Relative Humidity in the attic???

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 In the attic, it was 47F 90% DPD That seems to be a problem.
But at 5PM, it's outside 36F 44% DP Attic is 46F 76% DP9
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?
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Do you mean the attic or the roofspace ?

Only if you have good circulation between the attic and the outside and that isnt desirable in winter particularly.

Nope, not in winter.

So you clearly do have some circulation.

Because what circulation you do have evens the DP out eventually.

Not exactly the driest place on earth.

Likely because no one has asked that about DP before.

Dunno, never measured it myself.
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On 2/7/2019 8:51 PM, Mike wrote:

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.
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On 2/7/2019 8:48 PM, Ed Pawlowski wrote:

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?
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? My first impression is that the roof is heating and releasing moisture into the attic. Where is that moisture coming from?
Dew point inside the house is below the dewpoint in the attic, so that's an unlikely source.
Only place I can think of for that to happen is if there's liquid water trapped in the roof.
Sounds like an early indication of a leak?
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.
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True.

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

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

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.

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

Yes, that’s one obvious possibility.

Yes, but that doesn’t 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.
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On Friday, February 8, 2019 at 6:16:17 PM UTC-5, Mike wrote:

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True

The attic is not a sealed box and should in fact have good ventilation to the outside, so outside air is constantly exchanging with the air inside. If air comes in during the day at 70% humidity, 70F and it drops to 50F at night rather quickly, the attic will be at a higher humidity until it has time to move enough air for it to equalize again.

Do you have a vapor barrier between the living space and the attic? Have you tracked were all bath, dryer, etc vents go?
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I've seen issues where the sun hits one side of the roof and not the other. Moisture will tend to evaporate from the hot side and condense on the cold side. I added another side vent so there was cross ventilation and made sure to seal all the openings from the living space, especially the bathrooms. Paint the ceilings in the bath with vapor blocking paint. M
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