Attic Gable Vent Fan

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  #1  
Old 11-17-16, 05:03 AM
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Attic Gable Vent Fan

I installed a Gable vent fan in September. It has been kicking on and off when it was hot etc. But now that it is cold it runs in the morning a lot I noticed. I have the humidity set at like 80-90%. I know it needs to be set at a lower setting but don't want it running all the time in the winter. Attic is a bilevel house type attic and it is just over the living room area. So roughly 25ft long. There isn't a ridge vent but there are soffit vents and I pulled all the insulation they had smashed down into there back so I can see daylight again out of the soffit vents.

Should I just turn it off or do I need to have the humidity set properly to prevent moisture etc? Prior to me buying the house the attic had nothing. Even the soffit vents were covered with insulation. It just had a gable vent.

Thanks as alway!!
 
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Old 11-17-16, 02:20 PM
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The primary purpose for of attic ventilation is to remove unwanted moisture during the cold season. a fan to exhausting heat in the summer should not be necessary if the house is well air sealed and insulated.

However, your humidity setting is probably being affected by the temperature. Here is a link to play with.
Temperature, Dewpoint, and Relative Humidity Calculator

Basically, a fan will not do what you want during the cold season and you need to address the passive ventilation between the soffits and whatever you have for high vents.

Do you have just the one gable vent for high?

Bud
 
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Old 11-18-16, 08:25 PM
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Yes the highest vent is the gable vent. The humidity setting was at 75%. So it was running the other morning and I went up and turned it to 90%. I will need a new roof in the next 3 yrs or so, so a ridge vent will be installed when that happens.

Checked the link you shared. Confused cuz if I fill out the first two and click calculate...gives me the RH. Is that what I should set the fan at?
 
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Old 11-18-16, 10:57 PM
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Hi diyp,
I understand the confusion. The problem is, using a RH setting to control a fan in a space that has an uncontrolled temperature makes no sense. With summer temps well over 100 and winter temps near freezing, the RH number goes all over the place.

First, is the humidity really that high in the attic, 75% is a scary number? Using that calculator, if the temp up there is 50 when that RH = 75% then the dew point is 42. That means at 42 the RH will climb to 100% and deposit some water. To make the RH more confusing, a reading in the middle of the attic will be different from a reading next to a colder roof surface. Same amount of moisture in the air, but at different temperatures the capacity of the air to hold that moisture changes.

I would start by picking up a remote RH and temp sensor and install it in that attic so I can see from downstairs what is happening. The RH up there should track the outside RH adjusted for temperature. Example, if outside is 40 at 90% RH, dew point of 37, (just checked Lancaster) then that same air warmed up to 50 by the sun on the roof (use that dew point and the 50) and you have a RH of 62%.

Point to consider, on a rainy day the outside air is at 100% RH, do you really want the fan to be running?

What steps have you taken to air seal and insulate between the house and the attic. Inside air will be carrying a lot of moisture so must be sealed off as much as possible.

Get that remote RH and temp sensor and let us know what it is reading.

Bud
 
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Old 11-20-16, 03:34 AM
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Ok... will do. Thanks for the info.

Yea it's confusing. But have read articles on ice inside the attic. Don't think I want that. I just don't believe the humidity setting on that thing. I got the fan from lowes...seemed to have good reviews.

As for air sealing...The door which is roughly a 34x34" opening is pretty good. I could use some strip of foam to make it even better. As for between my kitchen Dining/living room and attic how would I check that? lol I'm not an idiot for asking that I hope. I did caulk outside the house where the soffit meets the aluminum siding.

I to have radiant electric ceiling heat (so essentially the drywall on the ceiling heats up and it radiates down) and I worried it was going to leak into the attic but it is freezing up there. Cold front came through and wow. There are insulation between the joists and on top. I moved some of the insulation to create storage so there is a section of only insulation between joists.

Here is an attic pic for now.
 
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Old 11-20-16, 04:08 AM
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@diyd, "lol I'm not an idiot for asking that I hope." not asking would be dumb.
I'll add a link on air sealing, those leaks are hiding everywhere. The most important ones are the big ones and those you can get to easily. Hard to empty an attic and start over, although it would be beneficial $$$ savings. That air that leaks is also a major heat loss.

Quick review of air sealing. Chimneys, vents, recessed lights, ceiling electrical boxes, drop ceilings over cabinets, and the tops of all of the walls where the drywall meets the wood.

https://www.energystar.gov/ia/partne...ide_062507.pdf

Bud
 
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