Removing excessive moisture that is trapped below a slab 4

[cuddlebunny]

Problem Statement: How do you remove the excessive moisture that is trapped below a slab but on top of the vapor barrier?

The slab in our house had a hot water line break about a year and half ago. Today, we continue
to have high moisture in the slab around the perimeter and also in some rooms. We have used electonic moisture meters that indicate up to 50-60 percent moisture content as well as had three moisture dome (calicum choride) tests that indicated 14 pounds of moistures over 24 hours per 1000 sq feet. We are looking into ways to remove this moisture from the slab because it continues to be a source of potential mold growth and mildew smell.


We have tried these techniques to solve or understand where the high moisture is coming from.

1. We cut a 18x12 inch hole in the slab in an area indicating high moisture. When the 2 to 3 inches of concrete were removed, a 12x12 inch area was revealed to contain sand that was moist. The other 6x12 inch area of sand was dry. This is what the effloressence powder indicated on this area before we cut into the slab. This test was performed to verify that moisture was indeed above the plastic and trapped with no where to go except to evaporate through the concrete.

2. We have run dehumidifiers with the house sealed off several times. This resulted in little moisture drop in the moisture areas. A concern of how these tests were run was that the house was sealed up so that the dehumidifiers could not vent air to the outside. Should the dehumidifiers vent air outside while running?

3. We have had several leak tests verifying that no pipes are leaking.

4. We have verified that no water is intruding from an outside source and finding its way under the slab.

5. We have verified that the water table is not high enough to have an effect.

6. We have looked into using sealers but have found that they are ineffective or have negative side effects. Is this true or are sealers a viable solution?

7. What is the effect of removing the slab, replacing the vapor barrier, sand and concrete? How long before we can put down cabinets, carpet and tile?


Because of the longivity of this problem and other tests we have done, we believe that water migrated during the leak on top of vapor barrier and is now trapped beneath the concrete slab with no where to go.

Thank you for any help you can provide on this subject. We are at our wits end...

 

[jheidt2543]

Many homes are built without a vapor barrier installed under the basement slab so, you are somewhat ahead on that score. I'm not sure what part of the U.S. you are in (or even if you are in the U.S.), but here are a few thoughts to consider:

1. I live in the upper Midwest and 60% relative humidity in my basement is not unusual during the summer and in the winter it will go down to about 40% or so. I have my dehumidifier running 24/7 at least 9-10 months of the year. I happen to have it empty into a pan that I empty twice a week. A better solution is to put a section of garden hose on it and let it drain to the floor drain. During the winter two months I shut if off because the humidity is low enough.

2. If you decide to put carpet down, use a jute backed carpet that can breathe. A rubber backed carpet will collect moisture between it and the concrete and ruin the backing. Also, vinyl tile, paint or any sealer is a no no too! The water vapor pressure will lift it off.

3. I run my furnace fan to circulate the air throughout the house. I added two supply grilles and two return air grilles in the basement to improve the air circulation. The higher air volumne prevents the mildew/mold smell.

4. Remember, the humidifier is trying to remove the moisture in the air, so I would think you would want to keep the house closed. The only time I would open it up was if the outside air had significantly lower relative humidity. But, remember also, relative humidity is temperture dependant, the higher the temperture, the more moisture it can hold.

Just some thoughts to consider. Good luck!

 

[Ron]

cuddlebunny...jheidt's comments are good and on target with most observations.

Concrete inherently retains some moisture (usually about 1-3% by weight at equilibrium...note that this is much higher than the vapor migration rate!). The vapor migration rate you noted (14 lbs/1000sf/24hrs) is a bit high. Tile manufacturers set a limit of 3lbs/1000sf/24 hrs for adhesive attached tiles. This value is low and is often exceeded in practice without significant detriment.

Vapor migration is controlled by several factors. As with any "relative humidity" condition, this one is temperature related. If the slab surface is warm, the vapor will condense near the bottom of the slab and remain there, only to rise and become a problem when the slab surface cools to a temperature lower than its bottom temperature. Moisture moves back and forth in concrete in this manner. Moisture migrates toward drier and/or cooler conditions.

One way you can force the moisture content of the slab downward is to use a surface drying agent (similar to the calcium chloride dessicant) in conjunction with reducing the slab surface temperature temporarily. I have done this for commercial applications with reasonably good results. Even clean, oven dried sand will help quite a bit if spread on the floor during a temperature drop. The sand can then be swept, vacuumed, and disposed (might help your landscaping!)

Be sure to completely remove all floor coverings and residue before you attempt to dry the slab. Additionally, after your remove the coverings and residue (don't use water to wash the slab, though!), you can then use a commercial buffer with an abrasive pad coupled with medium to coarse sand to abrade and open the surface texture of the concrete a bit. This will help in the evaporation process.

As for your dehumidifier...pipe the condensate outside but keep the space sealed as much as practicable to prevent an influx of outside, humid air.

 

[Rjeffery]

cuddlebunny - - - jheidt's and Ron's comments are on target.

If there is a large volume of water under the slab and you are sure that no other water in infiltrating the area, core an eight to ten inch hole through the slab and remove enough sand and other subgrade material to install a sump pump wrapped with filter fabric. when the pump is no longer needed (when no more water seems to be present) follow jheidt' and Ron's method of removal and restoration.

good luck and please keep us posted.