Attic vents

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I read with interest this exchange from an earlier post....
"Ridge vents work with soffit vents, and suck air in the bottom and out the top. Gable vents work with other gable vents, and air blows in one, and out the other. Mixing the two makes the situation complicated. A single gable vent and a ridge vent is certainly better than just the ridge vent, but probably not as good as either a pair of gables at opposite ends of the house, or a matched set of ridge and soffit vents. Do you only have one open gable? If you have two, and they're working, why are you installing a ridge vent? "
Our roof was replaced 12 years ago. The original roof had gable vents and soffit vents, no can vents. The outfit that replaced the roof installed can vents. One section of the roof already needs to be replaced. I had a different roofer come out to assess why. He said cause was condensation due to improper venting, and that the cause of the improper venting was that the gable vents were not blocked off when the can vents were installed. According to him, having both gable and can vents made both less effective. Being ignorant about all this, I'm trying to get some confirmation that what he told me is in the realm of possibility. The roof obviously need to be replaced in any case, but I'd like to be able to make sure that we don't just have the same problem. There's also the question of whether or not the people who installed the roof failed to do it properly.
Thanks in advance for your assistance.
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Look at the Building Science Corporation web site for climate specific recommendations based on research.
Your problem may be the result of moisture from plumbing vents ending in the attic or general moisture moving into the attic from the rest of the house.
TB
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That does seem counterintuitive.
I imagine the point was that the momentum of moving air created by convection is more effective than just passive diffusion through a larger ventlating area.
In an actual roof, convection, conduction and diffusion of heat would all play a roll so its hard to say if he was actually right or just mis-using some wisdom incompletely passed on to him.
Depending on where the condensation occured, any number of causes might apply. To guess would require writing a book.
My roof has soffit and gable vents with one having a fan on a thermostat and several can vents. I live in CA so it never freezes and condensation is a lesser concern. Just about anything that keeps the water out works here.
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The gable vents short circuit outside air to the roof vents. Therefore the air flow from the soffits is reduced. Block the gable vents and air flow from the soffits will increase and better displace the total attic air volume.
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Ronald' wrote:

Interesting theory. Has anybody done any scientific research to determine whether it's true?
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The roofer who told me this has been in business for more than 20 years and is a "master installer", whatever that means. That isn't scientific proof but at least lends some credibility to the theory. I'll probably have another roofer look at it and see if he confirms what the first roofer said.
Sid
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I don't know about any scientific research. But practical experience is there. The inlet and outlet air flow areas should be matched. The hot air escaping any vent, caused by outside air flow across the vent, creates a slight negative pressure in the attic. This causes inflow of cooler air through the soffits. If ridge vents are also present, some air will also flow into the attic through them. Thus reducing the inflow through the soffits and reducing the cooling over the entire roof area which results in heat damage to the shingles in the lower roof area. Even with a ridge vent fan only the peak of the roof is cooled by pulling air through the opposite ridge vent and the roof vents along with some reduced amount from the soffits.
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Ronald' wrote:

The devil is in the details, methinks. Air could as easily flow from both the gables and soffits to the ridge and result in a cooler attic temperature.
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I would think that air flowing from the gables to the ridge vent would be almost useless because it is hardly traveling any vertical distance and therefore leaving most of the attic air untouched. The gable vents are supposed to be where the air exits, not where it enters. Everything I've read says that the air should be entering through the soffits, rising along the inside of the roof, and exiting the vents. If the air coming in the gable vents is reducing the airflow from the soffits, I can see how this would cause the overall ventilation to be inadequate.
Sidney
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Sidney Schwartz wrote:

If it's replacing hot air with cool, it'll result in convection. That cool air coming in from the gables will drop as the hot air near the soffits rises. So it's not clear to me that anything has been lost.
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On Thu, 6 Apr 2006 21:02:03 -0700, "Sidney Schwartz"

But what differencd does it make?. The goal is to exchange the hot air in the attic with cooler air outside, the humid air in the attic with less humid air from outside.
For every bit of air that enters, some air has to exit somewhere.

Even if the air near the bottom of the attic wasn't moving, because the gables usurp the air flow to the ridge vent, iiuc, the humidity in the lower air would still disperse into the rest of the attic unrelated to air movement. Separate gasses, partial pressures, physics is a blur to me, but that's what I think.
Like the smell of a dish of gasoline fills a room even when there is no breeze. Does it do that? What about the smell of cooking. Doesn't that fill the adjacent rooms faster than the air as a whole moves to that room?
I realize that theory, even if reasonable sounding, doesn't trump fact.
To get facts, aren't there things that make smoke, like smoke bombs? Are there July 4th smokebombs or are they only found in the military? Can recreational smoke bombs, if any, be broken into smaller pieces?

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I don't know about any scientific research. But practical experience is there. The inlet and outlet air flow areas should be matched. The hot air escaping any vent, caused by outside air flow across the vent, creates a slight negative pressure in the attic. This causes inflow of cooler air through the soffits. If ridge vents are also present, some air will also flow into the attic through them. Thus reducing the inflow through the soffits and reducing the cooling over the entire roof area which results in heat damage to the shingles in the lower roof area. Even with a ridge vent fan only the peak of the roof is cooled by pulling air through the opposite ridge vent and the roof vents along with some reduced amount from the soffits.
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I don't know about any scientific research. But practical experience is there. The inlet and outlet air flow areas should be matched. The hot air escaping any vent, caused by outside air flow across the vent, creates a slight negative pressure in the attic. This causes inflow of cooler air through the soffits. If ridge vents are also present, some air will also flow into the attic through them. Thus reducing the inflow through the soffits and reducing the cooling over the entire roof area which results in heat damage to the shingles in the lower roof area. Even with a ridge vent fan only the peak of the roof is cooled by pulling air through the opposite ridge vent and the roof vents along with some reduced amount from the soffits.
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Makes complete sense to me. Hot air will rise and exit from the highest point (ridge or can vents) and cooler ari will be brought in to replace it. Since air like water and electricity will always flow along the easiest route, it would come from the gable vents as they would be closest.
If you want a scientific experiment, try a few well positioned smoke bombs on a hot but otherwise still day.

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PipeDown wrote:

That presumably relatively cool air entering from the gables will be subject to convection. So the hot air will still be what exits.

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air
Or you could take the approach I saw in a coastal town in Louisiana a few years ago. This fellow added a whole second roof, about five feet above the real roof. Basically, he built a no-wall pole barn over his house. Ugly, but I bet it did keep the roof cool. (No real trees around there, being barely-dry mudflat and all, so shade was a rare thing...) It'd be an interesting concept to build into a house from the get-go- make the upper floor a screened deck, with wide overhangs. Water control would be a problem. Rich people down south used to have huge sleeping porches under deep overhangs, just to be able to sleep at all.
aem sends...
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ameijers wrote:

That's an interesting idea, but I wonder if it develops much lift in a strong wind.
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Sidney Schwartz wrote:

when they did my new roof they closed the gable vents and put in ridge vents along with the soffits already there.
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That's exactly what I was told should have been done with our house but wasn't. :(
Sidney
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Many thanks to everyone who has answered so far. My next problem will be going to the roofer who did the job 12 years ago and trying to get him to make good on the work that was done incorrectly. Oh joy.
Sidney
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