Quick Attic Ventilation Question

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While calculating the number of soffit vents I am going to install in my house, I am operating on the priciple that I should have equal intake square footage (from the soffit vents to be installed) as exhaust. Now my question is do my existing gable vents count as intake or exhaust? And just to be sure I am using the right term, by gable vent I mean the two triangular vents at each end of my house that are right below the peak of the roof. It is my guess that after the installation of soffit vents that these gable vents would function as exhaust, but I want to verify that.
TIA for any help.
James
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Yes.

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James Bond wrote:

my
Now my

And
two
peak of

that
that.
You might consider them as half and half for this exercise. As an exercise in logic: The gale end vents will probably act in various ways depending on wind pressure. Soffit and ridge vents would usually act consistently based on the chimney principle. The gable end vents, because they are large and in a vertical surface, would act differently with different wind direction.
In any case, look at the Building Science Corporation web site for discussion of venting. This is researched material. TB
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Complicated fluid dynamics. At a layman's level, yes, they behave more like exhaust than intake in greater than 50% of the cases. But, the way gable vents work is that air blowing towards one vent creates positive air pressure and the air moving away from the other vent creates negative pressure so one vent is intake and the other is exhaust. Maybe on a perfectly calm day they would both exhaust. Now a ridge vent work similarly. Air blowing over the top of the vent creates a negative pressure area on one side of the vent that sucks the air out of the attic. The replacement air is supposed to come from the soffit vents, thus creating the cold air flow along the rafters that is the design effect. But with the addition of gable vents this make up air will more likely come from the gable vents as intake, not from the more distant soffit vents. That is why it is recommended to do one or the other but not both when it comes to attic venting. And none of this is true all of the time. There are many more variables like orientation of the house, pitch of the roof, design characteristics of each of the vents- alone and in combination, and more. Confused? Join the club.

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Thanks to everyone for their input. Of course, I am now more confused than ever. :)
So here is my exact situation. I have an older home in a warm, humid climate (Charleston, SC). Currently, I have gable vents and 2 14" wind turbines for ventilation. It is not enough as evidenced by extreme attic temps in the summer and some curling shingles (especially on the western end of the house, which is oriented such that its long axis is roughly NE to SW).
I understand that ridge vents are a great option, but adding them to my roof is cost prohibitive for me at the moment. My thought process was that there is probably little air flow in the lower attic due to all the current venting being up high. Thus, I thought soffit vents would be the answer. It appears, however, that there is a possibility that I could make the situation worse by doing so? Is that right? Should I install soffit vents and block off one or both gable vents? Any further advice would be greatly appreciated.
TIA,
James

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scribbled this interesting note:

Composition asphalt shingles on the South and West sides of a house (in the lower latitudes of the northern hemisphere) will always wear faster than those on the other sides of the house because of the exposure to the sun. This is not due to heat in the attic, rather the solar radiation breaking down the organic compounds in the asphalt.

I'd just install the additional soffit vents and be done with it. If, on the other hand, you were to install ridge vents and soffit vents, then you'd want to close off the gable vents because the ridge vents draw from the source closest to their level. Or you could install a thermostatically controlled power vent in one of the gable vents, on the inside, that would force air through the attic and out the other end of the house. There are many, many options.

-- John Willis (Remove the Primes before e-mailing me)
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WRONG. You can't have "too much" venting. Leave the gable vents open-- they will help.
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scribbled this interesting note:

Do a little more reading on the subject. While it is true that you can't have too much properly designed ventilation, the key is "properly designed."
For example, if you have ridge vents, it is best to close off all other vents except the soffit vents, if you want air to be drawn in from the soffits. Why? Has to do with fluid dynamics. There are plenty of sites where you can read up on the issues involved.
Here's one that does a good job if discussing many of the factors involved: http://www.factsfacts.com/MyHomeRepair/ventilation.htm
Have a good read...
-- John Willis (Remove the Primes before e-mailing me)
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WRONG AGAIN. Use your head: The best vented attic would be a suspended roof open on all sides.
You cannot have too many vents-- period. This "bypassing" that is talked about (turbines with gables with power fans, etc.) is not harmful to proper venting.
The key is to have AT LEAST THE MINIMUM REQUIRED VENTING at the soffits and at the top. However, you cannot have "too much venting", and "bypassing" is not a bad thing.
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proper
and
is
You must be one of the types that listen but don't hear. No one said anything that advocates minimizing the amount of ventilation. What is being tossed around is the best "design solution" to maximizing the amount of ventilation one can expect to get in an attic. In your comment "The best vented attic would be a suspended roof open on all sides"---So what, too simplistic and not an option in a house so forget it. You apparently don't know anything about fluid dynamics--air flow, pressure drop, resistance, path of least resistance etc.. You have no idea what the air flow path will be, without an analysis or experimentation, if you have more than one flow path. Think of an electrical circuit with two or more parallel paths. The current (air flow) will not be the same for each path unless they are all of equal resistance (or do you want to dispute that too). If you take the time to do a bit of reading (just goggle roof vents, or the like) you'll be lead to a number of sources that cover the topic. What you'll learn is that you cannot arbitrarily open up a bunch of holes and expect to get the best ventilation setup. This has been well thought out and one conclusion is that gable vents in parallel with soffit vents are not the optimum solution for a well ventilated attic. Of course if you disagree, that's not a problem but do more than blow your horn---provide some analysis, reference document or any source of information that we can look at----gut feelings don't work when you're trying to solve an engineering problem--and that's what this is whether you recognize it or not. MLD
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You can't handle the truth.
Even when the dreaded "bypassing" is occurring between two vents close to each other, the attic is still being vented.
I stand by my statement that it is impossible to have "too many" vents.
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If,
vents,
vents
plenty
suspended
talked
"bypassing"
resistance
of
do
when
I've worked with people just like you---Guess what, they were always the first ones laid off!!
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I've worked with people like you. They were the ones who read their horoscopes and asked for validation of their narrow-minded opinions from strangers on newsgroups.
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it.
in
is
of
in
resistance
bit
gable
a
but
or
is
to
Ooops, looks like I stuck a nerve. The truth shouldn't hurt unless it ought to----BTW, where did you meet these people, in the unemployment line??
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"Arguing with someone on the internet is like winning in the Special Olympics.....even if you win you are still retarded"
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Oscar_Lives wrote:

Autoflame A+. Props to you Oscar.
--
t 'I'm a winna'!' m

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ridge
vents
all
drawn
factors
is
What
amount
"The
option
fluid
a
for
document
work
this
close
from
Ambiguous comment--are you referring yourself? BTW, kind of curious at this point--which number is larger--Your hat size or your IQ?
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scribbled this interesting note:

While true, this still begs the question: Is this the best venting that can be obtained or is there a better design? Isn't it possible that a better designed system may, just may have the same or fewer vent openings, the same net free area, and do the job better?

Thank you for your personal opinion.
-- John Willis (Remove the Primes before e-mailing me)
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James Bond wrote:

That is right, but it is not common, in most case you will improve the results. A good roofer (not all are good) can tell you quickly with a high degree of accuracy.

--
Joseph Meehan

Dia duit
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James Bond wrote:

It is really a complex issue. Most of the time (assuming they are located high at the peak) they will act as exhaust. Depending on the wind, temperature of air and deck temperature who knows at any given moment. On occasion they can act as a short cut reducing ventilation.
--
Joseph Meehan

Dia duit
  Click to see the full signature.
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