In my last blog I described the three elements that mold, like the rest of us, need to grow. The elements are reasonable temperature, food and water. In most cases temperature is a given and food is available. So the limiting factor is often moisture.
Mold does not need liquid moisture. Liquid moisture is essentially 100% relative humidity, like when water condenses on your air conditioning coil because it has reached the dew point. But some molds will grow fine at about 70% moisture activity, which is well short of liquid water. This is why we often focus our attention on spaces where there may not be liquid water but where the relative humidity reaches above 70%. The two most famous spaces are basements and crawlspaces.
Why is humidity higher in these spaces? It all comes down to temperature. To understand the situation, let us review the relationship between temperature and relative humidity.
The amount of moisture that a given air mass can hold is a function of temperature. Very cold air simply cannot hold much moisture. That is why it does not snow much in the Arctic. The little bit of snow that falls does not melt (until recently) and so it tends to accumulate.
Conversely, the very warm air of the tropics and sub-tropics can hold a ton of moisture. Have you ever experienced one of those afternoon deluges in Florida? The air was holding a great deal of moisture, and as it heated up during the day it rose by convection. As the air rose, it expanded and cooled because of natural adiabatic processes, eventually releasing the waterfall that forces you to pull over on the highway.
This cooling also occurs on the west coast, where moist air from the Pacific Ocean flows inland and encounters the Sierra and Cascade mountains. The air is forced to rise, expand and cool. The moisture is released onto the west face of the mountains, which is where the Redwoods and Sequoias grow. When the air descends on the east side of the mountains, it compresses and heats up. The air can then hold more moisture but the moisture has already been released. Hence we end up with the high deserts, also known as the rain shadow arid areas.
So what in the world does this have to do with our basements and crawlspaces? Think of a nice hot, humid August day, the kind we call sweltering. Some of the discomfort is due to high temperature and some is due to high humidity. This late summer air is holding a lot of moisture.
When it enters our house, what happens? The air is cooled to the prevailing temperature of the indoor space. If that space happens to be a basement, the prevailing temperature is around 60 oF, as opposed to the 85-90 oF outside. What does this do to the relative humidity? It increases of course. If the relative humidity outside is 50%, it may increase to 70-80% in a cool basement or crawlspace. That puts the air in the range of the dry-tolerant (xerophilic) molds. Basements and crawlspaces are susceptible to mold activity simply because they are, well, basements and crawlspaces.
By the way, in the northeast the condition is just the opposite in winter. Cold temperatures are (or used to be) normal in January and February. Cold air cannot hold much moisture so there is usually not a lot of moisture available. When we heat that cold air it feels even dryer because warm air can hold more moisture but it is not available. The relative humidity of 50% outside will drop to 10-30% inside depending on how much moisture was initially available and how much heat we applied. We can moderate the indoor humidity by adding moisture and there are several ways to do that. But most of us simply acclimatize to the reduced humidity, after a short period of discomfort and scratching, by turning the thermostat down.
How do you reduce humidity in a basement in summer? You can either ventilate, which brings in warmer air that can hold more moisture (and thereby has lower relative humidity) or you can mechanically dehumidify. In either case, if you can maintain relative humidity levels below 60%, and preferably near 50%, mold will simply not grow. Ventilation has the benefit of not only exchanging indoor air, which is an automatic air quality improvement, but also being relatively cost efficient. Dehumidification has the benefit of relative ease (as long as you are not emptying buckets) balanced against the cost of running what amounts to an air conditioner.
Some folks have resorted to air conditioning their basement. This can be a problem in the context of humidity. Keep in mind that when air passes through an air conditioner coil, it is cooled to about 55 oF and usually reaches its dew point, which is the temperature where the air reaches 100% relative humidity (saturation). Even though water condenses on the coil, the air that emerges is at 100% relative humidity. In the upstairs living spaces, this cool air is quickly heated to the room temperature of say 75 oF. As the air warms up the relative humidity drops to an acceptable level, hopefully below 60%. In a basement, however, the air only heats up to 60-65 oF. The relative humidity remains high and mold grows. So in this case you thought the air conditioning was dehumidifying but in fact resulted in higher relative humidity.
And what about crawlspaces? They 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 by cross ventilation. This is generally a good thing and I successfully maintain a vented crawlspace in my own home. The caveat is that in the presence of excess moisture, natural ventilation may not be adequate and mold will still grow due to elevated humidity. This is particularly problematic when preferred food sources such as fiberglass insulation are present. One of the most common sources of mold activity we encounter is fiberglass insulation in a moist crawlspace. When you are planning to insulate a vented crawlspace, consider some of the alternative products such as polyurethane foam or foam board, which tolerate moisture and do not grow mold.
If you have any questions about mold activity, please call 609-371-2489.
AIR Consulting Services, LLC
301 E Ward Street
Hightstown, NJ 08520