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Why Is The Rim Joist So Important?

Tuesday, November 12th, 2013 by Brian Stewart


by air sealing and insulating this rim joist with spray foam, it ensures air won't escape out of this St. Louis homeFor many homes in the St. Louis area, the rim joist is a major source of energy loss and discomfort.  How does that happen?  First, it helps to know what a rim joist it. 

Most homes in our area have either a basement or crawlspace with foundation walls.  The foundation walls may be made of poured concrete, block, stone, or other materials.  Most of the time a piece of wood called the sill plate lays flat all around the perimeter on top of the foundation walls. 

The rim joist is another piece of wood that stands on end, all around the perimeter, on top of the sill plate.  In the case of a basement, the rim joist is a piece of wood that is the only thing separating outside air from air inside the basement. 

Fiberglass insulation isn't the best solution

Sometimes builders or homeowners cut fiberglass insulation batts and stuff them against the rim joist all around the perimeter of the foundation.  The same thing is often done in crawlspaces.  While well-intentioned, this generally has very little benefit because outside air flows through the seams around the rim joist, right through and around the fiberglass insulation, and into the basement or crawlspace. 

Why does outside air come in through the rim joist?  Outside air is pulled in by something called the stack effect.  Most of the time, the airflow in any house is from bottom to top.  This is caused by warm air rising and building air pressure at the top of the house.  This pressure at the top forces air out of the house and into the attic.  The loss of air at the top pulls outside air in at the bottom to replace it.  Where is the bottom? 

In most cases, the bottom is the rim joist.  In winter, cold outside air is constantly pulled into the basement or crawlspace.  In summer, hot humid air is pulled in.  All year round, the outside air pulled in through the rim joist brings dust, pollen, mold spores, dust mites, and allergens of all types. 

What are the options if I  have a leaky rim joist in my crawl space?

For many houses, air sealing the rim joist can be the one thing that makes the most difference in addressing problems with energy waste, discomfort, and indoor air quality.  What is the best way to air seal the rim joist?  Two-part spray foam is by far the most effective way to do the job. 

Two-part spray foam expands to fill all the seams and cracks around the rim joist.  The foam adheres directly to the rim joist ensuring no air space can exist between the foam and the joist.  In addition to creating an air seal that stops outside air from being pulled in to the basement or crawlspace, spray foam is also an excellent insulator. 

Two inches of two-part spray foam applied to the wooden rim joist will ordinarily result in an R-value of R-13.  In theory, this is same as an R-13 fiberglass batt stuffed against the rim joist.  In reality, the R-13 spray foam will perform much better than the R-13 fiberglass batt because the rim joist also creates an air seal. 

How do I know if my rim joist is faulty?

How can you find out whether your rim joist is leaking air?  Have an energy audit to include a blower door test.  Then you will know whether the rim joist is a significant problem for your home.  Air sealing and insulating the rim joist with two-part spray foam might be just the prescription for what ails your home.

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About the author
Brian Stewart is the General Manager of Dr. Energy Saver St. Louis. He has many years of experience making homes more comfortable and energy efficient.

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