I live in the northeast. Here's a (perhaps silly) question I have about
attic ventilation: On those days where the humidity outside is very high
(such as 100% on a rainy/foggy day (in any season), or a hazy/hot/humid
summer day) then I'm curious why that having more ventilation is said to
reduce, rather than increase, the attic's humidity? To see what I'm
getting at, lets suppose that, in theory, I could increase my attic's
ventilation to that of a screen porch. Wouldn't having so much ventilation
cause the relative humidity of the attic to get all the way up to the 90% +
range on those very humid rainy/hazy/foggy days, or am I missing something
here? For example, today the outside humidity is 100% but my attic's
humidity was around the low 50% range. Would adding more ventilation make
the humidity even lower or would it just draw more of that humid outside air
into the attic and make it even more humid? If not, then why not?

Anyway, I have a mold problem in my attic, in which the mold is growing on
the ceiling underneath the roof sheathing. A roofer has checked out my roof
and said there are no signs of leakage. The roof is less than 9 years old.
Everyone seems to suggest that increasing the ventilation would be of
benefit. Currently I have two gable vents on the sides and two square
passive vents at the top. I'm considering having a fan installed at the top
of my roof. Before I do so, I just need to make sure that adding a fan, or
adding more vents (such as soffit vents), would actually *reduce* the
humidity on those very humid days rather than causing my attic to become
even *more* humid (by drawing in more humid air). What do you think?

Anyway, if a fan is going to reduce humidity, and you recommend I have one
installed, then would having it controlled by a thermostat be adequate or do
I need a humidistat control too?

Thanks.

Jeff