Attic humidity.

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Old 12-07-18, 05:06 AM
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Attic humidity.

Hopefully this is the correct forum. What should an attic humidistat setting be in eastern pa. Now weíre moving into winter months I believe itís set at 45 and it seems like itís runnng fairly constant. It not wired wrong or stuck on as it does go off just running a lot Never had humidistat until this summer so looking for the happy spot to set it at. Thanks.
 
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Old 12-07-18, 06:32 AM
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I'm not sure that is a perfect science. RH will vary based on temperature and the temperature in your attic can vary wildly. The maximum it should be set at is likely 55-60 since its generally accepted that mold growth can occur if the humidity is higher than that. I wouldn't be afraid to play with it a little if you don't want it running constantly. But there is nothing wrong with that either... it's doing what it's supposed to do.
 
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Old 12-07-18, 11:37 AM
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We need your approximate location, nearest big city will do.
As Sleeper said, monitoring RH to control attic ventilation isn't really the best approach.

1. That RH reading will vary with temp as Sleeper said.
2. It will also track the outside humidity since that is basically the air that ends up inside the attic.
3. There are also times when you might not want outside air to be pulled in, like rainy days with RH=100%.

The bigger question is why are you power venting in the winter? If warm moist air from the house is leaking into the attic risking condensation on the bottom of the roof, the leaks need to be sealed. If not, adding a power vent will significantly increase that house to attic leakage.

Pick up a remote reading RH meter so you can monitor what is happening up there. If it also reports temp even better.

Here is a humidity calculator to try.

Bud
 
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Old 12-07-18, 05:30 PM
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Location is outside Philadelphia. As far as power venting. I can easy turn it off. I got a new one installed with roof replacement this summer and the version has a humidistat as well as thermo. Never had humidistat and trying to understand what it shouldíve set too. As I said it runs fairly constant if not benifiting from it Iíll gladly turn it off for winter.
 
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Old 12-07-18, 08:37 PM
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What other vents do you have is the power vent is turned off? Ridge vent? Gable vents? Soffit vents? Just checked Philly and temp is 30į with 53% RH. Since that is the air entering due to the fan it explains why it is running constantly. The attic exhaust fan is trying to cover summer and winter seasons. In Summer it should be controlled by a temperature setting. The RH setting is for winter, if you need it. Example, lots of warm humid air from the house leaking into the attic. When 70į at 40% RH cools down to 40į its RH goes exceeds 100% so that air you would definitely want to get rid of. But, if you have very little interior air leaking into the attic you would be better off with natural ventilation, that avoids the fall pulling from the house.
 
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Old 12-07-18, 09:11 PM
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I have soffit vents and 2gable vents. So yes itís pulling moist outside air in I would imagine. Do you think itís best to shut it off until summer. Iíd rather it not run constant and prior to I saw no signs of interior frost on roof. I plan to get up there tomorrow when itís just at freezing after morning so it will dip below freezing over night. And take a look for any frost build up. I think Iím in the camp if shutting it off for winter. But would love to hear opinions.
 
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Old 12-08-18, 03:51 AM
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Have you ever had moisture issues in the attic, wet spots, mold, or frost?
Do you get ice dams in the winter? An abundance of icicles would be an indicator of ice dams forming.

Still some information missing, like size of gable and soffit vents (net free area) but if no history of the above and adequate venting then yes, turn the fan off.

You can monitor the attic and watch for ice and if either problems appear, cycle the fan on.

Bud
 
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