To prevent mold growth in an attic is highly dependent upon consistent ventilation. This is a two part process of ventilating an attic. Air must flow in through the soffit vents, moving across inside of the roof sheathing, and then flow out through the upper roof or ridge vents. This process is depended on the warm air ascending to the attic. The Laws of Physics states that warm air rises, so when the heater is on in the home, most of that heat moves up into the attic.
The proper way that an Attic operates is when, this warm air finds an exit out of the roof (ridge vents) and flows outside. The air flowing out of the attic must be replaced with an equal amount of air from else where. When this happens it creates a mild suction effect which pulls air through the soffits. Depended on the wind speed it can affect this process by pulling the air out of the attic more then the normal air flow.
The reason why air is pulled through the soffits, is to move it along the underside of the roof sheathing, which helps keep the moisture from accumulating in the attic.
Mold growth occurs when the moisture accumulates in the attic and the roof sheathing begins absorbing the moisture. When increasing the intake and exhaust of fresh air will drastically decrease the chances of mold growing in an attic.
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Lots of attics are built to code and many even above code requirements, yet even with everything done right things still seem to go wrong. Some of the problem areas a good attic can go bad are when: Soffits get blocked by insulation, baffles installed in soffits are get crushed by insulation and in mechanical ventilation options.
Most attics that perform poorly do so because someone made an error during installation. A few examples would be that the venting was installed incorrectly, the quantity was insufficient, the insulation is blocking the vents, etc. These kinds of issues are very common, however, there are also many attics that are genuinely hard to vent right.
Two issues you can run into with your roof are Hip Roofs and Roof Valleys. These can cause many problems when it comes to proper ventilation of an attic.
Hip Roofs are a style of roof that severely limits the available space to install upper roof venting. Because the roof is essentially a pyramid, the ratio of upper roof space to lower roof space is quite small. This leads to wildly disproportionate ventilation. Worse yet, Hips are built with a tall rafter running along the length of the ridge/hip. This framing is usually quite tall and it impedes the movement of air up the roof line. You could have perfect soffit venting, but it wouldn't help because the air has nowhere to go. They’re not cheap to fix but, solutions do exist for hip roofs
Roof Valleys valleys are even trickier. A “valley” occurs where two roof lines intersect. This is the opposite of the issue found in a hip roof. The valley collects water and directs it down the roof, no ventilation in the immediate vicinity is possible. Solutions for roof valleys exist, but they are imperfect and not aesthetically pleasing.