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Community and Q&A

Insulating Basement/Crawlspace… among other things

CurryRojo | Posted in Energy Efficiency and Durability on

My house was built in the late 1800’s and is in climate zone 4C in Oregon. It has about a total footprint of 660 sq ft and is half basement/half crawlspace with a cripple wall all along the perimeter. Both basement and crawlspace are unconditioned with no venting. The grade slopes toward the house from the backyard. I’d say in the backyard the grade is half way up the 19″ high cripple wall and in the front yard grade is 8″ below the top of the concrete foundation.

There are a number of issues I need to address. There was dry rot on a number of old growth floor joists where the joist rests on the cripple wall. Many of these were sistered with what appears to be old growth floor joists (it doesn’t appear that any of the sistered joists were secured to the joist with dry rot, rather they were just placed next to them). Some water weeps in through spots on the foundation wall creating a tiny puddle or two in the basement and some areas in the crawlspace foundation have efflorescence. The dirt in the crawlspace is dry although there is some white fungus in spots on the dirt when peeling the vapor barrier back. The house is not tied to the foundation for seismic.

I know I need to address the drainage to get water away from the foundation and the grade so that the soil is below the top of the foundation in the backyard (this is difficult because my yard is not big), but this will hopefully be a summer project when it isn’t raining all the time. I will also encapsulate the crawlspace.

What I am concerned with right now is the humid air getting in through cracks, sistering up some joists, and seismic retrofitting the house.

This was my plan of action:

1. Air seal between the top of the foundation and the mudsill with silicone caulk or spray foam
2. Air seal top and bottom of rim joists and any gaps in the cripple wall boards. (stud bay behind electrical panel is inaccessible)
3. Insulate accessible cripple wall stud bays with 1 1/2″ rigid foam and spray foam along perimeter of rigid foam. (some drain pipes prevent insulation from being installed).
4. Install 1/2″ plywood for seismic sheathing with ventilation holes.
5. Install foundation plates for seismic
6. Sister cracked or rotten joists that aren’t already sistered.
7. Cut out dry rot sections on old joists because it looks terrible.
8. Install 2″ rigid foam in rim joists and spray foam around perimeter.
9. Install blocking in rim joists to secure floor system to top plate of cripple wall for seismic. I was told by a seismic contractor that I don’t have a traditional rim joist so I need to install blocking in the rim joists to be able to secure the floor system to the top plate of the cripple wall.

A few questions:

Is #3 a good idea, or a waste of time? I read that a continuous layer of rigid foam is ideal, but I need to install 1/2″ plywood with vent holes along the cripple wall for seismic, so I figured I’d insulate within the cripple wall stud bays and then install the sheathing from the mudsill to the top plate for seismic. I don’t think exterior insulation is feasible because the 3/4 of the cripple wall is stucco before transitioning to asbestos siding. I wouldn’t have a way to easily address insulating the entire cripple wall from the exterior.

For #8 and #9, should i flip them or is rigid foam good first and then blocking after rigid foam is installed?

Are there any better solutions from what I have outlined or does anything sound like a bad idea?

Eventually I want to insulate the foundation walls with 2″ rigid foam but I would like to fix weeping areas and efflorescence on foundation walls before doing that.


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  1. GBA Editor
    Martin Holladay | | #1

    You have a lot of issues here. I'm guessing that it's going to be hard for a community of GBA readers to provide you good advice over the Internet, based on your description alone.

    If I understand correctly, you have a wood-framed cripple wall that is below grade. My guess is that all of the lumber in that wall is damp or beginning to rot. So I would hold off on all work until you have addressed the grade problem. It's essential that you lower the grade in your back yard, so that the grade is at least 8 inches lower than the top of your foundation. You also need to create a swale that directs surface water away from your house.

    Only then can you assess how rotten your cripple wall is.

    In most cases, I would recommend replacing the cripple walls all the way around the house with CMUs (concrete blocks). But the seismic retrofit complicates things. I'm not qualified to provide seismic retrofit advice, so you are going to have to depend on local advice for that aspect of the work.

    Once you have lowered the grade around the house, created a swale, and completed the seismic retrofit work, you can catch your breath and come back here for advice on insulation. (Somewhere in there you'll need to replace a few more floor joists, I suspect.)

    Here's what I'm getting at: Insulation details are the least of your worries.

    -- Martin Holladay

  2. CurryRojo | | #2

    Thanks for the response. Yes, I'd say 1/3 of the perimeter of the cripple wall is partially below grade. It has stucco all along the exterior, which I'm guessing doesn't help much...

    Can I easily evaluate whether the cripple wall is rotten by poking it with a screwdriver?

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