Attic Fan

[Peter Stahl]

Anyone know what size (cfm's) fan I would need for a 1000 sqft attic? It's a hip roof with soffit vents. There is one in there now but it's old and getting noisy. No idea what size it is. Also any Pro or Cons on which brand to get? Thanks for any and all replies!

[Dan Hintz]

I imagine a 12" would be perfectly fine, though you can always get a 14"er if you feel the desire. Once you get some flow up there, you don't exactly need a dust collector's worth of flow to keep it cool.

[C Scott McDonald]

Take a look over here.

http://www.airscapefans.com/

Scott

[Jim Matthews]

I just went through the same process; gable fan/attic fan/whole house fan.

I ended up taking a passive approach instead.
Drip edge intake vents (immense surface area compared to the old soffit vents)
Larger exhaust ridge vent (installed with new roof)

Perforated foil radiant barrier over conditioned spaces and attached to rafter of roof.

I can't speak to the amount of sun exposure you have, but for us the problem
is the thermal gain from the roof itself.

At night, even with the whole house fan running the upper floors stayed hot.
No amount of air exchange could keep pace with the heat released from the roof
as it cooled overnight.

Even if you take the house fan approach, at least consider this passive approach.
It costs little more than the labor to install, and has no moving parts to fail.

http://energy.gov/energysaver/articles/radiant-barriers

[Paul McGaha]

Nutone used to make a very good roof fan. Particularly the model # RF59N.

Not any more though. Nutone was acquired by Broan. Similar to Porter Cable being acquired by Black & Decker. Several Nutone products seemed to have gone away, including all the roof fans.

I do highly recommend installing a roof fan though. Getting all that hot air out of your attic does wonders with keeping the upper floor of your house cool when it's hot outside.

PHM

[Fred Perreault]

If one uses attic fans over the gable vents, would you use one at each end of the attic? I have ridge vent the length of the attic. It's about 1100 sq. ft.

[Jim Matthews]

I had a single gable fan, on the North side of the roof.
It pushed cooler air in, and theoretically across the attic to the opposite gable vent.

In practice the ridge vent has considerably more surface area and should draw more air out.
The important part of a vented system is that it needs to pull air from the lowest point of the envelope.

If you have soffit vents, they must be cleared to allow an air path.
When we did our retrofit, the soffit vents were painted over.

This will not make any difference to the temperature of a conditioned space,
with a layer of snow over the top. Our roof is one of the few in our neighborhood
that did not form ice dams this past Winter, the other bonus of having a
well insulated attic.

[Dan Hintz]

Fred, if you have a ridge vent, why would you want roof-mount fans, too? If you did put fans it, you certainly don't want them right above the vents as that won't create any circulation.

[Fred Perreault]

Dan, I just thought that the thermostatically operating gable fan(s) would be more apt to exhaust more air out of the attic, thereby lowering the temps in the summer. I am not an experienced HVAC person, but I want to lower the attic temp in the summer, as it gets above 130 degrees. Wouldn't fans in front of the gable vents force more air to move than just the current static situation? The 12 pitch roof is facing due south, and gets really warm.

[Mel Fulks]

I have an attic fan like you describe set at 110 located at one gable louver and pulling thru other louver. Don't think it's really needed as it was bought to help cool down low knee wall area. When that did not help I closed off soffit vents an put in a bunch of insulation ,since that area was next to living space something had to work. Ridge vents are good but most people seem to think that if attic is insulated well it really doesn't matter if attic is hot.

[Dan Hintz]

Are you talking about fans situation above the vents, or fans replacing the vents? Replacing the vents with fans would certainly move more air through the ridge vent (higher pressure), but if there are any leaks into the living space (such as around the access panel), you'll push warmer air through those, too.

[Paul McGaha]

An example of a gable fan, they typically mount behind an existing gable vent and move hot air from the attic to the outside of the house:

http://www.rakuten.com/prod/dayton-10w199-gable-attic-ventilator-120v-1650-cfm/265285043.html?listingId=340322088&scid=pla_google_Zoro&adid=18172&gclid=CjkKEQjwwbCcBRCxvJn9-N6dorwBEiQAVriOil1cAWJ8Ml_NrCT4bDRCp-yZso5z7V7xGtgf3o_mF_zw_wcB

The soffit vents need to be in good working order for a gable fan (or a roof fan) to work properly.

PHM

[Jim Matthews]

Is there any persuasive survey of how much air moves passively through a ridge vent
compared to forced ventilation through gables?

+1 on intake vents being 1/2 the battle.

[Chris Padilla]

I installed two solar-powered roof/attic fans when I had the roof redone a few years ago. There used to be one electrical powered one that was noisy. Now I have two silent ones that I really honestly never know if they're running but every time I get paranoid/curious and look, they are both spinning just fine!

Next, I installed a Tamarack whole-house fan in our second story to help cool the second story off. It is a twin fan system, well-insulated, and a bit noisy but not bad, but it works nicely to suck the cool outdoor air in. We use it way way more than the A/C but there are times when it is still 80 degress at 10 pm and the conditioned air becomes a necessity. :)

[Mike Wilkins]

I don't recall the issue number, but Fine Homebuilding magazine had an article about this very same subject-attic ventilation. I will check when I go home for lunch and report the issue number back to you. You may be able to look it up in their web site.

[Andrew Pitonyak]

Fred, if you have a ridge vent, why would you want roof-mount fans, too? If you did put fans it, you certainly don't want them right above the vents as that won't create any circulation.

A contractor installed a ridge vent along the top, then, in case we needed extra flow, he installed a powered fan. Sadly, the truth is that if you turn on that powered fan, it pulls air from the top ridge vent rather than pulling air form the other intake points so it will not cool the attic. In other words, the contractor did not know what he was doing... but the three roofing people that did (who saw it later) all gave me the scoop.