attic fan/ gable fan

I have increased my airflow to the top floor of my house by sealing ducts, closing dampers, ect....
I have an open gable vent on each side of the house and also one of those vented shingle ridge vents on the peak of the roof.
Would an attic fan or a gable fan make much of a difference in the temperature of the attic, therefore helping the cool air be more efficient in my 2 upstairs rooms?
Would a gable fan do the job just as well or would the attic fan through the roof be better?
I don't know why, but I have hesitation about having someone cut a hole in my new roof. I'm afraid it won't be sealed right and I will get moisture under the shingles and/or in the attic.
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Postal68 wrote:

Well that attic fan will help the attic stay cooler, but only by drawing out the cooler air from the house to do it. A gable fan is not likely to really help much.
Do you have any under eave vents? How did the venting work before the ridge vent was added? How much insulation is in the attic floor? Do any of the supplies or returns for your HVAC go thought the attic?
Note: closing off vents to "force more cool air upstairs" may not be a good thing to do. It can reduce air flow to the point where not only is efficiency decreased, but it can damage the system.
If you are having a problem cooling the second floor, there is a design problem with the current system. You may want to have a professional take a look at it and do the calculations (manuals).
Good Luck.
BTW I have a bit of a problem with the second story as well ,but it is not bad, but I will have it corrected when it is time to replace my current system.
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Maybe not, given your gable and ridge vents, assuming you have some low vents too, eg soffit vents. I might block the ridge vent permanently and make some doors to close the gable vents in wintertime.

Why would it have to draw air from the house?
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

If you get snow you want the attic the same temperature as outside. If the attic is warmer it can melt the snow on the roof. Water flows down until it gets to the soffit which is not heated. The water then freezes forming an ice dam. The ice dam can force the water up under the shingles into the attic which can cause expensive damage.
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snipped-for-privacy@ece.villanova.edu wrote:

Please no. You want the attic to be as close to the same temperature as the outside as possible winter and summer. If you close up the attic moisture will tend to accumulate and cause damage in the form of rot and mold. The insulation should be between the heated parts of the home and the outside, that would be the floor of the attic. It is most efficient there.

The term "attic fan" is used two ways. One refers to roof or gable end fans that only ventilate the attic. In that case there should be no vents through the attic floor to the living area. In that case there is still some small amount of air drawn in from the living area, but not important. It is also used to refer to whole house attic fan which sucks air out of the living area and pushes it into the attic area to be released through any vents that may be there (roof, gable or ridge). It was the second type I was referring to, but I failed to make that very clear.

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Postal68 wrote:

considerably, but not result in much net energy savings due to its own power use. It'll relieve some of the load from your AC system, though, which is a good thing.
A radiant barrier can be very effective in my experience.
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On Wed, 16 May 2007 15:08:39 -0400, "Postal68"

Which is better I can't say.

I had that too, so I cut the hole myself. :)

It was actually pretty easy to seal. The top half easily slid under the shingles and the bottom half sat on top of them and iirc was nailed down.

I had the roof replaced a couple years ago, after 20 years that I had lived here, I saw it when the roof was off. Nothing special around the fan. No damage, as I expected since the installation is simple and well designed.
As to the attic, the rain hits the roof, bounces off, then some of it hits the screeen that came with the fan, and some of that gets past the screen, and a tiny bit of mist lands on the plywood I covered the center of the attic with. It's never been enough to more than sit on the surface, and after 24 years, the unfinished plywood has suffered no damage from this little bit of water. I guess I wouldn't put an antique under the fan, but even a grocery store cardboard box would probably be fine. We have verrrry heavy rains here sometimes.
To prevent the mist, I did look into putting a second (vinyl) screen inside the circumference of the housing, but A, I'm not sure how this would have cut down on the amount of water, and B it didn't seem easy to do. Then I paid attention to the wood and decided there was no need.
I think Joe must be thinking of a whole house fan when he said it would draw air from the house. That's why I call these things roof fans, and not attic fans, which name I think misleads a lot of people.
I don't have a gable fan or vent. I only have about 4 inches of fiberglasss and an inch or half inch of cellulose under that. I guess I should have more.
As to the fan as a whole, a lot of people poopoo roof fans, but I think mine is great. I have a townhouse that was built with a full width ridge rail, and full width soffitt vents, front and back, about 4 inches deep and as wide as the house is. I almost never use AC and when I bought the house, when I came home from work in the summer, it was so hot, I couldn't even go upstairs for a minute. I would make dinner, work and sleep in the basement, then go upstairs in the morning to bathe and get new clothes. I put the fan in the first summer.
After the fan, it was 10 or probably 20 degrees cooler. 20 degrees or maybe 30 cooler in the attic. I stopped sleeping in the basement, nailed a little fan to the window sill, kept the bed next to the window, and I only need AC 2 or 3 weeks a year, in Baltimore. I never thought to or tried to measure how much electricity the fan uses. I thought, It's a fan. It couldn't use much. But I'm going to look at the plate and see how many amps.
It runs from 10 or 12 in the morning to 6 to 8 at night, depending on how sunny and hot it is outside. On hot days it can run from 9am to 10pm or maybe 11. This means it's off by the time I go to sleep.
Plus I put in a swtiches to keep it on or off. I keep it off in the fall and spring, to use the sun to heat the attic to heat the house. It works well. I don't take humid showers so there is no need to keep it on to vent the attic, but it was easy to put in the second switch when I put in the first. The switch is underneath the fan, in the second floor hall, a double switch mounted sideways so it still goes up and down, although iirc up is off and down is on. I get confused what the normal setting is, so I drew a black line on both that shows when they are in the "normal" setting, thermostat controlled, no overrides.
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In restricting airflow you can do damage to your hvac system, one way is freezing the coil from reduced airflow. Attic fans can increase cooling costs by pulling out cooled air. No simple answer for you , best is a pro to look it over.
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I'd consider the radiant barrier solution which was suggested a few posts back before a powered fan. The radiant barrier is a metal foil that get stappled to the bottom of the rafters. It blocks most of the radiant energy so it doesn't heat up the attic as much. I'm considering one myself, as everything I've seen tends to say they work.
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You have a ridge vent without under-eave or soffit vents? Skip the powered fans, put in some soffit vents. A lot. You need more soffit vent space than ridge vent space.
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I think the power drawn by the fan would be better spent feeding the central AC.
Suppose your fan draws 400W and on a hot day would likely run continuously from late morning into evening (depending on thermostat settings). Suppose your central AC draws 4kW total (10 times as much). Running the fan cools the attic and reduces the heat load on the house a bit (depending on the insulation above the ceiling). Every hour of attic fan would buy 6 minutes of central air (one tenth of an hour). This is just armchair reconnoitering but somehow I think that 6 more minutes of real air conditioning will help me more than an hour of trying to keep the ceiling a little cooler.
That's not to say that ample attic ventilation is not a good thing. It is...as long as you don't have to expend power to do it.
There has also been discussion in this forum in the past about fires caused by faulty attic fans. I had one lock up because unbeknownst to me it had sleeve bearings. Motor was sitting there cycling on and off on its own thermal protection. I vowed I would not put it back but then relented and did (after polishing and relubing the bearings). For some reason I can't understand the fan itself slipped down the motor shaft--no idea why, the set screw was plenty tight--and caught on the supports, locking it from rotation. Again, saved only by the thermal protector. That was the last straw and I disconnected it. Makes a nice passive vent.
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