Of the places you might find mold in your attic, the insulation is the least common. This is because the area with insulation is relatively warm compared to others like your roof sheathing. However, just because it is less common to see mold growth here doesn't mean it isn't important to inspect. If the insulation is blocking soffit venting then air flow is limited, thus leading to mold. Other problems that can occur are when insulation in install incorrectly, such as upside down, or to close to electrical equipment. Also though not as common, mold can actually grow on the insulation itself.
Preventing mold growth on the insulation requires the same steps as outlined in the attic sheathing section. The improvements made to address the condensation on the roof sheathing will address the condensation and mold growth on the insulation as well.
First, it is important to remember that mold growth on attic insulation does not negatively impact the indoor air quality in the rest of your home. Due to the stack effect, the mold spores cannot migrate further into the home. Unfortunately, there are dishonest contractors who tell customers to replace all of their insulation, even when little or no mold growth is present. This can double the cost of a remediation project and the worst part is that it is typically not necessary.
In most cases, mold growth on the insulation is found when mold on the roof sheathing is present as well. In these cases treatment applied to the roof sheathing will also treat mold on the insulation. It is important to remember that this will not remove the mold spores or the discoloration. Therefore, in some cases, it is best to remove the affected insulation. This can be accomplished by either removing the top several inches of insulation or replacing it fully. This can add a significant amount to the cost of remediation and therefore, we typically do not recommend it unless necessary.
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Attic mold often appears as a thin gray or black layer on the very top of the insulation. It will also grow in uniform layers on top of the insulation. If the mold growth is due to high humidity and condensation, which is usually the case, then the mold growth will only occur on top of the insulation.
Technically, mold doesn't typically grow on the insulation itself as fiberglass is not a viable (or delicious) food source for mold. However, mold growth is often found on the top surface of attic insulation. How does this happen? Well, mold seen on insulation is not actually actively growing, but there because spores are falling from mold growth on the roof sheathing. The second option is that the mold is growing on settled dust that has accumulated on top of the insulation.