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A puzzle: Moisture Mitigation and Energy Efficiency in Inaccessible/Unconventional Crawl Space

Tessa_FFSFarmhouse | Posted in Energy Efficiency and Durability on

Hello–apologies for this long post. I have an unconventional, puzzle of a problem. 

Our house was built in the early 1900’s in zone 3c (Sacramento, CA) and has an inaccessible, dirt (heavy clay is the natural soil here–aka slooow drainage), and marginally vented crawl space. I think the original house was built directly on the ground, and then overtime the ground level around it was raised, rather than an excavated crawl space. The crawl space has anywhere from 12″ to 24″ inches of clearance between bottom of floor joists and dirt floor, and there’s old railroad ties in some places (not touching current joists) that would further obstruct movement of a person. The house is 1,100 sf with only three 12 in. x 4 in. vents. 

We’ve had the subfloor open in several spaces for the past 2+ years to fix major wood rot and termite damage issues and to redo plumbing. This has allowed us to observe the problem. 

Moisture! mostly around the interior perimiter. We used various sealants and caulking to fix all cracks in the stucco siding and sidewalk walkways. We thought this would solve the problem; however we’re still seeing moisture in the soil. Based on the floor pattern of the moisture, I believe this is ground water. We have not seen any pooling, and this year brought very heavy rains, including the highest single-day rainfall on record during which we did not have gutters. Still no pooling. 

Goals (in order of importance) 
1. Stop woodrot and mold.
2. If mold can’t be avoided in this situation, keep it from seeping upwards into the living space and walls.
3. As best as we can in the situation, insulate the floors for energy effiency. 

1. I think the “right” answer is to jack the house up so there’s an adequate crawl space, we can condition it, etc. This is not financially possible for us. 
2. I think the second-best “right” answer is to remove all the subfloor and do our best to lay down a plastic vapor barrier. However, we’ve already replaced most of the subfloor in the house as the prior owners had a hoarder/critter problem and the old OSB was soaked in dog/mice urine. Subfloor is T&G 3/4″ ply that was attached with liquid nails, so I think it would be extremely wasteful to rip all this up. Not to mention current inflation on wood. 

Things we’ve considered and questions: 
1. Given this is mostly a moisture issue and not a pooling issue, would a commercial style dehumidifer (or 2) be sufficient? I am considering building a raised box in the subfloor in 1 or 2 closets where the dehumidifer could sit in the crawl space, otherwise, I don’t think there’s enough floor clearance. I think this would only be needed during a few winter months. 

2. What about fans in the crawlspace? I am considering the fans linked below and building hatches in the floor where they are located so we can access them if they break:

3. Should we seal up the three piddly vents? If so, how will we get sufficient airflow to remove moisture? Will sealing up the vents create condensation or other issues? Should we go fully the other direction and add a ton more vents since they can only be about 4″ high? 

4. For insulation, I’m planning on “air sealing” the spots where subfloor meets wall sill plates, and over the sill plates with caulk, foam, or some kind of tape. Any suggestions? Also considering laying down some thick cork for temperature control with a moisture barrier attached. Any suggestions?

5. We have gutters. Anything else we can do outside? 

Thank you!

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  1. Expert Member
    NICK KEENAN | | #1

    Is the house on a foundation? What's it made of?

    1. Tessa_FFSFarmhouse | | #2

      Sort of--its patched together. The perimter is made up of 4x8s that are either sitting on poured cement that extends about 2 ft into the crawl space or piers spaced every 2 to 4 ft. They poured concrete all around the outside of the house at some point, but this isn't providing any structural support. The spot where there is the most moisture is the side of the house that only has a 4ft sidewalk. But the driveway side has some moisture too, which is part of why I think its groundwater.

      1. Expert Member
        NICK KEENAN | | #3

        What is the ground level relative to the bottoms of the 4x8's?

        1. Tessa_FFSFarmhouse | | #6

          1" to 2"

          1. Expert Member
            NICK KEENAN | | #9

            Above? Below?

            Here's where I'm going with these questions. I don't think groundwater is an issue. What I do think is happening is that rainwater is getting in there. The key to addressing rainwater is diverting it, getting it to go somewhere else. Rainwater follows the laws of gravity, it wants to flow down. It's much easier to deal with it if the thing you want to protect is above the ground level than below. So these details matter.

            I also think there are two other things going on. One is that moisture is entering your house's wood framing through capillary action. That's the tendency of liquid to move from wetter areas to dryer areas in a porous material. Capillary action is not affected by gravity, it works in all directions. You want the wooden parts of your house to be isolated from soil and concrete by what's called a "capillary break" which is basically a layer of waterproof material. That's straightforward if the house is above the dirt, more complicated if it's below.

            The third thing I think is happening is that moisture is evaporating out of the soil and raising the humidity in your crawl space. If you have any kind of temperature gradient that elevated humidity will lead to condensation, which will cause problems. This is easily solved by putting a vaporproof layer over the soil. Ventilating the crawlspace can also help but that can be tricky.

            So ideally, what you want to be thinking is grading the soil around the house so that it drains away in all directions, and all wooden parts of the house are above grade; inserting a capillary break between the wooden parts of the house and what they rest upon; and covering any exposed soil in the crawlspace with a vapor barrier, which could be as simple as plastic sheeting.

          2. Tessa_FFSFarmhouse | | #13

            Thanks @DCCContrarian. Unfortunately the foundation is below the ground level. It's helpful to think about what we can do outside to move the water away from the house. Will probably add in some french drains and do some grading at the end of the rainy season. Make sure all rain run off is at least 10 feet from the house.

            Since the foundation is below the ground level, not sure how we could add capillary breaks at this point. It does look like there is poured cement that was poured up against the redwood siding in most areas, but I know that cement brings in moisture. Not sure what else we could do. Maybe this is where we have to just accept the soil gets moist and turn on a commercial-style dehumidifer in the winter? In the summer, the soil gets bone dry. You can't get a shovel more than half an inch down.

  2. walta100 | | #4

    The type of construction you have described sound pretty sub standard today.

    It is amazing that untreated wood in direct contact with damp soil managed to survive 100 years.

    Seems to me you should be very careful about making changes to a system that has worked for 100 years. I think the last thing you want to do is seal the vents without a solid plan to condition air in the crawlspace, insulate the walls and add a vapor barrier over the dirt.


    1. Tessa_FFSFarmhouse | | #8

      Hi Walta,

      It is amazing--in some spots the walls were essentially floating because wood floor joists had rotted so bad. We think the sheer strength of these 8" redwood siding is what held the house together. At some point, it seems there was a fire and wherever they replaced the redwood floor joists with modern lumber it rotted away.


      1. walta100 | | #11

        “I think the original house was built directly on the ground, and then overtime the ground level around it was raised, rather than an excavated crawl space.”

        Is there concrete foundation under this house?

        It sound to me like it is just resting on some beams sitting in the dirt. If so It seems like the wrong way to build a house and something no one would do today.

        Yes red wood is rot resistant but in contact with the dirt it will rot away given enough time and sooner or later this house will need a real foundation you need to prepare yourself for that eventuality.


        1. Tessa_FFSFarmhouse | | #12

          It was originally beams sitting in the dirt. At some point , they put in a bunch of concrete blocks and now its beams on concrete blocks over the dirt. Yes, not a standard or modern way to build. But for environmental/waste reasons, I prefer working with what we have, rather than ripping down a house and starting over.

  3. Expert Member
    BILL WICHERS | | #5

    It sounds to me like that moisture is seeping through the foundation. That probably means no exterior damp proofing, which is difficult to really fix from the inside. A full encapsulation project (putting in a poly liner) will help, but you'll need to be extra careful to protect the wood structure where it's in contact with the masonry in this case to avoid issues with rot. You'd absolutely want to install capillary breaks.

    You would seal those vents after putting in the liner. You may still need a dehumidifier though. I would NOT seal those vents if you don't also line the crawlspace, since those vents are probably limiting the moisture in the space right now, assuming the source of the moisture is the ground.

    Regarding #4, you need to think about protecting the wood structure first. Anytime you have moisture issues, insulating can make things worse, so it's important to address the moisture problems BEFORE you add insulation and seal things.

    For #5, make sure those gutters are directing water away from the house. Make sure the ground near the house is sloping AWAY from the house. Do everything you can to limit the amount of water that can seep into the ground up near the foundation where you're having moisture issues. If you really want to go all out, you can escavate the exterior of the foundation, add damp proofing, and possibly add additional drainage too. This gets expensive though, and you have to be careful not to damage old foundations while doing this kind of work.


    1. Tessa_FFSFarmhouse | | #7

      Thanks, Bill.

      Since a full encapsulation project isn't possible (and would be really hard to do in spots where wood is in contact with masonry without trapping the wood inside "moisture zones"), do you think we should try to add some vents? If we go this route, I was thinking of using flexible drainage pipe that goes from the exterior wall, down through the sill plate and into the crawl space, and attaches to steel vent covers ( Otherwise, we have a lot of vents reallly close to the ground (see attachment).

      RE #5: I just looked up exterior damp proofing. Around the exterior of the house, there are sidewalks and driveway made of concrete. Rather than excavating, would it be helpful to use some sort of paint-on product around the perimeter of the house, covering the walkways? We also have a ton of old brick. What do you think about laying down mortar with a slope (like you would in a shower pan), putting a liquid proof membrane on top, and then laying brick over it? Too much?

      1. Expert Member
        BILL WICHERS | | #10

        More vents are unlikely to make the moisture problem worse, but you do run more risk of critters getting in.

        Damp proofing needs to be between the foundation and the ground, so painting it on things like sidewalks and driveways won't really help.

        I've seen people put "roofs" over ground close to their home, and it can help -- you just have to make sure any water shed by that roof doesn't just soak back up under the ground underneath.

        I'm not sure what you're trying to describe with the "flexible drain pipe" tied into steel vents?


        1. Tessa_FFSFarmhouse | | #14

          Thanks, Bill. With the flexible drain pipe, I was thinking that air would rise up through the pipes into the walls, and then outside. This is because the foundation crawl space is below or near-below the outside ground level, making vents hard. But upon more research, I dont think this will work unless it was attached to a fan system.

          1. Expert Member
            BILL WICHERS | | #15

            You'd get some convection airflow by doing that, similar to the draft produced by a chimney, but it won't be a huge amount of airflow. You'd need fans to really make it work. I'd just try conventional vents first though.


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