Q: I read your response here to an earlier question on dealing with mold in ventilation systems. I have a question on air/heating ventilation and I hope you can help me.

We are installing a furnace and air handler in the attic with the need to have some ductwork also run within and across the crawspace to the other side of the room. I have been told by some air/heating suppliers that this ductwork under the space is a magnet for mold that develops from the condensation that forms in the ductwork. Others say, that if the ductwork is insulated properly, there should not be a problem with condensation and the forming of any mold. Can you provide some clarity on this?

I also learned from your article that crawlspaces should be vented properly to avoid moisture and contaminated air problems. In my crawlspace, I have a window to the exterior that is sealed shut (but can be opened if necessary) and another opening (about 2ft. by 2ft.) into the adjacent basement that has no closure . Above the crawlspace will be a ceramic tile floor. How do I create the negative pressure you described and/or how do I assure proper ventilation to best reduce moisture?

A: Any time warm and humid air comes in contact with cold surfaces, moisture will condense. In order to prevent mold growth in ductwork the key is to keep the ducts clean. This requires good and properly installed filters. Mold requires nutrients as well as moisture. If condensation moisture occurs on a clean nonporous surface, no mold will grow and the moisture will evaporate into the flowing air. This also means that the return air ducts should be sealed properly and air from the attic should not be leaking into the system thereby providing humid and dirty air to the system.

Crawlspaces vary based on the moisture sources. The earth is cold and porous. If you have a lot of moisture from ground water in the crawlspace, ventilation is the best policy. You want to avoid having crawlspace air move into occupied spaces. However, you should also realize that pulling humid air into the basement will also condense moisture on its cold surfaces. If the crawl space is relatively dry normally, sealing it up to avoid humid air from entering and condensing is also an acceptable approach. Ventilating can be done in several ways. Most people simply have passive ventilation, which means having vents in several places around the crawlspace and allowing natural cross ventilation. If the crawlspace is so wet as to require negative pressure, then a powered window fan, perhaps controlled by a humidistat such as those seen in attics, might be the best approach. Much depends on the construction of the crawl space. Plastic sheeting installed on a dirt floor can do much to minimize water vapor entering the crawlspace and avoiding some of the problems. In some crawlspaces, sealing the space and dehumidifying the air might be the best response. Obviously the basic decision is to either ventilate or seal the space and this depends on the sources of the moisture present. No paper or wood debris should be left in a crawlspace because that would  provide the nutrients that support mold growth.

Mark Goldman, CMC, Senior Air Quality Scientist of EFI Global 

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