November 2020 EnergyWiseSM Tip: Rim Joists



By: Energy Efficiency Program Manager Cory Fuehrer

While it’s still dark outside, do you ever sneak into the kitchen or bathroom before everyone else wakes up and notice how cold the floors are? Worse yet, are you the last person up in the morning and still find your floorboard, tile or linoleum is just as cold as it was for the first person who got up? Since the overwhelming majority of Nebraskans don’t live in igloos, you may wonder why. Ask yourself, "Is my rim joist sealed and insulated?" If your home was built prior to the 1990s, you might want to find out.

Now, you may be asking, "Where do I find my home’s rim joist?" If you have a basement or crawl space, you need to look directly underneath the exterior perimeter of your main floor. If your basement is finished, it may be difficult to check because of a drop-in ceiling, paneling or drywall. On most homes, the rim joists, in combination with the sill plate, are the first pieces of wood that set on your home’s masonry or concrete walls or foundation.

Prior to the 1970s, many builders assumed the weight of the house would put enough downward pressure on these boards and the home’s foundation to keep the elements out. A decade later, most contractors installed a felt or rubber gasket between the two surfaces to reduce drafts and cut down on the number of bugs slipping through any gaps. In the end-cavity spaces between floor joists, they installed fiberglass insulation in an attempt to reduce heat loss. Unfortunately, outside air and humidity still infiltrated past these materials, causing heating and air conditioning systems to run longer.

Today, many older homes have huge energy-saving and indoor air quality opportunities associated with properly sealing and insulating rim joists. A Department of Energy case study revealed that the test group of older homes with rim joists later sealed and insulated had a 11.4%reduction of outside air infiltration. This translated to average annual heating and cooling cost savings of approximately 19.3%.

The "retrofit" practice of simply insulating rim joists with fiberglass batts is no longer recommended. Because fiberglass batts are air permeable, they do nothing to prevent warm, humid interior air from contacting the colder rim joists in winter. When this warm, moist air comes in contact with the joists, water vapor condenses. As the problem continues, this condensation can eventually cause mold and/or rot in the rim joist area.

To prevent these problems, this area must be properly sealed and insulated. The entire job can be done with spray polyurethane foam or impermeable rigid foamboard in combination with an expanding foam sealant.

For the best results, a two-part insulation/sealant foam can be professionally installed or Do-It-Yourself kits are available at most hardware or home project stores. A two-inch coating will provide at least an R12 insulation value. At the same time, these products provide a vapor-proof seal between the wood and masonry. This approach is the most convenient as it combines sealing and insulating into one step. Unfortunately, this approach also tends to be the most expensive.

To reduce costs for this improvement, rigid foam board can be cut into rectangles slightly smaller than the space between floor joists. Once inserted into the cavity, expanding spray foam is applied around the foam board’s perimeter to hold it in place while providing an air-tight seal. As with the previous method, a minimum of two inches of insulating material should be applied.

Southwest Public Power District and Nebraska Public Power District want to help you make the most of the energy that heats and cools your home. Contact them or visit if you would like information on other ways to keep warm this winter. In addition, you may be eligible for EnergyWiseSM incentives to reduce the cost of energy efficiency improvements.

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