Sealing and Insulating the Rim Joist

Dec 10, 2020

The rim joist, for those who are not familiar with building terms, is the edge of the wood floor framing system. It sits on top of the foundation walls, secured to the sill plate.
In a typical home, the rim joist area is a huge source of energy loss. It often lacks proper air sealing, so outside air is consistently leaking in through the wood joints, corner, gaps and openings drilled in the rim joist for electric wires, plumbing and ducts.

When the rim joists are insulated, it is often with pieces of fiberglass batt, which is not the best choice for this type of application. Fiberglass doesn’t stop air flow, so the air still leaks in. Also, fiberglass loses R-value when it is damp. Basements tend to be naturally-damp environments due to ground water evaporating into the space and condensation issues.

The results of poor rim joist air sealing and insulation can be felt throughout the house in the form of cold floors, drafty rooms and high energy bills.

The best materials for rim joist insulation are those impervious to water. When the rim joists are easily accessible, pieces of rigid foam board can be cut to snuggly fit on top of the rim joist, between the floor joists and then air sealed with spray foam.

In this particular job, however, Dr. Energy Saver chose another great option to insulate rim joists on top of stone basement walls: two-part polyurethane spray foam.

Two-part polyurethane spray foam not only provides superior R-value (R-7 per inch), but is also expands to fill gaps and holes in the rim joist, stopping air leakages in one simple step.

In most homes, even new constructions, there is always an area that can use some improvement in terms of energy efficiency. It can be the rim joist, or your attic that need better insulation. It can be doors and windows that are leaky or air ducts that need to be cleaned, sealed, balanced or insulated. There is often an opportunity to save money and energy. Your local Dr. Energy Saver dealer can help you discover ways to improve your home’s energy efficiency, while making your home more comfortable and affordable to own.

For more tips on how to save energy at home, watch our “On The Job” video series and subscribe to our YouTube channel for the latest updates.

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