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Extra venting, house has gable vents

BlueSkyGuy's Avatar

Join Date: Jun 2005
Posts: 27

02-12-07, 10:44 AM   #1  
Extra venting, house has gable vents

I'm pondering the wisdom of using solar, electrical, or whirly vents on my home. The house already has a few gable vents. If I were to add vents at the top of the roof line would they be drawing in air from the gables instead of the eve vents thus defeating the purpose of drawing cooler air and expelling hot.

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Concretemasonry's Avatar

Join Date: Mar 2005
Posts: 6,125

02-12-07, 01:34 PM   #2  
Extra venting, house has gable vents

Do you have soffit vents to supply air into the attic?


resercon's Avatar

Join Date: Nov 2001
Posts: 1,873

02-12-07, 07:48 PM   #3  
The natural buoyancy of warm air will always cause cooler air to be drawn into the attic (chimney effect). Regardless if you have induced ventilation. Furthermore, equilibrium applies not only to the difference in humidity level between the outside air coming into the attic but also to the temperature differences between the incoming air and the air inside the attic. In other words ventilation reduces attic temperature in two ways during the summer. The first is obvious that is by bringing in lower temperature air into the attic. The second that is not so obvious that is by lowering the humidity level inside the attic.

Induced ventilation (solar, electrical or whirly) accelerates this process and in no way interferes with adequate free venting.

nap's Avatar
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02-12-07, 07:53 PM   #4  
If you have soffit and gable venting now, the air enters the soffits and exits the gable.

If you add other roof vents, the air will enter both the soffit and the gable and exit the new powered vent. The air volume will be divided as fluid dynamics dictate. Any air entering the gable will actually be undesirable.

If you use ridge or powered roof vents, lose the gable vents and be sure you have adequate soffit venting.

XSleeper's Avatar
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02-12-07, 08:29 PM   #5  
My opinion is based on the articles that I have read on the subject, written by experts in their field of study.

Unless there is some special need based on local codes, your attic temperature or humidity, and/or local climate, it is rarely wise to have additional venting in conjunction with soffit/ridge ventilation.

One article I could find offhand can be found at: http://www.jlconline.com/cgi-bin/jlconline.storefront/45d12cf4001572bc27177f00000105d2/UserTemplate/82?c=96a18b307ffe22b519699ab97907b48b&p=1&q=1

One must take into consideration that many types of increased ventilation would have negative effects in cold weather, so the question is whether it makes sense to add more vents and ventilation, or whether it would be a wash... saving a little A/C in the summer while possibly costing more to heat in the winter.

Additionally, many researchers have found that power vents can sometimes pull makeup air from within the conditioned space- again, something that is counterproductive, energy-wise.

If you live in a warm desert climate, then perhaps more ventilation would be desirable. Rather than giving a blanket answer, ventilation questions ought to be answered on a case-by-case basis, based on the climate, needs and design of the home. As such, the advice you get here on the topic is certainly not written in stone.

resercon's Avatar

Join Date: Nov 2001
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02-14-07, 10:45 AM   #6  
First and foremost every standard for attic ventilation ONLY mandates minimums.

Second, the SOLE purpose of attic ventilation is to bypass the low vapor permeability of roofing materials.

Third, attic ventilation during the winter in colder climates is more important than in areas with milder climates due to structural implications.

Fourth, adequate free attic ventilation is based on a natural process known as "Convection". Its SOLE intent is to achieve "Equilibrium". This explicitly implies that the purpose of attic ventilation is to have the conditions inside the attic space to be the same as outside.


The above site is from the US Dept. of Energy; Energy Information Administration. It shows a list of Energy Conservation sites inside and outside the US. My website is on that list and it can be easily noted because it is the only independent listed.

There are two schools of thought, New School and Old School. I will publicly admit, I am Old School. Not because I am older than most of the men and women who belong to that school, it is because they have fail to defend their positions in front of their peers. Furthermore, their papers clearly include facts that are NOT pertinent and OMIT facts that are pertinent.

There are a lot of things that the New School disputes, the Fourth, that is mentioned above is an example. The former three above are either ignored, overlooked and/or misinterpreted in their papers. New School refers to attic, basement and crawl spaces as "Partially Conditioned Spaces". Old School states "You either want to heat or cool a space or you DON'T! No such thing as a partially, unintentional or buffer space."

Old School leans towards processes that mimic nature and things that last. Adequate free ventilation and gable vents are examples. New school is very fond "Ridge Vents". Besides being an energy conservationist, I am a licensed Home Inspector in my State. Ridge vents with mesh inserts clog up so easily it is pathetic. Damage by installers with nail guns is rampant. Those without the mesh inserts are worse. Forget about insect infestation. Wind driven rain forces water through the vent saturating structural members such as trusses, substrates, rafters and ridge boards. In cold climates when water freezes it expands about 12% in volume resulting in the cracking of these structural members. And regardless of the climate if the substrate is plywood, when it gets wet, the laminate fails (structural implication - detriment to load distribution). Clogged mesh vents results in inadequate ventilation which also has a structural implication regarding laminates (black stains around roofing nails). And last but not least, Has anyone ever seen a gable vent covered with snow? Which will impede attic ventilation.

Induced attic ventilation means the acceleration of "Equilibrium". It in no way interferes with adequate free ventilation. It is assumed, especially by experts in this field, that depressurization be avoided by properly sizing of the fans. In other words, if you have an attic fan that moves 2,000 cfm, then you must have vents capable of allowing air into the attic as much or more than 2,000 cfm.

If air leakage is present in the ceiling below the attic floor, it will be present regardless if you have induced ventilation or not. While it may be true that the induced ventilation will increase the heat loss/gain, it is certainly not the source of it. In other words, disregarding the use of the induce ventilation does not resolve the heat loss/gain from air leakage, air sealing does.

If induced ventilation accelerates Equilibrium, then how can the attic temperature be lower than the outside during the winter? It should be the same temperature, but if you have adequate free ventilation, the temperature will probably be lower than outside. The GAS LAW states that for a liquid to change states into a vapor, that liquid must be absorbing heat. For a vapor to condense into a liquid, the vapor must be giving off heat. The amount of heat energy needed to change a liquid into vapor is equal to the amount of heat energy given off when the vapor condenses into a liquid. For those that fully understand this Law of Physics, you already know why the temperature in the attic is lower than the outside during the winter.

The moisture that gets absorbed in the attic from inside the home evaporates in the attic. Which requires heat to accomplish and this in turn lowers the temperature inside the attic. The air inside the attic absorbs this moisture vapor and attic ventilation expels this moisture to the outside. Let's say you don't want the attic temperature to be lower than the outside and want that heat energy released inside the attic. All you have to do is stop the ventilation. You also get an added feature, icicles hanging from your rafters.

I would like to revise my position to, in order to avoid any misinterpretation, "There is no such thing as having too much attic ventilation."

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