Causes Of Attic Mold

Most attics provide the three necessities for mold growth. Warmth, moisture, and abundant food sources. Add poor ventilation and an “out of sight, out of mind” location–which means mold infestations can grow for years without being noticed. Attics are one of the most common locations for mold growth.

Causes Of Attic Mold

Ideal Mold Environments

Mold spores are everywhere–floating in the wind until they find a home. Some types of spores can be dormant for hundreds of years before the right conditions enable them to come to life. Attics provide an abundance of those conditions.

  • Warmth. Mold grows best in temperatures above 40 degrees F. Attics are usually warmer than that for most of the year.
  • Food Source. Mold grows on wood and dust–both plentiful in attics.
  • Moisture. Moisture is the catalyst needed for mold to grow. Dry mold spores do not grow.


Attics are warmed by the sun and by warm air rising from the living areas of the house. Even well-insulated attics experience heat gain from below. Insulation reduces thermal conductivity. It does not stop it completely. Most houses in the US are under-insulated–allowing more heat into the attic.

Food Source

Mold feeds on organic material found in attics. Timber framing, plywood, dust, cardboard boxes. Most insulation materials do not get mold but attract and hold the dust that the spores feed on. HVAC ducts, plumbing, and vent pipes in the attic often have mold on them. The mold is feeding on dust–not the ducts and pipes.


Moisture gets into attics from many sources. Once inside it is difficult to remove. It soaks into insulation and can become attic rain. Warmth turns moisture into a vapor that rises to the underside of roof decks. Cool roofs condense the vapor into water droplets that rain back down on the insulation.

Sources of attic moisture include:

  • Roof Leaks. Even a tiny leak produces enough moisture for mold growth.
  • Humidity. Warm humid air leaking into the attic from below through gaps, cracks, light fixtures, etc. is the most constant source of attic moisture. Humid air also enters through ventilation openings.
  • Exhaust Fans. Bathroom and kitchen exhaust fans that terminate in the attic instead of outside the building pour warm humid air into the attic.
  • Rodents. Rats and mice in the attic leave urine and feces that promote mold growth.

Inadequate Ventilation

Relative humidity under 60% inhibits mold growth. Warm moist attic air is moved out and replaced with cooler drier air from outside. The International Building Code (IBC) requires a minimum of one square foot of venting for every 300 square feet of ceiling area–split evenly between soffit intake vents and roof-mounted exhaust vents. Local building codes may require more venting. Insufficient venting allows moisture to build up in the attic and promotes mold growth.

Consequences Of Attic Mold

Attic mold can be responsible for health problems if it reaches the home’s living areas. Mold reduces the effectiveness of insulation–increasing energy costs. If left long enough mold will cause rot in framing and plywood roof decks.

The monetary costs of mold remediation can be very high.

  • Replacing insulation.
  • Structural repairs.
  • Drywall repair.
  • Reduced home sale price. May even cause a loss of sale when the building inspector finds the mold.

Inspect the attic at least twice yearly to find and stop mold early.