passive vs active attic ventilation

Was just talking to a HVAC guy and he referred to some study that said the cost of running power ventilation exceeded the cost of just turning down the AC. Someone shared the idea of solar powered vents which I've looked in to but I think my first best move is to improve the passive ventilation. I have only vents at the eaves currently so will add soffit and maybe, in time, ridge vents.
What do you think?
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wrote:

The best passive ventilation is a functional cupola
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On Jun 25, 12:24 pm, snipped-for-privacy@aol.com wrote:

So, add a dome to my roof...????
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my ridge vent helped a lot
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my ridge vent helped a lot
I think ridge vents depend upon the part of the world in which you live. In hurricane country I'd avoid ridge vents, which have proven to be a weak spot in hurricanes. My contractor referred to them as "roof zippers."
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wrote:

It is more like a tall dog house with vented sides. This draws the hot air out by convection.
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wrote:

Avoid powered fans, if at all possible. I'd put in more insulation before considering a fan.
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If you want ventilation get a fan. More insulation is always good

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Big Jim wrote:

The problem with a fan is that it must exhaust massive amounts of air to be effective. Assuming an 1800 sq ft house with a 6' attic at the peak, we're talking (roughly) 6 x 1800 x 0.5 = 5,400 cu ft. An attic exhaust fan should change the air every 2 minutes, so you'll need a fan that moves 2,700 cu ft of air per minute. That's a good sized fan.
"For well-insulated ceilings (i.e. insulation levels of R-19 or above) it is doubtful that a power ventilation can be justified economically. "
http://factsfacts.com/MyHomeRepair/ventilation.htm
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HeyBub wrote:

Why on earth does a fan have to change the air every 2 minutes to be helpful?
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Bob F wrote:

Because if the area is being heated faster than you can remove the heat, you don't accomplish anything.
I don't know that 2 minutes is the optimum value - it may be one air exchange every half-hour or it may be twice per minute. I'm sure somebody has figured it out. Research is called for.
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Based on the comment about the power required by the power ventilation, I'm abandoning all thoughts of that. However, the HVAC guy also said that the standard for insulation was R19 but not it's R38. I need more insulation!
While I was installing the soffit vents, my thermometer next to me was reading 108! I looked online and the heat index was 119. Guess that's why the AC is struggling. :-)
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BradMM wrote:

The goal of attic ventilation is to get the attic down near ambient; you can't expect to do better than that -- insulation and A/C have to do the rest.
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BradMM wrote:

Always a good idea, BUT there's a point of diminishing returns. The R19 advocate says effectiveness drops off at about the R19 point. That is, you get the biggest bang for your buck at the (up to) R19 level.
I wonder if this idea might provide some insight: Install a remote-sensing thermometer beneath all the attic insulation, next to the ceiling drywall. This might provide an indication of exactly how effective the insulation really is.

Keep going! You can't have too much soffit venting!
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HeyBub wrote:

Wish I'd read that before I responded to the original. :-)
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HeyBub wrote:

Where does that come from? It sounds pretty aggressive to me. I think an attic fan can make a big difference even if it's only capable of changing the air a couple of times an hour.
IMHO, an attic fan isn't a panacea, but it can be useful in the mix. I know mine has relieved some of the stress on my (much more expensive to replace) A/C unit.
I'm also a big fan (no pun intended) of radiant barriers.
so you'll need a fan that moves 2,700 cu ft

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