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Attic Mold – The Facts

Attic mold is a common problem in many cold, damp climates. The majority of issues occur due to condensation forming on the underside of the roof sheathing. This moisture, combined with the cellulose structure of the roof sheathing, provides the necessary conditions for fungal growth.

Attic mold is often surrounded by misinformation and outdated advice. However, we’re here to set the record straight. Below, we’ve outlined the latest research and recommendations.

Is attic mold dangerous?

Fortunately, attic mold growth is not typically associated with negative health effects of the building occupants. This is due to the ‘stack effect’, best understood as the upward movement of air throughout a building. As warm air rises up through a home, it finally reaches the attic and escapes through the venting into the outside air. The stack effect is a strong force, and only in very unusual circumstances would these air currents be reversed. What does this mean for mold growth? Those same air currents moving up and out of your home will prevent mold spores from reversing course and moving downward into your living spaces.

Ok, but are there exceptions?

Occasionally, we’ll encounter homes with leaky return ducts in the attic space. These ducts are under negative pressure and will suck air, and mold spores, from the attic into the ventilation system. Fortunately, a proper fitting furnace filter will grab the majority of spores prior to entering the living space.

What causes attic mold in western Michigan?
  • Blocked soffit ventilation. Often we’ll encounter attics with great soffit vents, but they’re completely buried in insulation.
  • Bad ridge vent. Ridge venting is very popular among roofing, however, it varies widely in its ability to exhaust air from the attic.
  • Disconnected ducting. Missing exhaust fan ducts are a major source of attic moisture.
  • Poorly designed roof structures. Many newer, high end homes come with complicated hip roofs. While visually pleasing, they offer dramatically reduced ridge area ventilation.
What unique conditions in western Michigan contribute to mold growth?

The western Michigan lakeshore and other cool, coastal climates suffer from high winter humidity. This is in contrast to cold, dry climates such as the Rockies with very low winter RH. For Michigan, this creates conditions in the attic spaces that are very conducive to condensation. As warm, damp air migrates from within the home up into the attic space, it quickly interacts with the cold surface of the roof sheathing. Often, the dew point is reached and water condenses out of the air and onto the surface.

How often does the roof need replacement?

Less than 10% of the projects we’ve investigated require a roof replacement. This is because the majority of the time attics suffer from mold growth rather than wood decay fungi. The latter is typically associated with plumbing leaks and other highly saturated environments. Two conditions call for replacement rather than treatment:

  1. If the structural integrity of the roof is compromised
  2. If the cost of treating the mold problem exceeds the cost of replacement

The latter is especially applicable when the roof is at the end of its service life and requires replacement regardless. The additional cost of replacing the roof sheathing may be less than remediation.

How are attics suffering from mold growth remediated?

  1. Encapsulation. First, it involves applying a chemical treatment to kill any active spores. Second, an encapsulating layer is applied to cover the remaining staining. Of course, no treatment should ever be performed without first addressing the underlying moisture issue.
  2. Physical removal. Occasionally contractors will attempt to physically remove not only the surface mold growth but also the remaining staining. Popular techniques include dry-ice blasting and hand sanding. Both methods are extremely costly and leave behind a vulnerable surface. Because of these negatives, these approaches have fallen out of favor in attic environments.

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