Venting the attic - powered fan or not?

Home was built in 1960, located in New Hampshire. One story house with a gable roof/Hip.
In the summer the temperature difference between the living area and attic was in the region of 25F-30F, and upto 35F-40F on the hottest days.
Went up in the attic and found that the ridge vents were OK but soffit eaves were blocked by the second layer insulation. So I pulled back the insulation from the edges and inserted "prop-a-vents" along the sides.
About 2/3 of the work was done while it was still hot and I noticed that there was a small temperature-difference drop on most days but on hot days the differential was still 30F or so.
Now I've just finished the rest of the work, but it's not hot enough to compare the temperature difference -- it's about 17F today on a cool day. However, it is easier to get contractors to come out and do work in the attic or on the roof during the cool season :) So I'm wondering, should I:
1. Have a roof vent installed - it's just a small, molded, vented plastic square that goes on the roof and vents air to help the ridge vents, and will that help on hot days, or
2. Have a power vent installed? The fan itself is not too expensive, but it would require running a new power circuit to be run up from the basement to the attic, which will raise the total cost quite a bit.
Opinions?
-- Himanshu
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I had a thermostat powered fan in my attic. It would come on low speed about 10am, high speed about noon, and run until about 10pm or so. It made no difference in my power bill, it just made me feel better thinking my attic was not as hot as it would be without it. If I had to do it over again, I'd just have a couple of good vents and leave it alone.

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I'm running a power vent as a band aid until I can get around to properly venting the soffits, I have a ridge vent 3 gable vents and it still runs often till the wee hours of the morning. I'm on my second motor, (I've got to get motivated to finish the venting). i was able to tap an existing line in the attic and split off a wire for it, as a bonus I now have lighting when I change the motor.
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A 30 degree diff between the living area and the attic on a hot day is excellent. It's not unusual for attics to reach 110+ on a hot day, even with excellent venting. That's well within the capability of the insulation to handle. Would you be concerned if during the winter the outside temp was 30 deg below the inside temp? Attic ventilation is important to keep humidity down and to prevent really high temps, like 140, which are possible with poor ventilation.
So, I wouldn't do a thing. Plus, installing a powered fan raises the possibility of a considerable amount of air just being pulled in via the ridge vent and blown out by the fan, effectively short circuiting what is already there. Also not clear that adding a few more box vents is going to do much either.
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I think the latter is the key point -- a powered fan may not be be much help because of cross currents with the ridge vents. Many years ago I installed a powered vent at my house in Ohio and immediately noticed a significant decrease in my need for air conditioning. For that house, the powered vent was worth it.
Since then, insulation has gotten much thicker and electricity more expensive. Reportedly, tests now show that a powered vent will use about as much electricity as it will save in a/c costs and that the cost-effective way (for houses with soffit and ridge vents) is to add more insulation. Nevertheless, I did put in a powered vent (I have soffit vents but no ridge vents here in hurricane country). I can't quantify a savings in my electric bill, but now if I have to get something from the attic the temperatures are bearable, even here in Central Florida. Regards --
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Thank you Ook, Eric, trader4 & Jim R for the replies. Looks like most of the replies aren't favorable towards the powered vent, so I'll probably stay away from that.
One thing I forgot to mention is that there's no air-conditioning in the house, so the attic staying hot is a major discomfort to the wife & infant in the living area, which is why I am trying to address the problem. I did install a window unit, but in some areas of the house where there are larger, connected, open spaces, I am not sure what to do. I'm considering a split-type a/c in the future.
Anyway, back to the attic ventilation:
Would any of you recommend installing a whole-house fan? Keep in mind that I'm in New Hampshire and July & August are probably the months that get really hot.
And if yes to the whole house fan, is a powered vent required on the roof? I was under the impression that eaves+ridge vent would be enough.
Again, the fan itself is not too expensive, but the installation & labor makes me think twice. As an example, the fans can be $200-$400, but the installation estimates are between $1000 (without powered vent for the roof) & $1800 (with powered vent).
-- Himanshu
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We have a whole fan here in San Jose and works well without AC. For this to work well, the outside temperature in the evening time must be comfortable, or it is no use to turn on the fan. The idea is to pull in cooler air than inside the house and vent the hot air in the attic. Pulling in hot air into the house is not what you want.
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I picked up what was probably sold as a gable vent fan for a few $ at a thrift shop. I slipped foam pipe insulation over the 4 mounting bars that came off the fan. This I then set over the attic access hole in my house, blowing up into the attic. I plugged it in through an X10 appliance module, which allows me to turn it on/off remotely. When I want to cool an area, I turn on the fan and open a window in the area to be cooled. Or, I open a window downstairs so that air will be drawn through the whole house. This thing makes a big difference in the comfort of my home (in Seattle). I'm sure that part of its effect is due to the air it blows up into the attic, cooling the attic at the same time it vents the house.
It's not a real "whole house fan", but is an effective substitute. I don't really need anything more.
Bob
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In my first house (located in Ohio) I installed a squirrel cage out of a furnace in the finished attic vented through the gable. I plugged it into the over head light and when we wanted to cool the house I just opened the attic door and turned on the light switch. It was so powerful if you didn't block the door open it would suck it shut. We never had A/C and never missed it, the constant flow of air made it very comfortable.
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Himanshu wrote:

It sounds like you have done all you should do and your venting system is working well. I doubt if adding power venting would help anything other than the bottom line of the vent manufacturer and the electric company.
--
Joseph Meehan

Dia duit
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A whole house fan would probably be very useful in your area, where there are many times the temp drops down nicely overnight. This would quickly cool the house down close to outside temps and it could stay reasonably cool till at least mid day.

All that is needed is enough exit area for the fan rating. That is specd by the manufacurer.

Sounds high to me. The whole house fan is more involved, because it involves cutting/framing an opening in the ceiling. But $800 for a powered attic fan is way high, unless the install is complicated for some reason. You can get a fan for $100 that will install either in a gable or in a new hole cut in the roof. I'd say it should cost more like $400 installed.
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I love mine.
On Mon, 04 Sep 2006 20:15:15 GMT, Himanshu

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