Mold in Attic

Last year I was in my parents house getting some stuff out of the attic, and there's sporadic, but a good amount of, mold on the roof plywood. It's black and almost looks like staining, in some areas there are typical mold spots. Oddly all the rafters are clean. There are no water leaks but a couple things I want to mention. This is a regular colonial house (built in '75), no living space under the roof (bedrooms don't have slanted ceilings). 25 years ago, my father installed soffit vents along with those pink styrofoam channels on the front side. He also put two fans on the peak vents (1 fan on each, those vents on the sides of the house near the peak, not sure what these are actually called) blowing outwards for 12 hrs / day. Those fans ran for 25 years before I told him to get rid of them last year because I saw the mold. There is equal mold on the front side and back side. There is no ridge vent. There is a bathroom fan venting in the attic but it hardly ever gets used.

I have a similar house, no ridge vent or soffit vents, 2 bathroom fans that vent directly into the attic and get used regularly, and my roof plywood is so clean you could eat off it. Houses are same age. The only difference is theirs has plywood on the floor and mine has nothing.

He doesn't remember if there was or wasn't mold 25 years ago, but I bet there wasn't and he thought the fan thing was a good idea, and it created the problem. What do you guys think?

 

You are in MA, are the houses near the shore, humidity question?
It is impossible to to answer why yours is different as there are a million combinations no one has time to resolve. Your best approach, both houses, is to follow proven and tested guidelines and get those attics ventilated properly.

As a note, 2 powered exhaust fans would have been pulling warm humid inside air from the house into the attic. Air sealing is now recognized as the most important step to keep moisture out of the attic.

There are guidelines for the recommended amount of high and low vent areas and along with the air sealing and moving those exhaust ducts to the exterior his house should be fine. As for cleaning up what is there that can be discussed later.

Another note, bath exhaust fans have an internal damper that opens towards the exhaust direction. Between natural convection and those fans air has been flowing for all 25 years.

Bud

 

Yeah we're only 25 miles apart, and as you know the summers have jungle humidity. Oddly enough, the roof section between the rafters that is above the vent fan has no mold, haha.

Are soffit vents pointless if you don't have a ridge vent?

 

High and low vents aren't particular as to their style. Soffit vents distribute the incoming air to be sure all rafters benefit from the flow but high vents can be almost anywhere as long as they are "high".

Here is a link for reading, I bookmarked it for myself.
Rethinking Ventilated Attics | GreenBuildingAdvisor.com

Bud