"Mike" wrote in message
...
On 2/7/2019 8:48 PM, Ed Pawlowski wrote:
On 2/7/2019 8:51 PM, Mike wrote:
I have a temp/humidity sensor in the attic.
The attic and outside temperatures vary, but the
DEW POINT should be the same.

Often, they are.
But it's been wet this month and they don't always track.
At 2PM, the outside was 41F 41% DP=19
In the attic, it was 47F 90% DP=44
That seems to be a problem.

But at 5PM, it's
outside 36F 44% DP=16
Attic is 46F 76% DP=39

Other times the inside/attic dew points are about the same.

I've been using this site:

http://www.dpcalc.org/

The 90% number is not good for mold growth.
The 76% number ain't so bad.
The attic is below freezing at night, so may be
collecting moisture as ice crystals.

I don't have a historical log, but I haven't been alarmed in the past.

House is 47 years old ranch in Portland, Oregon. Two layers of asphalt
shingles. Wasn't leaking when I put on the second layer.

The insulation is fiberglass. I expect it holds some moisture
and there will be some delay in the humidity dropping as the
outside humidity changes.

I called the roofer and asked about humidity in the attic and was
told to email the data. The response basically ignored what I wrote
and gave no answer.

There's a lot of insulation in the attic and I can only get at about
half of it without messing up the insulation. I see no evidence
of leakage around all the stuff sticking thru the roof and no
evidence of water damage, but I can't see much of the wood on the
inside of the roof.

Am I worrying too much? Or should I put on the rubber boots
and trudge thru the insulation to get a better look.

What humidity should I expect to see in the attic?

My guess is worrying too much. I don't know of anyone that has ever
tracked that data. Just a guess, the air in the attic does not change
nearly as fst as outside air so even though the temperature is changing,
especially from solar load during the day, the air is not changing like
outside and moisture levels lag.

In any case, keep it well ventilated.

My basic assumption is that Dew Point is constant.
I enclose a volume of atmosphere in a container.
I heat the container.
The temperature goes up, the relative humidity goes down,
the Dew Point stays constant.
True or false?

True.

If it's true,
and the attic dewpoint is 20F higher than the outside dewpoint
and sunshine causes the attic dewpoint to increase further,
where does the moisture come from?

If the outside air has a higher dew point, by some
of that getting into the 'attic' by normal ventilation.

My first impression is that the roof is heating and releasing
moisture into the attic. Where is that moisture coming from?

It can have condensed onto attic surfaces when its cold overnight in winter.

Dew point inside the house is below the dewpoint in the attic,
so that's an unlikely source.

Its more complicated than that if exhaust fans move
the air from inside the house into the attic instead of
outside the house, particularly with showers in the
winter and with cooking that involves boiling stuff.

Only place I can think of for that to happen is
if there's liquid water trapped in the roof.

That can certainly happen too, most obviously
when the gutters leak and even with normal
rain with a non continuous roof like shingles.

Sounds like an early indication of a leak?

Yes, thats one obvious possibility.

I make the same measurements in the crawl space under the house.

It's hard to imagine that nobody else ever considered measuring
attic humidity. There's gotta be some real data out there.

Yes, but that doesnt mean they ever published it where
you can find it using google or that they are in the tiny
handful of people that read in here.