Fills hard to insulate areas
• Does not sag or settle over time
• Protects against moisture
• Adheres to existing structure