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How to seal off and insulate a sloping, tall crawlspace?

Hi guys,
I'm in Oakland, California. My 1-story house is built into the side of a hill. The crawlspace is tall enough for a person to stand in at the front and rises to the mud sill at the back of the house. The cripple wall is 4 ft tall at the front.

I want to build an unvented crawlspace to improve the air quality in the house. I will be installing poly sheeting and insulating the cripple wall and foundation as if it were a basement without a floor.

The problem is that the dirt is 2 ft below grade at the front of the house and floods when it rains. I don't think a poly sheet is going to stop this from happening.

Should I back fill to exterior grade and call it good or install an interior french drain and sump pump? Both?

The foundation is also concrete-capped brick. It's in good condition and I'm waiting to see if I will have to replace it due to seismic concerns. There may be some kind of sealing and insulation opportunity there if that has to happen. I'm not interested in the cost of building out a real basement or lifting the house for a second floor, but who knows maybe I'll end up spending just as much doing this basic stuff!

Asked by Mateo Hao
Posted Jul 7, 2018 10:35 PM ET
Edited Jul 10, 2018 8:39 AM ET


3 Answers

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Answered by Steve Knapp CZ 3A Georgia
Posted Jul 8, 2018 1:00 PM ET


Hi Mateo,
You have a complex problem you are dealing which I'm also familiar with because I have an identical situation I'm dealing with and my house is also in Northern California on a hill. First off, living here is a different situation than the eastern USA. The humidity here in summer doesn't warrant sealing the crawl space. That's only for regions that have high humidity in summers.

My feeling is that the water flowing into the crawl space will be exacerbate poor conditions if you seal it. The previous owners of my house elected to do what you are thinking of doing in regard to putting in a french drain in the crawl space. It didn't work and the crawl space still flooded badly in winter. I ended up removing it and putting in hydraulic cement to fill the hole remaining in the uphill foundation wall after removing it. The problem is that one really has to execute a French Drain perfectly if it's routed under the house. If you don't execute it perfectly it then becomes a method for water to ENTER the crawlspace instead of EXITING it. That's what happened to my house.

A better plan if its within your ability is to create a barrier to the water draining in from the uphill side of the foundation. In other words, dig down to the bottom of the foundation on the uphill side on the exterior side of it and hire someone to seal the foundation on that side. I doubt that you'd want to do that yourself. You will still have hydraulic pressure on the side when it rains so a french drain should then be placed where the water would otherwise back up and then slope it so it goes around to the side of the house and out to daylight following the slope.

If it isn't feasible to do that then you can still do the interior French Drain but it will not be a total solution. At least it sure wasn't for my house. The poly sheeting over the ground in the crawlspace and up the interior foundation wall is a very good idea. Best of luck.

Answered by Eric Habegger
Posted Jul 9, 2018 1:50 AM ET


1. If you are waiting to find out whether you will need to replace your entire foundation due to seismic concerns, then you shouldn't do anything about the water entry problem until you get an answer to your structural questions. If you end up getting a new foundation, your foundation contractor can provide good drainage details at the time the new foundation is installed.

2. If your foundation is good as is, then I think your suggested solution of installing an interior French perimeter drain, drained to daylight, along with installing crushed stone inside your crawl space to raise the level of the crawl space floor to be equal with or higher than exterior grade is a good plan.

If you go this route, install a layer of polyethylene above the new crushed stone, followed by a rat slab.

For more information, see "Fixing a Wet Basement."

Answered by Martin Holladay
Posted Jul 10, 2018 8:45 AM ET

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