question about attic ventilation & humidity

[jeff]

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

[Speedy Jim]

You're correct; it's a complex situation:

Jim

[spacekase]

attic ventilation is a complicated issue. i run a lab in MI that tests and id's mold in homes, attics, other (as well as lots of other things too). if you are getting mold growth on your roofing boards and the joists without leaking, you are having humidity accumulation, i.e. condensation. if the mold growth is active, green and fuzzy, the problem is a current issue. if the growth is thin and black and looks like charcoal, the issue is most likely a different season, as this is the 'dormant' stage of mold growth. you've got to understand that condensation occurs when temperatures change faster than the humidity. increasing ventilation allows the excess moisture to escape the attic during air flow (created by temp differentials, pressure diffs, etc) before it can condense. increase ventilation - using fan force if necessary - decrease reoccurance of condensation. other things... make sure bathrooms, dryers, and sewer vent are not directed into your attic - ever! many homes i am called to have these venting into the attic. very stupid. also if the mold is fine black dry stuff and it gets wet, it can bloom very fast damaging wood and causing serious allergy effects on people even if you dont have allergies... yet...if you clean this stuff up ( and u should if there is lots of area with growth) dont use bleach, use quaternary ammonium compounds in a detergent (read labels!) dont drip (catch it all), dry it thoroughly, make sure insulation is DRY!!! good luck