Contaminated HVAC Ducts: Fungal Growth And Its Role In Indoor Air Problems

One recent study, which looked at hundreds of duct liner samples taken from "problem buildings," found that nearly half of the samples were contaminated. This presumably means that contaminated material was left in place until it either caused or contributed to ongoing IEQ problems - problems severe enough to trigger an investigation. The conventional wisdom is that dirt + moisture = fungal growth. However, another study by a major US environmental laboratory showed that some liner supported growth with just moisture. The same study also indicated wide variations in growth among different brands of similar insulation, as well as between many liners of the same brand, making it difficult to tell under what conditions building managers should consider removing the liner.

Bare Metal

In all fairness, even unlined ducts will support microbial growth if water and nutrients are present, leading some advocates - most notably the North American Insulation Manufacturers Association (NAIMA) - to claim that the task is to focus on keeping ducts dry and clean, rather than pointing fingers at the lining material. NAIMA, and others, point out that the insulation provides sufficient benefits to outweigh the dangers. However, this doesn't mollify others in the indoor air community.

Growth on Various Liners

Another recent study examined the potential for fungal growth on various types of liners under different humidity conditions. The researchers looked at how the liners performed at what would normally be considered high humidity situations - between 85% and 97% relative humidity (RH). They studied the materials in high humidity, when wetted, soiled, and clean.

The materials they studied included:

- Fiberglass (FDL) A and FDL-C: >44%-98% fibrous glass, 1%-18% urea polymer of phenol and formaldehyde or urea-extended phenol-melamine-formaldehyde resin, <0.1% formaldehyde;

- FDL-B: 82%-98% fibrous glass, 2%-18% ureaextended phenol-formaldehyde resin (cured) or urea-extended phenol-melamine-formaldehyde resin (cured), <1% nonwoven, Foil-Skrim-Kraft or vinyl facings or vinyl or latex coatings;

- Fiberglass duct board: 85%-96% fibrous glass wool, 4%-15% cured binder, <1% formaldehyde; and

- Fiberglass insulation: 90%-95% refractory ceramic fiber, 0%-10% phenol formaldehyde.

New materials were purchased from commercial vendors. The researchers also studied used materials taken from old buildings. The used materials were similar in appearance to the new materials, but researchers couldn't determine their origin. First, the researchers looked at all five samples after placing them in an environmental chamber at 97% humidity and measuring microbial growth. Then, they wetted the samples to see what effect that had on contamination. Finally, they soiled the samples to see how that affected the growth. During the time the new material samples were at 97% RH alone, only FDL-A supported the growth of the test fungus - Penicillium - that had been placed on the samples. FDL-B, which had been manufactured with a "permanent biocide," actually showed a 1-log decrease. The testing period lasted for six weeks. When the materials were wetted with sterile water, FDL-A exhibited fungal growth similar to what was found during the high RH period alone. FDL-B showed an even greater decrease in growth. FDL-C, however, underwent a 2-log change over the test period, while the other materials again exhibited no growth. When the materials were moderately soiled, all samples showed significant increases in fungal growth.

Effects of RH Alone

When the FDL-A was kept in a chamber at various RH levels - 85%, 90%, 94%, and 97% - the samples exhibited a slight drop in fungal growth during the first week. From the second week on, the samples kept at 97% increased steadily throughout the period. However, the sample kept at 94%, while it initially followed the same pattern, began exhibiting growth during the sixth week. When the researchers repeated the experiment on FDL-A samples that had been heavily soiled, using RH levels of 85%, 90%, and 94%, those at the two higher levels began showing growth after the first week. The sample held at 85% initially showed decreased growth, but began showing growth by the fifth week. Used duct materials, from old buildings, were all able to support growth immediately when kept at 97% RH.

What This Means

The research was not intended to cast insulation in a bad light, but rather to provide a screening tool to allow managers to select materials with the lowest susceptibility to fungal growth. The research brought up several important points. The first is the tremendous variability among liners in their ability to sustain microbial growth. Another finding runs counter to the common wisdom that both dirt and moisture are necessary but some of the liners showed growth with just the addition of sterile water. The issue is not so much high humidity as it is areas of the HVAC system where water accumulates, either through water accumulation or because the air stream encounters surfaces that are below the dew point, causing condensation.

Recommendations For Acceptable Indoor Air Quality

- Insulation not be used in areas where moisture can be expected;
- Cold surfaces be insulated to prevent condensation; and
- Advising against materials that can biodeteriorate or trap dirt and moisture.

The standard calls for designers to avoid placing insulation within one and a half feet of the outdoor air intake, as well as the duct area from the coil to the downstream end of the drain pan. The standard also restricts insulation use to within one-half inch of surfaces that may become wet. As to the type of insulation that's acceptable in areas with cold surfaces, the standard reads: The thickness of insulation shall be as required to prevent condensation on cold surfaces. Insulation that is subject to damage or reduction in thermal resistivity if wetted shall be enclosed with a vapor retarder sealed in accordance with manufacturer's recommendations to maintain the continuity of the barrier. Special coating that can be shown to inhibit condensation may be used in lieu of insulation if approved by the authority having jurisdiction. In an appendix dealing with microbial growth, the standard further discusses the insulation issue: Microbial growth on HVAC systems' internal surfaces in or near moisture-producing equipment (e.g., dehumidifying cooling coils, humidifiers,etc.) is an important problem for occupant health and comfort as well as system maintenance. Microbial growth causes damage to biodegradable materials and metal corrosion. Microbial growth is dependent on the presence of both moisture and a nutrient source. Consequently, materials that are used to line the airstream surface near moisture-producing equipment (e.g., dehumidifying coils, humidifiers, etc.) should not contribute to biodeterioration, should minimize the accumulation of dirt, and should not absorb and retain moisture.