Crawlspaces can be convenient storage spaces for many homeowners but because they are typically underground and most often not finished, they are a haven for moisture where mold can become a real problem. Many older homes have dirt crawlspaces that create an on-going problem pertaining to moisture. If you have a dirt crawlspace and you think it is dry, take a small trowel and dig just a little below the surface. You will more than likely discover that while the dirt is dry on the surface, just below there is indeed moisture lurking and creating problems.
The problem with having moisture in a crawlspace is not just the mold but it also affects the flooring above the space. For instance, when warm air rises, replacement air is forced from the lowest part of the home. This is called the stack effect and is a natural movement of air. In fact, if you have a fireplace, this is exactly how the chimney works. The problem is that the air at the lowest part of the home will eventually make its way to the living space and if there is any mold or even radon, which is a problem with many older homes, it is being forced into the portions of the home where you live and breathe.
Okay, so how can you repair your crawlspace to eliminate these problems? Well, first the crawlspace is important to fix. Did you know that a problem with a crawlspace is a much bigger problem than what you would be faced with if you had a wet basement? To fix your crawlspace, you need to first have it fully sealed, isolating it from the outside and the ground. To do this, you would place a 20 mil, 7-ply sandwich of both low- and high-density polyethylene along with a polyester cord for reinforcement on the floor of the crawlspace and up along the walls. Another option that works well is to use actual pool liner. This material is strong and durable and will seal off the crawlspace blocking out any moisture or vapors.
Another option to consider although it can be a little more expensive and time-consuming is to have concrete poured into the crawlspace. Just remember that for this to work the concrete has to be the kind that offers a barrier against vapors. Otherwise, it simply is not a good solution. Even with the vapor barrier on the floors, you still have a problem with water-vapor diffusion that would come through the walls. In this case, you could use the concrete on the floor and then the polyethylene or pool liner on the walls.
Just keep in mind that before you do anything, you should check on the current building code, which could quickly identify what you are and are not allowed to do. For example, in some states, the code reads that there has to be one square foot of ventilation for every 150 square feet of dirt floor in a crawlspace. For this reason, you need to be careful, as some vapor barriers would lower the requirement for ventilation to one foot per 1,500 square feet of floor space. The purpose of the code is to make sure the humidity is vented from any exposed dirt.
However, even the venting can create a new problem. For instance, in the cold winter months, you run into the situation of having cold floors, which results in higher heating costs while in the hot summer months, the vents would allow moisture in from the warm, humid air. If warm air enters into the crawlspace and reaches dew point, moisture becomes condensation. Therefore, a solution for this problem is to close off the vents from the outside, and then sealing them on the inside with a two-inch piece of foam insulation, secured with polyurethane caulk.
Draining water from crawlspaces is another common problem. It is common for some crawlspaces to have a problem with water leaks. In this case, you would need to have a drainage system installed. Additionally, check all of your downspouts outside to make sure they are clear and pointing away from house. For the drainage system, you can install a sump pump that has a sealed lid. However, if your crawlspace has a lot of water, you can build a swale in the direct all around the perimeter of the crawlspace so water will channel to the sump pump and be far more effective.