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We have 2 gable vents in our roof (approx 2' high or so), one at either end, and upon inspection it does not appear that there is really enough overhang in the roof for soffits. I still have to inspect it closer from the outside, but they are for sure not installed at this time. The roof is also slate, so going in through that is not really an option we'd like to pursue. If we install a attic gable fan on one end, will there be enough air flow through the attic to make this project worth our while in terms of heating/cooling costs, or should we go another route?

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I don't think the fan would help with heating costs. As for cooling, ideally you'd first invest in more insulation on the attic floor. –  DA01 Jun 18 '12 at 22:23
    
@DA01 true, it is mostly cooling. However, we are using the space for storage now and there is plywood floor cover. So I'd ideally want to take all that off to find out what's there first before doing that.. which is a bit of a chore. –  Aaron Jun 21 '12 at 16:50

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I can tell you from experience in my house a slate roofed colonial, it will make a difference. Even with the fan mounted in an attic window that is that is 3 inches off the floor of my walk-up attic the temperature drop in the attic was 30 degrees.Most fans will have a spec sheet telling how many square inches of ventilation opening they require. It is important to remember that you are calculating square inches of unimpeded opening so subtract for louvers and other obstructions. In my case I have a large peak where hot air tends to get trapped. If we have several days of very sunny and hot weather I place a small table fan on the attic floor. With the small fan pointing up it circulates the air to remove the hottest air trapped at the ridge.

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In terms of most efficient usage of money, I suspect weather stripping will have a bigger impact on an older home. Sealing all the gaps around doors and windows will cut down on drafts. If fixing a window involves replacing it with a new energy efficient model, all the better.

Once the drafts are solved, insulation is usually next. But that can be expensive, especially in an older home where blown insulation is about the only way to get it in the wall, and that also has a habit of settling over time.

A fan in the attic will be a lot cheaper than insulation, and I'm betting that it will be a net positive. If I were to install it in your situation, I'd do the following:

  • Check the predominant wind direction in your area, particularly during summer evenings. You want to supplement the wind, not fighting it.

  • Use a GFCI in the circuit. I would consider the attic fan as exposed to the elements just as a garage area is.

  • Next, include a timer switch in the chain. You only need to run the fan at in the evening. In the middle of the day, you'll be spending money to run a fan that's pulling in hot air.

  • Finally, include a thermal switch in the chain. If the attic is 100°F, you want to run the fan, but not if it's 80°F.

For the timer switch, I'd install that somewhere easy to access, such as near the attic entrance inside your home or close to your thermostat. This way you can turn it off completely when the forecast calls for cooler weather.

Here's my ascii art of the layout:

power  --- W ---  GFCI  --W---+------ Thermal --W-- Attic
source --- B --- Outlet --B-+ | +-R-- Switch  --B--  Fan
                            | | |
W = white (neutral)         Timer      R = red (switched)
B = black (hot)             Switch 

For the GFCI/thermal switch, you can probably get both of those in a double J-box next to the fan for a clean looking install.

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Running the fan in the heat of the day may also be beneficial. While it may be pulling in hot outside air, it's likely the outside air is still much cooler than the air in the attic. –  Tester101 Jun 21 '12 at 17:09
    
@Tester101 very true, I just don't know how efficient the fan is over natural circulation when you factor in the electricity and wear and tear on the fan. The reason for the timer is that thermal switches don't work on a temperature difference between inside and out, only an absolute temperature of the inside. So if it's 100°F outside and in, the fan would run needlessly. –  BMitch Jun 21 '12 at 17:56

Do you have continuous eave vents and continuous ridge vents? It is recommended to actually close off gable vents if you have continuous vents because it messes with the convection flow along the underside of the sheathing.

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Continuous eave vents would be soffit vents, which Aaron doesn't have. And since he doesn't want to disturb the slate roof, that would imply no ridge vent. –  BMitch Jun 19 '12 at 1:30
    
Not disturbing the slate doesn't mean there isn't a ridge vent there already. –  DaveNay Jun 19 '12 at 1:36
    
Fair enough, I may have been basing my assumptions on external knowledge: chat.stackexchange.com/transcript/message/4975658#4975658 –  BMitch Jun 19 '12 at 2:03

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