Causes of Attic Mold
Attic mold is due to poor ventilation, roof leaks, and HVAC problems. In most attics, there’s an abundance of exposed wood and insulation on which mold can grow. The fiberglass material in the insulation does not support mold growth, but the paper backing does.
In a cold climate, poor ventilation of the attic space may lead to excess humidity as moist air from the warmer rooms below infiltrates into the cooler attic around plumbing pipes, the attic hatch, or even recessed lighting fixtures. The moisture in the air then condenses on surfaces that are below the dew point resulting in attic mold.
Even with the best attic ventilation, you can still have a serious attic mold problem if too much moisture enters the attic from the house. For example, bathroom and dryer exhausts vented into the attic can lead to condensation. If mold is growing in your attic in just a few of the rafter bays above the bathroom or laundry area, you most likely need to vent the bathroom or dryer exhaust to the outside.
If there is mold growing on most of the attic sheathing and the attic is well ventilated, you must find and eliminate the sources of moisture. Attic ventilation is important, but controlling the leakage of home moisture into the attic is more important.
If you live in a warm climate and air-condition your home, and the AC system and/ or ducts are located in the attic, moisture from humid outdoor air, used to ventilate the attic, may also condense on the outside of the system’s components, such as the condensate trap or lines, or the suction line to the AC coil.
Roof leaks can be caused by anything from damaged shingles to improper flashing installation around chimneys, exhaust pipes, and dormers. Flat roofs have their own issues because they lack the advantage of a clear slope to shed water. Sometimes gable-type sloping roofs collect mold on their lower edges, especially on the north side of the house, where the sun doesn’t have as much opportunity to dry the wood out after a rain leading to attic mold growth.
Examine the area around all vents for attic mold, as well as all roof decking, rafters, and attic floor joists. Check all penetrations including chimneys and pipes going up through the roof to exhaust furnaces, the hot water heater, plumbing, fireplaces, etc.
Is Attic Mold Toxic?
The black growth on attic sheathing and rafters consists primarily of species of Cladosporium, Alternaria, or other genera of micro fungi, but rarely, if ever, the potentially toxic Stachybotrys mold (black mold), because attics are not consistently wet enough to sustain this kind of mold.
It is however, common to find black mold on the attic sheathing of a gable roof (one shaped like an inverted V), especially toward the lower edges above or near the overhang, or even along one of the gable end walls, especially on the north-facing roof slope because it receives little sun during the day and doesn’t warm up enough to accelerate evaporation and drying out.
Attic Mold Abatement
Treating attic mold with bleach isn’t really effective, because the wood surfaces are porous and rough, and it’s impossible to kill all of the growth. Where the mold is superficial (microfungi), there is no structural damage to the sheathing and rafters, and the growth is in the low part of one or two rafter bays, you should HEPA-vacuum the surfaces and then paint the affected wood with an alcohol-based primer, which will generally kill most of the spores and seal them into a paint film. Keep in mind that just painting over mold does not cure the underlying moisture problem, which must still be solved.
For attic mold, follow mold removal guidelines discussed on this site.
Remember that attic mold growth is most often due to genera of microfungi (Cladosporium, Stemphylium, and Ulocladium, all black, or Penicillium, which may appear to be white). These fungi usually only affect wood surfaces, whereas macro fungi destroy wood’s structural integrity. If you have mold in your attic, consult with an experienced roofer, or a structural engineer for a second opinion before hiring a remediator or tearing the house apart.
In many cases we find that one entire side of the attic is black with mold growth, but the sheathing is intact and the rafters are not damaged. In such cases a professional remediator can, under containment, clean the surfaces by soda-blasting or dry ice-blasting them. Then the wood can be sealed.
Attic sheathing in newer homes can be made of plywood. If, because of moisture and attic mold growth, plywood sheathing is delaminated and weakened the affected sheathing must be replaced. Of course, this means removing and replacing roof shingles. Any rafters that are significantly decayed from macrofungal growth (most often due to leaks) may have to be repaired or replaced as well. Occasionally the entire roof structure has to be removed and rebuilt, though in some cases I think this work has been done because people overestimated the significance of the damage or because rebuilding was less expensive than professional mold abatement.