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#1
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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? |
#2
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passive vs active attic ventilation
On Jun 25, 12:24*pm, wrote:
On Thu, 25 Jun 2009 09:32:02 -0700 (PDT), BradMM wrote: 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? The best passive ventilation is a functional cupola So, add a dome to my roof...???? |
#3
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passive vs active attic ventilation
On Jun 25, 2:04�pm, BradMM wrote:
On Jun 25, 12:24�pm, wrote: On Thu, 25 Jun 2009 09:32:02 -0700 (PDT), BradMM wrote: 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? The best passive ventilation is a functional cupola So, add a dome to my roof...???? my ridge vent helped a lot |
#4
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passive vs active attic ventilation
On Thu, 25 Jun 2009 09:32:02 -0700 (PDT), BradMM
wrote: 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? Avoid powered fans, if at all possible. I'd put in more insulation before considering a fan. |
#5
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passive vs active attic ventilation
If you want ventilation get a fan. More insulation is always good
On Jun 25, 4:18*pm, Phisherman wrote: Avoid powered fans, if at all possible. *I'd put in more insulation before considering a fan. |
#6
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passive vs active attic ventilation
Big Jim wrote:
If you want ventilation get a fan. More insulation is always good 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 |
#7
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passive vs active attic ventilation
HeyBub wrote:
Big Jim wrote: If you want ventilation get a fan. More insulation is always good 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. Why on earth does a fan have to change the air every 2 minutes to be helpful? |
#8
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passive vs active attic ventilation
Bob F wrote:
HeyBub wrote: Big Jim wrote: If you want ventilation get a fan. More insulation is always good 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. Why on earth does a fan have to change the air every 2 minutes to be helpful? 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. |
#9
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passive vs active attic ventilation
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. :-) |
#10
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passive vs active attic ventilation
HeyBub wrote:
Big Jim wrote: If you want ventilation get a fan. More insulation is always good 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, 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 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 |
#11
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passive vs active attic ventilation
HeyBub wrote:
Bob F wrote: HeyBub wrote: Big Jim wrote: If you want ventilation get a fan. More insulation is always good 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. Why on earth does a fan have to change the air every 2 minutes to be helpful? 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. Wish I'd read that before I responded to the original. :-) |
#12
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passive vs active attic ventilation
BradMM wrote:
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. :-) 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. |
#13
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passive vs active attic ventilation
"bob haller" wrote in message ... On Jun 25, 2:04?pm, BradMM wrote: On Jun 25, 12:24?pm, wrote: On Thu, 25 Jun 2009 09:32:02 -0700 (PDT), BradMM wrote: 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? The best passive ventilation is a functional cupola So, add a dome to my roof...???? 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." |
#14
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passive vs active attic ventilation
BradMM wrote:
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! 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. 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. :-) Keep going! You can't have too much soffit venting! |
#15
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passive vs active attic ventilation
I live near NYC and the attic fan (a small one, as the shape of the
side vent made a bigger fan of no greater benefit) made a huge difference in attic temperature and how hot it was in the upstairs of the house. Without it the window units I had at the time wouldn't have been able to keep the house at a tolerable temperature. My attic used to be probably 110 or 115 degrees, now it's roughly the same temp as outside. It's on a thermostat and I can hear it go on and off so I know it's working and I'm getting enough air exchange. |
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