Insulation of the Rim Joist Cavity: An Essential Home Upgrade

This is a writing sample from Scripted writer Kelvin Doerksen

Insulation of the Rim Joist Cavity: An Essential Home Upgrade

In northern climates, the space between the inside of the rim joist and the inside of the concrete wall is not just a cold zone, it's a frozen one. Homes with wood frame construction and a concrete foundation or basement must not only insulate but also seal this area with a proper vapor barrier. This area is a major source of heat loss and in today's modern, well-sealed houses, moisture in the form of frost and condensation can collect here, causing problems with mold, mildew, and eventually, rot.

The following method is a straightforward and practical solution for this area, regardless of whether the rest of the basement or crawlspace area will be insulated.

Step 1 -Clean the Rim Joist Cavity.

Thoroughly clean each joist cavity. Brush away debris and clean any mold or mildew with a product such as Concrobium Mold Control. Follow the instructions thoroughly.

Step 2 -Insulate and Seal the Rim Joist with Rigid Foam Board Insulation.

Use 2" thick rigid foam; for example, Foamular. This will provide an R10 insulation value, which is enough of a thermal break to function as a vapor barrier. If you have access to one, a small 9-inch bandsaw is perfect for cutting the small rectangles and odd shapes required.

Adhere the rigid foam directly to the rim joist. Be sure to use an adhesive specifically for rigid foam, like LePage PL 300 Foam Board. Once this is done, seal the perimeter of the rigid foam insulation with a foam sealant, such as Great Stuff Insulating Foam Sealant.

Step 3 -Fill the remainder of the Cavity with a Mineral Wool Insulation Product .

This type of insulation is superior to regular fiberglass insulation in basements for several reasons:

  • It does not absorb water.
  • It will not promote the growth of mold or mildew if it does get wet.
  • It is non-combustible and is a good choice to cover the rigid foam, which is a combustible product.
  • The R-value of mineral wool insulation is higher than fiberglass insulation.
  • Mineral wool also has excellent sound absorption qualities.

Mineral wool insulation may be covered with drywall or another wall covering if desired. Even if the basement is left unfinished, the rim joist area will be well insulated once this procedure is completed.

Written by:

Kelvin Doerksen
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Kelvin has been a Technical Writer and CAD Designer for the last 12 years. He has an extensive background in manufacturing, metalworking, and woodworking. Along with these technical skills, he also has a strong interest in history, current events, technology, and the general knowledge of things. Kelvin is also a dog lover and an avid outdoorsman.
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