The most common cause of all attic molds is improper ventilation. Let's start by explaining that in most attics the mold does NOT grow in the summertime. This is because during the spring summer and early fall months the daytime temperature of the attic is far to warm for the mold to effectively grow and the humidity levels inside the attic are typically too low to allow for mold growth. Most molds will only grow between 40 and 90 degrees Fahrenheit. Some molds that belong to a special class of mold (the ones usually affecting northern climate attics) called the Chrysophile molds will grow in temperatures as low as the 20's.
During the winter months when we heat our homes, some of the heat is inevitably lost into the un-heated attic space. This hot air mass then moves upward where it will dissipate outside if the attic is properly vented. If however the attic is poorly vented the hot trapped air will encounter the cold surface of the underside of the roofing boards or sheathing. This hot -air- meets- cold-surface results in dew-point being achieved. This dew-point created condensation and is often more noticeable on the nails as the nails are more efficient at conducting the cold temperature from outside. The condensation then forms water droplets which can drip onto the floor of the attic or worse sometimes run down into the wall cavities below resulting in potentially "hidden” mold which can be the source of frustrating to diagnose health problems.
What is the best way to vent the roof?
Roof ventilation is divided into two categories: Incoming air and Outgoing air.
Incoming Air: The most effective way for air to enter the roof is through vents that are placed in the soffets along the eves. Continuous strip vents provide the most reliable ports as well as rectangular vents. Round ventilator plugs are easy to install but are usually to small to provide adequate air flow.
Outgoing air: Stale or heated air escapes through the top of the roof. There are many ways to achieve sufficient ventilation including: Ridge Vents, Turbines or Powered Vents, Box Vents and Gable End Vents. Continuous ridge vents are the preferred method for eliminating built up hot air but may not be sufficient to ventilate short ridged roofs like hip roofs.
Roof leaks always need to be addressed by a professional roofer and special attention needs to be paid to flashings around chimneys and indeed any penetration found in the roof decking. Flashings often require maintenance between roof shingling jobs and are extremely important to maintain. Roof leak can often develop into festering mold problems both in the attic cavity or worse inside of hidden wall cavities in the home.
Ice Damming occurs along the eves immediately above the plate-line of the exterior walls. Leakage in this area easily penetrates into wall cavities where it can become trapped between exterior finish materials and interior vapor barriers. A surprisingly small amount of water can raise the relative humidity of a wall cavity to damaging levels. Mold growth may eventually become visible along the wall base as it penetrates from exterior to interior wall surfaces. More critically spore and VOCs (Volatile Organic Compounds) released by mold can aggravate serious health problems in the houses occupants. Especially at risk are the very young and the old as well as Allergy -sensitive or immunes-compromised adults. In some mild cases ventilation corrections are enough to cure the problem in severe cases the roof shingles must be removed and strips of "Ice - Guard" be installed along the eves to prevent damming. In severe cases the exterior wall drywall must be removed and the wall cavities treated as well.
Improperly vented bathroom fans: Bathroom fans and dryer vents can generate a lot of moisture. They should always be vented directly up and out through the roof. Often times builders or do it your selfer's will discharge fans or dryer discharges directly into the attic, greatly increasing the overall humidity. Do-it-your selfer's often try to connect more than one fan into a discharge and will commonly run the discharge out to the eves or soffets rather than straight up and out through the roof. The remedy is simple one fan one roof vent directly vertically up through the roof overhead. On the outside the vent has a hat and is not a risk for water leaks
HOW DO I TREAT THE MOLD?
Mold spores can grow at alarming rates. One mold spore can develop into 12 million in twelve hours! Also the mold is feeding on the boards, and what you can see with the naked eye are millions and millions of them clumped together. What you can't see are the "roots" The organism itself. The roots of mold grow into the substrate at least 3/16 of an inch. What we think of as the mold the clumped spores are actually the fruit or reproductive body like an apple to an apple tree. You cannot hope to solve a mold problem leaving these roots and treating only the surface of the wood. Also the majority of the chemicals being used on the market today contains known cancer causing chemicals and worse do not work on porous surfaces (every surface in an attic). The mold certifying authorities recommend putting the attic under negative pressure. This means using expensive equipment to suck more air into the attic than escapes from it.