Thread: Extreme attic humidity

3 yrs ago I had a contractor replace my roof and install soffit and ridge vents. He messed up big time and didn't drill enough soffit ventilation and also didn't pull back the insulation so the soffit vents were totally blocked. Fast forward 2 yrs and my attic was covered in mold. Took the contractor to court and won and he had to pay another contractor to fix the ventilation and to have the mold removed. There were 2 layers of R13 fiberglass insulation original from the house (1961) and the mold removal guys took off the top layer and I later found that they incorrectly replaced it with another R13 faced layer. Thats the first problem. Then, I realized that the house was extremely humid too...around 65%. Turns out the humidity control on the thermostat was turned way too high. Probably been like that for a very long time. I turned it down and within a day, the house went down to 40% humidity. I had been monitoring the humidity in the attic for atleast a month before I realized that the humidity in the house was so high and a week since then and the humidity in the attic has consistently been between 80%-90% and hasn't dropped since I turned down the humidity in the house. Its a digital humidity sensor that I've compared outside to the weather channel and am sure its accurate. And I'm pretty sure that the soffit vents are now adequate. So I'm going nuts to figure out where the humidity continues to come from. My guess is that the attic has absorbed a lot of moisture from the house and due to the incorrectly installed insulation. The bottom layer of insulation is totally compressed and discolored but doesn't actually feel damp to the touch. Nor do the joists or rafters or plywood on the roof. Some more points:
1. I borrowed a thermal imaging camera from a friend and there are definitely hot spots, but nothing huge.
2. There are two bathrooms with showers, but neither has a vent period. The house is 50 yrs old with both original bathrooms and no signs of mold in either bathroom. I can see the humidity in the attic jump by about 5% when the shower gets turn on but then it slowly drops back down and plateaus in the 80s%.
3. There is a drop down staircase into the attic, but I have an insulated box covering it.

Is there any way to tell if the insulation is wet and if it is, how long could it take to dry out. I need to pick up a moisture meter for the wood.

I live in Pennsylvania and its been between 30-50 degrees Fahrenheit for the last month. Any ideas what might be causing the humidity. If the attic has absorbed humidity, how can I get it to dry out when its so cold? I don't think dehumidifiers work at lower temps.

Thanks much! This has been driving me nuts! Not sure what else to try.

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Dan, what area of Pennsylvania are you in? I'm in SE Pennsylvania between Collegeville and Pottstown.

Hi Nick. I'm in Phoenixville. I see that you're in Spring City. We're practically neighbors!

I've been reading up on humidity. My humidity monitor is reporting relative humidity, not absolute humidity. Is it possible that 80+% RH isn't that bad at 40 degrees? The RH does drop quite a bit on warmer days but only to about 60% Still much higher than outside though. When report indicate what the attic humidity should be, are they in absolute or relative humidity?

Dan

Dan,
There is a lot of good info regarding building science here.
BSD-102: Understanding Attic Ventilation — Building Science Information

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try the calculator and you'll better understand
Dew Point Calculator

how to use Dew Point Calculator

“The humidity in the attic has consistently been between 80%-90% and hasn't dropped since I turned down the humidity in the house. Its a digital humidity sensor that I've compared outside to the weather channel and am sure its accurate. And I'm pretty sure that the soffit vents are now adequate. So I'm going nuts to figure out where the humidity continues to come from. My guess is that the attic has absorbed a lot of moisture from the house and due to the incorrectly installed insulation. The bottom layer of insulation is totally compressed and discolored but doesn't actually feel damp to the touch. Nor do the joists or rafters or plywood on the roof. Some more points:
1. I borrowed a thermal imaging camera from a friend and there are definitely hot spots, but nothing huge.
2. There are two bathrooms with showers, but neither has a vent period. The house is 50 yrs old with both original bathrooms and no signs of mold in either bathroom. I can see the humidity in the attic jump by about 5% when the shower gets turn on but then it slowly drops back down and plateaus in the 80s%.”

These bathrooms are adding humidity to your home, just because you do not see mold growth in the bath is no indication that they are not a factor. They should be vented!


“3. There is a drop down staircase into the attic, but I have an insulated box covering it.”

“Have you also weather-stripped the bottom to seal it?

“Is there any way to tell if the insulation is wet and if it is, how long could it take to dry out. I need to pick up a moisture meter for the wood.”

Insulation will dry out as long as the moisture is not trapped between vapor barriers

“I live in Pennsylvania and its been between 30-50 degrees Fahrenheit for the last month. Any ideas what might be causing the humidity. If the attic has absorbed humidity, how can I get it to dry out when its so cold? I don't think dehumidifiers work at lower temps."

Dehumidifiers work at most temperatures but it is likely that the outside air in PA has little humidity most days this time of year. Also you are on the right track concerning relative humidity; lower temperatures will make the humidity rise if the same amount of moisture exists.


Lastly, I would re-examine the work you had done last, are the ridge vents open or does the felt cover some areas, where they installed correctly, are there enough soffit vents (you cannot over vent a soffit) are there baffles installed to allow the air to travel freely from the soffit up the roof?

Can you explain how the vents were fixed? The best way to prevent moisture build up in the attic is to prevent it. So the source should be located prior to doing any work. If the source was not located by the contractor how can you really fix anything?

So we know the bathrooms and interior are a source from you own test. Add fans of at least 60 CFM that vent to the outside and run them for at least 20 min after a shower. A kitchen fan should also vent to the outside. That is the first step. Since you see moisture in the attic after a shower this will help. The other step that should be done is air seal the attic to the living space. Since the insulation is installed wrong have a qualified weatherization contractor move all the insulation to one side air seal then do the same for the other side and reinstall the insulation. This should do allot for adding moisture from the interior.

The next area to look at is a crawlspace if you have one. If your crawlspace is damp it can make it's way to the attic. Especially if there is a direct chase way to the attic. The crawl space needs to be 100% sealed with plastic. Any chases ways should have been fixed with the attic air sealing.

I suspect the attic is still not vented totally right. I would suspect you still have more ridge area than soffit vent area.

Additional moisture loading from:

Make sure dryer is vented to exterior.
Occupant load will ad quite a bit of moisture.
Cooking ads moisture.
Aquariums.
Pets.
Hanging laundry to dry.
Not using exhaust fans.

As mentioned above moisture migrates from moist to dry. It is possible to have quite a bit of moisture migrating up from crawlspace dirt floor, even though the soil appears dry.