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#### Bud9051

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On a recent thread, http://www.diychatroom.com/f9/conde...s-damp-roof-sheathing-next-gable-vent-353857/ ,the topic of keeping or closing the existing gable vents came up. Since I didn't want to drift on the OP's thread I've started a new one.

If you have heard the reasoning for removing those gable vents when installing a ridge vent, affectionately referred to as the short circuit theory, I'm here to explain why it is entirely wrong. Now, before you shoot the messenger, I do understand that 99% of the construction population that has read that reasoning, agrees with it. I did as well until I went looking for the actual pressures that move air through our attics and once you determine the pressures, you see why and where the air will flow.

Here's a brief description of the short circuit theory.
"When the hot air rises and exits the ridge vent it will pull in its replacement air. If the gable vents are left open they will provide a more direct path for that air flow, (the path of least resistance) thus short circuiting the desired path from the soffits."

My wording may be a bit different, but essentially this has been the explanation for decades and it has been accepted as gospel by some of the best in the industry so correcting it, which I started over 2 years ago, has been a challenge. By necessity I will try to be brief and add more information as I respond to questions.

There are three principles in their statement that we need to consider.
1. The hot air rises.
2. It pulls in its replacement air.
3. And the gable vents will provide a short circuit path because the air will follow the path of least resistance.

All three of those statements in are "wrong".
1. We often see hot air moving up, but it does not do so by itself. In our attics it is being pushed up by the cold air flowing in through those soffits.
2. The replacement air that is entering as the hot air is exiting is not being pulled in, but rather pushed in by a greater atmospheric pressure below the soffits.
3. For air to flow you need a pressure difference and a path. Despite the gable vents being closer to the ridge vent, the pressures across the gable opening and the soffit opening are completely different. The gable vent may be an intake or exhaust or a little of both, but leaving it open or closing it does not affect the pressure across the soffits, thus the air flowing in the soffits will continue regardless of where it exits, ridge or gable.

There are reasons why someone might want to close off the old gable vents, but the short circuit theory is not one of them.

More as needed,
Bud

Windows on Wash

#### steppinthrax

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Wow,

First time a thread I started was used to create another thread LOL.

I guess one way to put the theory to the test is to blow colored smoke into the soffits and see where it goes?

With gable vents open and gable vents closed.

#### Bud9051

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Hi steppin,
I didn't want to drift on your other thread, but looks like it would not have been a problem, zero response from the community. It is a difficult topic to discuss as the construction industry has been using the old school thinking for so long they can't imagine it was wrong.

From my research I have determined how the pressures are created due to the difference between inside and outside temperature. Once you know what the pressure differences are, you can easily see which direction the air will flow. Leaving the gable vents in place actually increases the pressure across the soffits and thus increases the ventilation.

Eventually I will return here to discuss the real reasons one should or should not keep those gable vents.

Bud

#### craig11152

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From my research I have determined how the pressures are created due to the difference between inside and outside temperature. Once you know what the pressure differences are, you can easily see which direction the air will flow.

PatChap

#### steppinthrax

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I imagine the DOE, EPA or ICC (International Code Council)

Probably has some incite one what is supposed to be done.

#### Bud9051

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Craig, limiting this discussion to keeping or covering the gable vents, there are many sites that still advise removing the gable vents when installing a ridge vent, I'm sure you have seen them. Over the last 2 to 3 years there have been some sites that have reworded their advice, some very good, others kind of half way. I hesitate to provide links and point a finger as those sites can change in a heartbeat, thus I have focused on how natural attic ventilation works.

Our attics develop a high and low pressure in the same way a house develops a stack effect, the attic version being much simpler. If our high/low vent areas are equal and we have a 35° delta T (as an example) with a 10' high attic we will have a stack effect pressure of 0.0067 (H) (T) = 2.4 pascals. You can find that equation on page 4 of the SeeStack manual at this link: http://energyconservatory.com/wp-content/uploads/2014/07/See_Stack_Trainers_Manual.pdf

The 2.4 pascals is the total pressure available and it is divided half high and half low. That 1.2 pascals across the soffits does not care where the venting originates, ridge or gable (other than height and area). As part of our decision to keep or close the gable vents we should consider their location and the resulting total vent area, but there is no short circuit effect.

Steppin, I'm an avid reader of building science corp and gave those links a quick scan, but did not see anything related to this topic, but thanks.

Bud

#### steppinthrax

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Craig, limiting this discussion to keeping or covering the gable vents, there are many sites that still advise removing the gable vents when installing a ridge vent, I'm sure you have seen them. Over the last 2 to 3 years there have been some sites that have reworded their advice, some very good, others kind of half way. I hesitate to provide links and point a finger as those sites can change in a heartbeat, thus I have focused on how natural attic ventilation works.

Our attics develop a high and low pressure in the same way a house develops a stack effect, the attic version being much simpler. If our high/low vent areas are equal and we have a 35° delta T (as an example) with a 10' high attic we will have a stack effect pressure of 0.0067 (H) (T) = 2.4 pascals. You can find that equation on page 4 of the SeeStack manual at this link: http://energyconservatory.com/wp-content/uploads/2014/07/See_Stack_Trainers_Manual.pdf

The 2.4 pascals is the total pressure available and it is divided half high and half low. That 1.2 pascals across the soffits does not care where the venting originates, ridge or gable (other than height and area). As part of our decision to keep or close the gable vents we should consider their location and the resulting total vent area, but there is no short circuit effect.

Steppin, I'm an avid reader of building science corp and gave those links a quick scan, but did not see anything related to this topic, but thanks.

Bud
I think it might be more having to do with bringing roofs up to "standard". In new construction I almost never see gable vents. Usually they put in a ridge vent and continuous soffit vent (if you're lucky a attic fan). But if you "convert" a old attic by putting in soffit vents and ridge vent. They feel they need to "make it match" with what's being done with current roofing.

Not sure if this makes sense.

#### Bud9051

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New construction will avoid any extra cost that isn't mandatory and if they can install a ridge and soffit combination, there are only a few reasons why they might also add a gable, like snow covering a ridge vent. I have no problem with the ridge/soffit being at the top of the decision list for new construction.

But, the roofing industry (a few exceptions have evolved) has long used the "short circuit theory" (sct) as an argument to tell contractors and home owners that the gable vents must be covered. For a home owner adding a ridge vent, they should have the correct reasons for making that decision and the sct is not one of them. In some cases, that added vent area is necessary and beneficial.

We should have a good discussion on why we keep or remove those gable vents, but after we have erased the sct from the list.

Bud

#### Colbyt

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Considering the fact that the heat pouring out of the gable vents almost choked me in late summer I don't need any convincing. I can't imagine what was coming out of the ridge vents.

Bud9051

#### craig11152

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Craig, limiting this discussion to keeping or covering the gable vents, there are many sites that still advise removing the gable vents when installing a ridge vent, I'm sure you have seen them. Over the last 2 to 3 years there have been some sites that have reworded their advice, some very good, others kind of half way. I hesitate to provide links and point a finger as those sites can change in a heartbeat, thus I have focused on how natural attic ventilation works.

Our attics develop a high and low pressure in the same way a house develops a stack effect, the attic version being much simpler. If our high/low vent areas are equal and we have a 35° delta T (as an example) with a 10' high attic we will have a stack effect pressure of 0.0067 (H) (T) = 2.4 pascals. You can find that equation on page 4 of the SeeStack manual at this link: http://energyconservatory.com/wp-content/uploads/2014/07/See_Stack_Trainers_Manual.pdf

The 2.4 pascals is the total pressure available and it is divided half high and half low. That 1.2 pascals across the soffits does not care where the venting originates, ridge or gable (other than height and area). As part of our decision to keep or close the gable vents we should consider their location and the resulting total vent area, but there is no short circuit effect.

Steppin, I'm an avid reader of building science corp and gave those links a quick scan, but did not see anything related to this topic, but thanks.

Bud
Bud, it doesn't seem to me your link has anything to do with the topic either. Your link is about air exchange in a interior conditioned space. The difference between conditioned interior space and an attic is huge. The goal in an attic is to try to come as close to outside temperature as reasonably possible. Short of a power fan that is done with passive venting to exchange warmer air for cooler air. An attic for all practical purposes is almost always warmer than outside the attic. The 2nd law of thermodynamics suggests heat moves to cold. So the warmer attic air wants to move outside to the extent outside is cooler.
Your link, which talks about interior space, is a whole different animal. The goal inside is to maintain a more or less constant temperature regardless of outdoor temperatures. In order to do that when the difference between indoor and outdoor is significant, a heating or cooling system is going to create inside pressure unlike anything an attic remotely reaches. And the goal is to maintain that temperature difference even while exchanging some indoor air with outdoor air.
Your suggestion that an unconditioned, vented attic, is any sort of a microcosm of an interior conditioned space with the pressure that comes from mechanical heating and cooling seems far fetched. I didn't see any suggestion in your link otherwise. It seems to me you are just claiming that the same principles apply despite the significant differences I pointed out.

#### Bud9051

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We need to distinguish between heat and warm air. The physics says "heat moves to cold", but hot air is controlled by gravity. Drawing the conclusion "So the warmer attic air wants to move outside to the extent outside is cooler." Is not correct. That warm air is only applying a downward pressure as its weight relates to the atmospheric pressure.

The link I provided was to reference the equation for the stack effect pressure in the attic that is responsible for the air flow and that equation only looks at the height and the temperature difference.

As for the heating or cooling system creating an inside pressure, all they do is change the weight of the air. Air outside a house weighs approximately 3.5 Pascals per foot of elevation (I'm suggesting 35° air). Inside, the 70° air weighs 3.25 Pascals per foot. It is the difference between those weights (which vary with temperature) that create what we call stack effect in our homes, and those same temperature differences (different numbers) produce the pressure differences in our attics in exactly the same way.

Heating and cooling do not otherwise pressurize a home, it's full of holes.

Bud

#### Mingledtrash

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So this house is built in the desert it gets up to 120F in the summer no soffit venting, as soffit is stucco solid, no gable end vent as there is no gables to vent. What do you think about this.

#### Bud9051

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Well, it doesn't look like they have to worry about shingles . I'm not sure what you are asking, but IMO they should find some way to incorporate natural ventilation.

Bud

#### Windows on Wash

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+1

Are there soffits? There are integral vents that will work with tile roofs.

As far as the main subject matter, I am less concerned about the short-circuit theory as I am with the fact that most homes are under-vented at the soffits to begin with. Leaving gables open in conjunction with a ridge just exacerbates this condition and drives the further loss of conditioned air via negative pressures.

#### Bud9051

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WoW, the negative pressure you mention is one of the details I wanted to answer when I jumped into this searching. A typical attic with balanced high and low vent area will experience one or twp pascals of pressure at the ridge and at the soffits. If we use the one Pascal number for high and low (2 Pascals total attic stack effect) and adjust the the ventilation to the 60% low and 40% high, the pressures change to -0.8 Pascals low and +1.2 Pascals for high, with respect to outside. But, with respect to the ceiling level inside, which might be at +2 Pascals, our delta pressure across that ceiling went from 3 Pascals to 2.8 Pascals, a 7% reduction under winter conditions. Whether that is a meaningful change depends upon the ceiling plane leakage as always, but unfortunately, the same adjustment goes the other direction during the ac season. In a southern climate, one might install 40% low and 60% high. In any case, to put that 0.2 Pascals into perspective, the pressure required to lift water in a straw 1/4" is 60 Pascals. So 0.2 Pascals would raise the water far less than the thickness of a sheet of copy paper. Bottom line, wild guess, 60/40 vs 50/50 would amount to a couple dollars per year at best. Better math is welcome.

Bud

#### Windows on Wash

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I just like the 60/40 split on intake to exhaust because the intake side is always overestimated as compared to how it is done.

Just built in fudge factor.

#### Bud9051

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Not sure if you've seen the paper by Bill Rose (below) looking back to find the origin of our current ventilation guidelines, but I think it opens the door for a lot of informed fudge factors. Some obvious ones are: a 4' high attic will have far less natural ventilation than a 10' attic; Windy areas may get by with less; and climate conditions, rain, snow, and humidity. The simple 1/300 or 1/150 guideline isn't doing the industry justice.

Bud

#### Gary in WA

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Well, I doubt you'll get much "discussion" around here, lol. Saw the delivery and the swing, and expected the answer given... I remember you were here back in '09 or so... and your own; http://homeenergypros.lbl.gov/forum...ntId=6069565:Comment:84076&xg_source=activity

I've used this a few times here; http://www.finehomebuilding.com/design/qa/close-off-gable-vents.aspx --------------------along with; http://en.allexperts.com/q/Roofing-1598/Ridge-Gable-Vents.htm

And this for the basic- heat "sucking" the cold air in; 2 good ones; http://learn-science.20m.com/tmyths.htm#myth2

and the follow-up one, that anyone can understand; http://www.usatoday.com/tech/columnist/aprilholladay/2005-02-18-wonderquest_x.htm

Attic ventilation/ceiling heat flux w. larger soffits, more/less insulation, etc. simulation;http://digitalcommons.unl.edu/cgi/v...y rising` attic air from passive ventilation"

I think this has a lot of merit; http://www.philwrites.com/AW_Manual_12_1_all_LR.pdf

You will like this, my "convective loop/60/40 cooling loss" results; oops... PM me.

Gary

#### Windows on Wash

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Bud,

Where we are, we are better served by erring on the side of additional intake ventilation.

Again, my point is that the intake side of the equation is often underdone. Installers will quote the NFA total and not note that the wood soffit is not completely open or otherwise obstructed.

Shooting for 60% on the intake as a default probably gets us closer to 50/50 more of the time.

I previously quoted the short-circuit theory but thanks to bright guys like you and Gary, have changed my position via the fact you have supplied.

Even if the venting is as stated and perfectly in line with the numbers, I would prefer to err on more intake cold air in the winter to minimize what we know to be common failures in the attic (poor insulation levels at the outer top plate and air leakage).

The most common issue we see is what we know to be most important....a tight and sealed ceiling plane. Without that, the venting issue will always under perform.

Thanks for the good data and same to Gary.

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