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Community and Q&A

Below-Grade Rim Joist Protection and Insulation

home4daly | Posted in Energy Efficiency and Durability on

Hello GBA community,

I have an 800sf single story house in Ann Arbor Michigan (climate zone 5). The house was built in 1948 and has a block wall basement foundation.

The basement has a nice ceiling height, about 8 ft, but it seems as if the builders dug the hole for the foundation too deeply: the house sits so low in the ground, that the bottom of the rim joist sits about 4 inches below grade. I believe additional top soil was added over the last 10-15 years, probably just before the house was “flipped”.

The exterior walls are covered with cedar siding all the way down to the block foundation (so the last two rows are now below ground.) Someone added a row of Hardy plank over the cedar siding where it is below ground, but that doesn’t actually provide much protection since the bottom of the cedar is still exposed to the soil.

I want to remove all of the cedar siding and add ZIP-R sheathing and then some type of cement siding. I know the ZIP sheathing cannot be placed below grade so I am currently planning to stop the sheathing at the top of the rim joist.

I also want to dig a few feet around the perimeter of the house and install 2 inches of EPS insulation over the block wall.

I am thinking of extending this EPS board up to the top of the rim joist. (So it would run from about 2 feet below ground along the block wall and continue over the rim joist and up to the top of the rim joist.) I intend to install a Z-flashing between the ZIP-R and the EPS board.

I am concerned about creating rot or mold issues. (I have not noticed any existing issues in the rim joist yet, but I have not removed the cedar siding yet either, so I don’t know what it looks like… I have found rot in other parts of the above grade wall – due to rain.)

Also, I believe the wood simply sits on the block wall without any type of gasket between the block wall and the rim joist.

Any comments or suggestions would be appreciated.

Thank you,


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  1. Expert Member


    I can think of two ways to deal with this.

    - The first is to excavate around the house and then build a short retaining wall about 12" away so that the grade was about 6" below the top of the blocks. You are then able to use any of the ways of insulation the rim joist area that are commonly used on houses.

    - You can excavate, remove the siding, replace with a waterproof membrane and covered by some type cladding as protection. If you choose this option you will have no drying to the exterior, so the wall should be designed to dry to the inside, and to keep the rim-joist area safe, should have an adequately thick layer of exterior foam.

  2. Expert Member
    BILL WICHERS | | #2

    I 100% agree with Malcolm's Recommendation #1 -- excavate and remove enough of the ground so that the rim joist sits ABOVE grade. Slope the ground away from the house while you're at it, so that water runs away from the house.

    If you can't do the excavating, then you're stuck with the retaining wall method. If you go that route, I'd dig it deep enough to put in some drain tile, then fill with washed gravel. This will limit mud splashing up onto your siding, and will provide drainage so that you don't have any water pooling up around the foundation of your home. You'll need to drain that drain tile to somewhere though...

    Ideally you want any perimeter insulation to go down to at least the frost line, which is around 4 feet deep in Southeast Michigan (I'm a bit North of you near Clarkston). The easiest thing to do would be to just bury entire 4x8 foot sheets sideways, with the top up to the top of the foundation wall. XPS is a better material to use here than EPS. I would insulate the rim joists with EPS to allow for a bit of drying. On my own home, I put the EPS on the interior side of the rim joist since I have foil faced polyiso over the exterior side, and foil faced polyiso doesn't allow for any drying at all.

    A capillary break between the top of the masonry foundation and the bottom of the wood framing will help prevent moisture/rot issues in the framing. This isn't as much of an issue if you insulate the exterior of the foundation as it is if you were to insulate the interior of the foundation, but it's still something that you really want to have in place.

    Lastly, treating the rim joist with Coppercoat -- a wood preservative -- will give you a little bit of extra insurance from rot, but only a little. Coppercoat should not be considered to be a substitute for any of the more involved things that have been recommended to you so far.


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