Mold Activity in Basements: Where Does the Water Come From?

We have discussed mold activity at length in previous posts and videos. Mold contamination has become the most common indoor air quality issue we encounter, both in commercial and residential buildings. And the lower building areas, such as basements, crawlspaces and slab on grade areas, most frequently develop mold amplification. Since we know that moisture is the limiting factor in mold growth, we arrive at the following question:

Why does mold like the lower building areas and where does the moisture come from?

Mold does not need liquid moisture. Liquid moisture is essentially 100% relative humidity, like when water condenses on your air conditioning coil because the air passing over the coil has reached the dew point temperature. The dew point is the temperature where a given mass of air is holding as much moisture as it can hold. The warmer the air, the higher the dew point. In the summer in the northeast US, typical dew point temperatures are in the range of 50-65 oF. So if a warm, summer air mass at say 80 oF encounters a cool 65 oF space, its relative humidity will automatically increase.   And guess what: Most basements and crawlspaces maintain temperature around 65 oF, so by definition they are more humid than other interior building spaces.

And how much moisture does mold need? Some mold species, such as Stachybotrys chartarum and Aspergillus fumigatus, require near 100% water activity, meaning liquid water. But some molds will grow fine at about 70% relative humidity (we call the humidity level in a material such as drywall or insulation “water activity”). Obviously 70% RH is well short of liquid water.

To avoid mold growth, therefore, we must maintain relative humidity levels well below 70%.

Because of their cool temperatures, the relative humidity in our basements and crawlspaces is normally in the 60-70% range, which is right on the moisture boundary where mold can grow. If we add any additional moisture we almost always push humidity levels above 70% and presto chango, we start growing mold.

How do you reduce humidity in a basement in summer? As long as you don’t have the added moisture condition, you should be able to control with either ventilation or dehumidification. In either case, if you can maintain relative humidity levels below 60%, and preferably near 50%, mold will simply not grow. So as long as the space is not taking in additional water, humidity control should be straight forward with a properly sized and rated dehumidifier.

Crawlspaces are a different beast for the simple reason that they are usually vented to the outside. The venting is intended to provide the natural ventilation that will remove some of the moisture. But in the presence of excess moisture, natural ventilation may not be adequate and mold will still grow due to elevated humidity. There are debates whether it is better to increase ventilation or to close up the space and dehumidify. That is a discussion for another time.

The serious wet basement problems almost always involve the intrusion of additional water into the already humid basement. By far the most common intrusion pathways are the foundation walls. One must remember that the basement is simply a masonry box in a hole in the ground. The concrete or cinder block walls are porous and would readily admit water if they were not coated with some kind of damp-proof coating on the outside surface. The most common damp proof material is an asphalt slurry, although various rubber coatings and membranes are becoming more readily available.

The limiting factor on all coatings, just like most other materials and all people, is that they wear out. Over time the coating is eroded and you can occasionally see a black residue develop on the bottom inside of the foundation. The residue is commonly mistaken for mold growth, but it is not. The black discoloration is a remnant of the asphalt coating that has eroded by water activity and carried to the interior. It is clear evidence that there is enough water activity around the foundation to cause the erosion and raise the humidity level in the basement.

Another sign of water movement through the foundation are the white salt crystals known collectively as efflorescence. The crystals are actually composed of cement (lime) that has been dissolved out of the masonry foundation by water and migrated to the interior surface. When the water evaporates, the crystals are left behind. Also sometimes mistaken for mold, the beautiful crystals are simply more evidence that there is water movement into the basement.

The final sign of water movement is dampness on the bottom couple of courses of foundation blocks. Because the blocks are hollow, they frequently fill with water and appear as darker in color. This accumulation of water increases the water pressure on the foundation. With chronic water pressure, the foundation can begin to crack. This allows additional water to enter the basement and also degrades the integrity of the foundation. Neither outcome is desirable.

The presence of water activity around the foundation, therefore, is responsible for a large number of the wet basements in New Jersey and surrounding states. In its most benign form, it adds moisture to an already cool and damp basement space, likely raising the relative humidity level above the 70% level that mold needs. This foundation condition is most effectively remedied by relieving the water pressure with an interior drain system, and moving the water out of the foundation into a sump. A good interior drain system will lower the humidity level, keep the floor dry and allow for finishes to be installed safely, with little risk of mold germination. In essence, a good system gives you a large living space at a bargain basement price.

In its cruelest form, the water pressure cracks foundations and causes deflection that requires more intensive remedies. The remedies may include installing masonry piers to help support the foundation, steel rods that reinforce the masonry and help resist the water pressure, or even an entire new basement.

None of these solutions is cheap and can frequently be avoided by managing the water activity early in the process. By far the most cost effective way to manage the water pressure is the installation of a good interior drain system.   The newer systems, which place a square pipe on top (not bottom) of the footing, will manage the water activity for a long time. In those cases where we have installed the Basement TechnologiesWater Trek Aqua Route®, our customers have stopped worrying about water. The Water Trek Aqua Route® is a superior drain system because it captures water from both the foundation wall and the subfloor soil areas. It carries a lifetime warranty because intruding water is captured as soon as it enters the basement.