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Community and Q&A

Controlling Bulk Water and Radon in Crawlspace

ylekyote | Posted in Green Building Techniques on

I have two major problems I’m looking to solve. I have Radon gas infiltrating from my open crawlspace floor and into my house, and I have water penetrating through my crawlspace walls and/or floor from the yard. I have a natural spring that saturates the soil above (and north) of my house and flows toward my house in the Spring to mid Summer months, and in the Winter/Spring the snow pack/melt I think also saturates the soil around my crawlspace. It saturates about 20″ of the soil and makes it too damp in the crawlspace for 4-5 months out of the year.

I’m trying to fix the drainage and the crawl space radon problem as passively as possible while also cleaning up the air quality in the crawl space, insulate it a bit, make it drier, and make some storage room for seasonal items. I’ve already plugged the 3 small vents and rebuilt the crawlspace door to make it pretty tight from the exterior openings.

I’m trying to solve the issues as passively as possible, without installing unnecessary materials and/or electrical devices (like sump pumps or radon fans).

As-is, on the wetter years like this past one I get about 1″ of standing water on the part of the crawlspace clay floor that borders my west facing yard. it stays close to that westward wall, about 36″ from it, and only about 3/4 to 1″ deep at wettest time of the year, for about 2 – 3 months. I don’t ever see water on the crawlspace walls, but I can see the salts leaching on them on the north and west sides, where the yard is really wet and higher in elevation.

I bought an electric radon meter. The radon reads between 15 and 30 in the crawlspace. Not super high but above the EPA’s random 4 measurement.

I think I need at least a 20″ deep PVC/gravel drain dug and installed in my yard, about 75 to 100′ length, around the northwest corner of my house. It needs to divert water away from my crawl space walls/floor. This water comes from the natural hillside spring, as well as snow melt that comes from above the home’s elevation. The drain can empty a few feet away from my home’s southwest corner. I’ll use it to water those small trees and such I have that can use the water. Maybe even fill a small 4′ pond for the dogs.

After the drainage is fixed, I need my 36″ to 40″ tall crawlspace (that’s floor to top of wall height), which is a 26′ x 31′ rectangle (or 806 SF) to be leveled enough to place 2 layers of 10 mil poly down and pour 3″ of concrete throughout its floor. My crawl space floor is relatively clean and level at present but it will require some material to be moved around to pour the slab level, perhaps. I have saved some scrap plastics to put down below the new 10 mil plastic is laid to help protect it from punctures and provide extra insulation between the dirt floor and concrete.

So if the yard drain takes care of 90% of the yard water from reaching the crawlspace walls, but I still receive some slight water from underneath the crawl space floor, will the dual layers of plastic and 3″ of poured concrete keep water from coming into the crawlspace from underneath? Or will drainage need to be installed on the inside of the crawlspace also, underneath the slab?

I want to avoid a sump pit and pump if possible. My crawlspace floor has a natural slope towards the south, so maybe a sump pump is not needed and the water can just be directed to drain through the CMU wall or under it, into the yard?

Suggestions on what is likely the best long term and most economical and simple fix for these issues? How’s my plan sound? Thanks. Andy

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  1. GBA Editor
    Martin Holladay | | #1

    Considering your goals and your description, you definitely need to install a drain pipe in your crawl space floor. This type of drain is usually called a French drain. You need to dig a trench inside your crawl space, along the side of the foundation where you think the water is entering. You half-fill the trench with 3/4-inch crushed stone, and then you install a dead-level 4-inch perforated pipe. Then you fill the remainder of the trench with more 3/4-inch crushed stone.

    At some logical point in your crawl space foundation, you have to dig under the footing for a trench that is big enough to slide a solid (non-perforated) 4-inch pipe under your footing. The interior side of this drain pipe is connected to the French drain with solid pipe. (Of course, you've planned ahead so that the French drain is at a higher elevation than the penetration under the footing.) The exterior side of this drain pipe runs to daylight, at a slope, and is also non-perforated.

    To adjust the slope of your crawl space floor, use 3/4-inch crushed stone, not dirt or clay.

    On the exterior of your house, adjust the grade to slope away from your foundation. On the uphill side -- presumably, that's the side with the wet area or spring -- create a swale. Then install an exterior French drain (or plastic catch basins, or both) in the swale, draining to daylight.

  2. qofmiwok | | #2

    Do the french drains inside the crawl have to be put around the perimeter? They just laid them in my foundation and they just run from a series of floor drains to a sump pump.
    Another set of piping has been run for the radon system.
    But now we're realizing the water drain system can introduce more radon. The drain to the dry well is open. Should it have a check valve to allow water out but gas not in? And I see there are sealed sump pump caps but should the based also be closed at the bottom? I believe ours is open to the ground.

  3. walta100 | | #3

    Is your home existing or yet to be built?

    It is best to capture and drain any water to daylight on the exterior whenever possible.

    Interior drains with sub pumps are a poor second choice.

    To me your questions have little to do with the 5 year old questions so a new topic may have been a better choice.


    1. qofmiwok | | #4

      Under construction. We have foundation drains on the outside going to a drywall. We also have interior drains with a sump pump going to a drywell, because there is a chance of high underground water every 7'ish years during high snow runoff. We believe we are well above that level but the Geotech can't tell for sure. No way to drain to daylight, thus the drywells.

  4. walta100 | | #5

    With a drywell the question becomes who designed the system and for what size rain event.

    If Billy Jo Bob dug a hole and threw in whatever tank he happened to be available it may work or not it is a matter of luck. It is a different matter if an engineer drew plans for a 100 or 500 year event for your watershed and put his stamp on the plan.

    Do you know what flood zone you are in?

    What will flood insurance cost for your property?

    Call me paranoid but I choose to live at the top of the hill.


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