Attic mold is a common problems in homes. It is caused chiefly by moisture getting into the attic either from roof leaks, air leakage into the attic from inside the home, or bathroom fans discharging moisture laden air into the attic rather than to the outside. In our opinion at Insulwise Energy & Comfort Solutions in Pittsburgh, PA, mold is the symptom of the problem. The real problem is excessive moisture getting into your attic in the first place.
After we've ruled out roof leaks being the problem, the way that we solve attic mold problems in our Pittsburgh climate is by 1) venting all bathroom fans to the outside and 2) air sealing the attic floor to prevent warm, moisture laden air from escaping upward into the attic space during the winter through the process of Stack Effect.

Video Content
00:00 - Improper Ventilation Consequences
00:53 - Steps for Locating Moisture Sources
01:57 - How to Find Attic Air Leaks
03:10 - Evidence of Air Leakage
03:35 - Sealing Air Leaks with Foam

Mold typically forms in the attic during the cold weather months. Why? Because that is when the roof deck gets cold, and water vapor can most easily condense on it. By sealing the air leaks hidden under the insulation in the attic floor, and then killing the mold, we've eliminated the live mold, made the visible mold stains disappear, AND eliminated the source of future mold blooms --- moisture.
In the video I mention another potential cause for attic mold problems, and that is when ridge vents are installed on homes that do not have soffit vents. When a ridge vent is installed on a home that does not have soffit vents, ie a source of outside air to feed it, it can have the effect of "depressurizing" the attic. The effect is most acute during the winter months when heated air from the furnace wants to naturally rise and escape upward into the attic through hidden air leaks. When a ridge vent is installed along the top of the roof, this can exacerbate the already potent force of Stack Effect within the home, causing an even greater upward rush of warm, moist air into the attic from inside the home. Some of the moisture will make it out through the vent, but every home's attic ventilation attributes are different (depending on wind exposure, direction the home faces, etc), but some of the water vapor also condenses on the underside of cold roof decks. For some this idea will be contentious, but over the years we have talked to many home owners who have lived in their homes for 10, 20, 30 years who never had an attic mold problem until a ridge vent was installed. And this problem seems most consistently to occur on homes without soffit vents (the theory being that lacking for a source of intake air, the ridge vent instead draws/ pulls air from inside the home).
Many mold remediation companies try to solve attic mold issues merely by killing the mold that is there, and then maybe adding some additional ventilation or painting an anti-mold coating onto the underside of the roof. This is an incomplete solution in our opinion because it DOES NOT ADDRESS the source of the primary component that mold needs for growth - MOISTURE. We have seen mold grow back in attics where these types of solutions were done. Don't make this mistake! Address THE root cause of mold growth -- moisture. If you live in the Pittsburgh, PA area -- call Insulwise!