"Duane Bozarth" wrote in message
...
Roger Shoaf wrote:
...
At 70% humidity dew will not form on the ground, so it should not form
in
your attic.

Not necessarily so...dew may well form on the ground just not in the air
as fog and may OP may well condense moisture out in the attic on cold
surfaces even if the bulk air temperature is above ambient.

At 70% humidity the only way to get the water to come out of the air is to
lower the tempriture of the air to the dew point. If the air is in motion,
it will not be incontact with the colder surface long enough to cool and
therefore condense.

In an attic you have problems if the air in the attic gets colder and the
humidity rises. If the warm moist air goes out the ridge vent and it is
displaced my colder air coming in from the soffet vents the air will warm a
little and the humitity will drop. If however the attic is full of air at
70% humidity and has nowhere to go and the tempriture drops, then the
humidity will rise to the point where it will condense on to the cold parts
of the roof.

Rather than the ice water on a warm day consider foggy windows in your car,
and a disfunctional defroster. If you roll down the window and drop the
tempriture on the inside of the glass to equal the outside, the fog goes
away, if you try and just wipe the window the fog comes right back. If your
heater finally kicks in, and you can raise the tempriture of the air in the
car enough the air can hold enough moisture and your window stays fog free.

With your example of the cold glass on a warm day if your glass was in a
sufficient air flow the air tempriture would not drop fast enough to reach
the condensation point and therefore the outside of the glass would stay
dry.

--

Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
they come up with this striped stuff.