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Community and Q&A

How do I stop moisture between foundation and slab?

jfincherok | Posted in Green Building Techniques on

I have a homeowner who is having moisture issues in one of her bedrooms. Everything we build now has a capillary break and vapor barrier under the slab and a WRB on the exterior sheathing of the home, but this one, like hundreds we’ve built over the years and countless thousands in our market doesn’t have either. This home has a pier and grade beam foundation with a floating slab on about 16 inches of fill sand. We are getting some moisture at the base plate from the outside (I assume) and some coming up through the joint between the slab and the foundation and it’s causing a strong musty smell. I haven’t had this happen before.

After reading an article here, I think the water at the base plates could very well be from solar vapor drive as this room faces south and west. We’ve had a lot of rain and we also have strong winds from the south which blow water from the sprinklers on the brick keeping it wet frequently. I think I’ll have to take off the brick veneer, install a drainage plane on the wall sheathing and make sure the air gap (between the brick and wall sheathing) breathes. I’ll also seal it off well to form a good air barrier.

I’m not sure how to handle the water coming up between the foundation and the slab. We had caulked the joint, but the caulk is wet to the touch and coming out. I considered digging down and placing aggregate on the outside of the foundation for a capillary break, but I assume this will be ineffective as the moisture can still go under it and back up through capillary action. I also considered finding some kind of waterproofing membrane over the joint and a concrete sealer in the rest of the room. Any suggestions? There is pretty fair drainage away from the house. No standing water. Thanks.

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  1. wjrobinson | | #1

    Stop sprinkling the home and nearby area?

    Tea tree oil is good for musty smells when you go to clean up.

    Go to Walmart, and get a bottle, shake it up in a spray bottle full of water before spraying.

    As to your rebuild, seems like you will need to do something if you don't stop sprinkling the home.

  2. wjrobinson | | #2

    High Performance Silane / Siloxane Water Repellents could possibly help you.

  3. Armando Cobo | | #3

    I once found that the weeping holes in the bottom row of the brick were plugged and it prevented from draining properly. They should be open every 3 to four bricks apart. Sometimes bricklayers use too much mortar when laying brick and some of that mortar falls behind the brick wall and never gets cleaned. That is a common way for water to move to the wall or to allow “flooding” behind the brick wall, it’s worth checking before tearing a whole brick wall. You could take out a brick or two from the bottom row to make sure is not plugged up too.

  4. jfincherok | | #4

    Thanks for the responses. Upon inspection I found that the weep holes were missing entirely and are being put in today. This will probably help the moisture at the floor plate, but I don't think it will eliminate it. We've torn off a brick wall from this same bricker previously and found ridiculous amounts of mortar not only at the bottom, but in the field also such that it had contact with the sheathing. It's infuriating. I still think I'll have to tear down the wall and put a WRB up.

    I still haven't figured out the best way to stop the moisture coming up at the joint between the floating slab and the foundation. As I stated previously, caulking did not work. Perhaps I can talk the homeowner into laying plastic down in the flower bed and running a drip system (at my expense). Is there any way to reduce capillary action under the slab at this point? It seems to me that even if I put gravel 4 feet down around the foundation that water would still wick down, under and back up to get under the slab. I picture the soil and fill around and under the slab as a giant sponge. Even if you take a chunk out of the sponge, the whole thing will end up wet when set on top of water because of capillary action. Am I thinking about this correctly?

    I'm still doing research so if I find out anything useful I'll post it. Thanks.

  5. GBA Editor
    Martin Holladay | | #5

    If I read your original post correctly, the problematic home, "like hundreds we've built over the years and countless thousands in our market," doesn't have a capillary break or a plastic vapor barrier under the slab.

    As you're now learning, this is one detail that is impossible to retrofit. It is very surprising to me that in some areas of the country, apparently within the last 20 years, some builders were still neglecting basic details like vapor barriers under slabs.

    I have a 1955 copy of a book on residential construction. The book is Your Dream Home and the author is Hubbard Cobb. Cobb recommended the following details under a slab: 6 inches of granular fill as a capillary break, topped with a 1/2-inch layer of cement grout coated with a layer of asphalt felt laid in hot-mopped asphalt. Once that was set, you were ready to pour your slab.

    These details have been well understood for over 50 years.

    Good luck -- this problem is tough to fix.

  6. jfincherok | | #6

    As I'm learning more about building science, I'm surprised what builder's (including myself) have done in the past and continue to do. The three main reasons things like vapor barriers and capillary breaks aren't being used in my market are: 1. It's not required by code so most builders won't do these things to save money, 2. It's what every other builder has always done and it's rarely been a problem (as far as I know). and 3. There's a huge lack of knowledge and understanding. My main reasons are #'s 2 and 3. I didn't know what I didn't know. I'm a less government kind of guy so I don't like the idea of more regulation so I'm trying to make a difference by creating a market demand for what I build which will cause other builders to do the same. That's actually been working pretty well. I can think of at least 6 builders who now build Energy Star Homes to try and compete with me.

    I started building only green certified homes about 2 years ago because I wanted to build better homes and I thought it would give me a marketing edge. To the best of my knowledge, I'm still the only builder in the state that is specing all my homes certified green. That will change if it hasn't already, but it goes to the point that there's not much building science going on here. We're in a more moderate climate than say Florida or Maine which is probably why it's been in less demand here.

    So am I to deduce from your previous post that in your opinion the moisture under the slab and foundation will be difficult if not impossible to fix? Do you or anybody else out there feel that I can reduce the amount coming in? I don't think at this point I can completely stop it from coming in, but reducing it down would probably be adequate. How about sealing plastic to the foundation and running it horizontally a few feet from the foundation under the flower bed? This home is one we built years ago before we did air and moisture barriers. I would assume it's fairly leaky by performance home standards. Thanks again and sorry for the long post.

  7. Anonymous | | #7

    There are specialty basement water problem companies. Maybe if you check there are specialty slab water issue companies.

  8. Anonymous | | #8

    Cut intersection wider and use hydraulic cement.

    Add your perimeter French drain to daylight or sump.

    Use the proper sealer and fix the weeps.

    Adjust watering and you should be all set.

  9. Armando Cobo | | #9

    I'm glad to hear the weep holes was the issue; that's an easy fix and I don’t think you need to take out the whole brick wall. I would take out every 2 or 3 bricks at the bottom, clean as much as possible to make sure you have clear venting and let it dry for a week or two w/o rain. I would make sure there is a small separation between the brick and the brick molding at the top of the wall, under the soffits, so air can circulate from the bottom up.
    On your foundation, I’ve never seen that much wicking unless you don’t have proper drainage, which you said you had. But here are some tips:
    a) I would make sure to remove all vegetation with in 3’-4’ form the house, install gravel until it dries well, then if they want any vegetation, I would use drip irrigation only.
    b) Install gutters if you don’t have them. If you do have them, make sure it the downspouts discharges the water at least 3’-4’ away from the house through splash blocks or underground drainage system. Better yet, collect that water for irrigation.
    c) Make sure there are 6"-8" clearence between the bottom of the brick ledge to the ground. Hopefully you have the slope to do it.
    A wild thought could be that your 16” of fill sand is totally soaked from all the rain and it doesn’t have a way to drain out. You may need to do a French drain digging under the corner, and/or around the house, and away from the house if you have the slope or install a sump pump next to the foundation and drain it away from the house.
    First, I would try to fix all these problems by the least intrusive and cost approach to the highest.

  10. David R Walters | | #10

    How do I go about putting in a french drain and how can this help the mosture comming threw the brick wall.

  11. GBA Editor
    Martin Holladay | | #11

    David Walters,
    You haven't yet described your problem. Is this a slab-on-grade home with brick veneer? Or is this a home with a basement experiencing moisture problems in the basement?

    A French drain is created by digging a ditch and filling it full of crushed stone wrapped with landscape fabric. Near the bottom of the trench is a perforated drain pipe; the pipe is surrounded by the crushed stone. The pipe slopes to daylight or to a drywell.

    French drains can be installed outdoors (in damp ground, under the drip line of a roof, or in a swale). They can also be installed in basement or crawl space (usually around the perimeter of the space). When installed indoors, they usually drain to a sump.

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