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?

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...????

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

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.

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.

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

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?

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.

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. :-)

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

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. :-)

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.

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."

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!

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.