sserrels wrote:
My house is one half of a prefab duplex from the 80's. Last week I went
into the attic and found mold covering the roof sheathing. It had not
been there a few weeks earlier. The attic is supposed to be vented by
soffit vents and ridge vent, but I found that the air is not flowing up
from the soffit to the ridge like it should. The only moving air in the
attic is the air blowing in from the ridge vent. The soffits are
blocked by fiberglass batts and an additional layer of blown in
insulation. So there's practically no ventilation.

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Shawn: Congratulations on your very clear explanation of the problem.

It just proves what has frequently been considered and referred to, by
some, as 'just a theory'.
i.e. That it is important to ventilate the attic roof space.

We have even had those who have asked "Is it OK if their dryer vent (
i.e. hot air loaded with moisture) can vent into their attic". Strewth!

Many building standards not only mention percentages of vents in
relation to the area to be ventilated they also importantly say
something along the lines 'arranged to permit cross ventilation'.

Based on what you have described I think you are on the right track
proposing an electric fan to exhaust damp air, both summer and winter
as a practical solution. Cost of the fan and a bit of electricity minor
compared to replacing a roof and more!

But as someone mentions you may be able to run fan slower at times to
avoid negative pressure up there sucking warm air out of the house?

Anybody here have a number for how often the air up there should be
exchanged?