Wet Basements

Published on February 13th, 2013 | by Henri


Reducing basement moisture–what works and what won’t

Q.  The basement of our 60+ year-old house is currently mostly dry, but only through the daily, year-round hard work of a dehumidifier and sump pump. We believe we have high groundwater.

The walls are bare concrete block and do not appear to have been treated in any other way than to have been painted on the inside. The paint has come off only in one small square in the SW corner. The floor is a thin slab of concrete with some cracks in it.

There is also a mysterious small hole in the slab on the north side that may be functioning as some sort of below-floor drain to the sump pump on the south side. The sump pump usually comes on a few times a day for a few seconds each time.

Occasionally we do get some water on the floor during heavy rains or if the sump pump fails for a few days, but usually there is no visible water. The ceiling is insulated with R-19 fiberglass batting because of stapled-up radiant heat installed under the first floor hardwood floors. We would like to make our basement less damp, mostly because of the high energy costs (we estimate $30-50/mo in electricity). However, our budget for doing this is tiny. I have 4 questions for you:

  1. First, I am planning to get rid of the mold myself using a borax solution (I have a small child and am concerned about indoor air quality in our pretty tight house). Mostly, I can only see a little bit of it (black) around the bottom blocks of the wall on the north side. However, there is also some in the drywall beneath the stairs that go down to the basement. I have read about a non-toxic sounding product called Caliwel. Would you recommend using borax on the block walls and encasing the drywall using Caliwel?
  2. Next, we discussed several options for making the basement less damp with a structural engineer. He recommended putting down Stego Wrap + concrete up to ground level. (We are never going to finish the basement and so probably wouldn’t dig up the slab–just have a slightly low-ceilinged basement.) However, he said if we don’t have too much hydrostatic pressure it might be possible to get away with just applying epoxy. Do you ever recommend epoxy for fixing a damp basement and if so, is there a good brand? We also looked into something called SaniTread–I believe it was rubber rather than epoxy–which appealed to me because of my concerns about offgassing etc.
  3. If not, can you comment on the Stego Wrap + concrete over it option. A contractor told us he wouldn’t do that unless he were first   installing a French drain, but I don’t remember the engineer mentioning that.
  4. Finally, we have also considered insulating the basement walls with fireproof foam board to raise the temperature down there and therefore increase the amount of moisture the air can hold at 50% relative humidity. The native soil is supposed to be a silty loam and therefore the structural engineer recommended only doing so above ground level in order not to cause the foundation to fail. Would you recommend putting in the foam board?  Throughout the recent cold snap the basement has been quite cool–at one point we measured the temperature at 43 degrees. Nothing has frozen however.

A.    Since the concrete block walls are painted and paint is only peeling in a small area, it would indicate that the leakage is not through the walls themselves. It also seems as if the water is mostly coming through the floor and not much, if at all, at the joints of the walls and floor since the sump pump is able to take care of it except in exceptional circumstances. If water were coming in at the joints of the walls and floor, you would probably see it more often than you indicate.

It is also important to know that hydrostatic pressure is hard to resist and to control by trying to cap it. For instance, if leakage occurs at the joints of walls and floor, adding a membrane on the old concrete and covering it with new concrete may not do anything. There are other ways to deal with the problem.

But it is also important to understand that grade problems may be contributing to the leakage by allowing deep penetration from surface water(and perhaps roof water, if you do not have gutters) to overload the capacity of the soil to contain it and of the pump to handle it.

Check grade around foundation first

So the first thing that you should do is to check the grade around the foundation, as most foundation leakage problems are caused by flat or negative grade, low spots, mulched beds, hollows created by the discharge from downspouts or dogs looking for cool soil; driveways, patios, walks that direct water toward the foundation. Any such contributors to leakage should be corrected, including possibly expensive work to change the slope of masonry or asphalt elements.

Once this is done, if leakage persists, you may either have a rising water table following heavy or long rainy periods or snow melt, or an underground spring that swells from these occurrences.

Controlling the water table

The control of a seasonal water table or swelling spring may be done in one of several ways depending on where the leakage occurs. If the water comes through the floor, there may not be a sufficiently thick stone bed to act as storage while the sump pump is trying to discharge it. It is likely to be the case in a 60-year-old house with a thin and cracked concrete slab.

It is also essential to know that the water discharged by the pump is disposed of properly. Often, the discharged water is recirculated back to the basement because of grade problems that do not move it away from the foundation.

I am not clear about your comment that the structural engineer suggests using “Stego Wrap plus concrete up to ground level”. It can’t mean filling the basement with concrete to grade.

A membrane covered with a new 3-inch slab may repair the thin and cracked slab, and contain the water coming through the floor, but would the hydrostatic pressure force the water out to the perimeter where it would show up again?

Exploring underlying conditions

It may be worth exploring the underlying conditions by cutting a small area of the concrete to determine the thickness of a stone bed, if there is one at all. One possible solution is to break the existing slab into small pieces and use it as a stone bed. It may be necessary to add some stone to obtain a bed 4- to 6-inches thick. The stones should be covered with a vapor retarder, which can be the Stego Wrap or just heavy plastic. To ensure the proper curing and finishing of the concrete, the industry recommends that a 2-inch thick absorbing bed (coarse sand, crusher run, etc.) be spread over the plastic to absorb the extra water in the pour. The new slab should be at least 3-inches thick and should have at least wire mesh reinforcement. It should also have control joints every eight feet to prevent irregular drying cracks.

Removing mold

You may also want to consider removing the mold with an environmentally-sound oxygen bleach such as Oxy-Boost (www.ecogeeks.com.)

Painting the thin floor with epoxy does not sound like a worthwhile thing to do. All the cracks would have to be repaired and the old concrete may not be able to hold the paint against the vapor drive from the soil through the slab.

Although I have no personal experience using Caliwel coatings, it sounds like a good product to use on the drywall, as its anti microbial agents are claimed to kill bacteria causing the growth of mold. You could also use it on the block walls.

Draining, if necessary

I would be leery about using SaniTred as a way to waterproof the basement floor. I question its ability to withstand hydrostatic pressure, and would be concerned about mold growing under it.

The contractor who said that he would not use Stego Wrap and concrete has the same concern I have about hydrostatic pressure pushing water up at the perimeter of the slab. Installing a subslab drain leading to the sump pump is good insurance and advice. The drain can be installed around the perimeter of the basement when the present slab is broken into small pieces to improve the storage and drainage ability of the substrate.

Insulating a basement

The consensus among building practitioners is that, if you do not know the composition of the backfill around your foundation or you know that it is heavy native soil, you can safely insulate basement walls from the ceiling joists to three feet down below grade. If the backfill was done with coarse, well-draining material and you have a functioning foundation drainage system, you can insulate all the way to the slab.

Fireproof board insulation is fine, but it is really only fire-resistant and more likely made with fiberglass and not foam. But you can use flammable rigid foam insulation as long as it is covered with fire-resistant material such as drywall.

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