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Venting


Chad Fabry
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Gable vents were once the primary form of attic venting, but have proven to be ineffective or counter-productive in creating the necessary airflow.

As Kurt says, the particulars of a given house would be important, but first I would want to know: "Proven" by who? When? How?

I've done houses just under 200 years old with nothing but gable vents, and found no ventilation-related problems.

Brian G.

I'm Not From Missouri, But Show Me Anyway [:-magnify

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Hi Chad,

If we're talking about ventilation theory in general I'd say that, in theory, gable end vents shouldn't work well when used in combination with upper and lower roof vents and should defeat the system.

I'd say that scientists who deal with this stuff have, in many cases, proven them to be, if not ineffective then marginally effective, and have proven that they can actually be counterproductive, because they'll reduce the amount of convective draw from the eaves and this, in turn, can sometimes cause portions of the underside of a roof plane to remain damp instead of drying out as it should.

I'd say that there's plenty of documentation to back that theory up and it can be found on various websites and in various texts; Dr. Joe's site and the ORNL site are good places to start, if you're looking for something in writing. You could also look at study done by the University of Illinois (or whoever the hell it was) that did the study for Certainteed that backs that position.

I'd say that the effectiveness of gable end vents can hinge on a number of factors, with building orientation, age, and type of construction all playing a role in how well, or how poorly, they work.

I'd say that, for everyone that wants to argue about how many homes they've seen with gable end vents where there aren't any issues, you'll be able to find an equal number of homes with gable end vents where you will find problems.

I'd say that I think it's convenient in this business to argue for gable end vents, given the fact that it's difficult to argue against them when they're still being installed so widely by so many builders, and because so many inspectors will talk about how they've never found them to be problematic. However, I think that building scientists will argue to the contrary and be able to show that, in theory, they do defeat roof ventilation when you are relying on convective airflow through eaves/soffit vents to ridge and/or jack vents to move air through a roof plane and keep the underside of a roof plane dry.

Have I said enough?

ONE TEAM - ONE FIGHT!!!

Mike

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However, I think that building scientists will argue to the contrary and be able to show that, in theory, they do defeat roof ventilation when you are relying on convective airflow through eaves/soffit vents to ridge and/or jack vents to move air through a roof plane and keep the underside of a roof plane dry.

I agree. Don't mix systems. My question wasn't about mixing systems though, it was whether a statement like the one I quoted could be proven true.

However, (we'll talk about a simple structure) in my opinion, a home built with two gable vents will have an attic space that's ventilated. Any air movement outside the house will cause the gable vents to be at different pressures and air will move from one to the other through the attic.

Are there poorly designed gable vent systems? Sure.

Are there buildings that aren't suited to be gable vented? Absolutely.

But for many classic building styles, gable vents have been proven effective in some of the leakiest homes ever built... homes where you're positive conditioned air is making its way into the attic.

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Hi,

Yes, I think your statement can be proven true. However, it's also possible to prove it false. I think you'll find that there are fewer problems related to accumulated moisture in older leaky homes than there are in newer "tight" homes. Have you ever seen a ventilation issue in an old turn-of-the-20th-century balloon-framed vernacular home without any attic vents? I haven't.

I think that a lot has to do with the building materials used. Around here, I've looked at thousands of homes built before the 60's with plastered walls. Most have one-by T & G wall sheathing (some, no sheathing), little to no vapor barriers in the walls, and nothing more than gable end vents. Some don't have any upper vents at all, and have only eave vents instead. Nonetheless, I think that the overwhelming majority of them are doing fine.

Conversely, I've looked at newer homes where the walls are covered with modern gypsum that's primed with a moisture-barrier PVA primer, there's an emulsion coated vapor barrier on the interior side of wall insulation, and they're sheathed with plywood or OSB, where one sees lots of indicators of moisture on the underside of the roof plane. Obviously, some of these modern materials tend to increase stack effect instead of allowing moisture to diffuse naturally outward as with older homes.

Most of these new home configurations look fine; others don't. Last March I looked at a new home delivered in November 2006 where the underside of the roof was covered with what I initially thought was green paint. It turned out to be mold. Eave vents and ridge vents, but also gable end vents. Almost no air convecting through the attic. The house sits in a little hollow surrounded by trees that screen it from sunlight. The gable end vents are oriented east-west but the wind blows primarily south-to-north here. The house's whole-house air change system was not functioning at all. It's hard to say whether it's the whole-house system's failure or a combination of vents and poor orientation that caused the underside of the roof to get like that in such a short time. One might blame it on the failure of the air change system; but, then how do you account for the exact same configuration in a home without an air change system, where the house is oriented to the sun and wind differently, and everything is fine?

There really is no one-size-fits-all solution to any of this stuff. It's one of the reasons that it's so important for folks in this business to actively study as much building science as they can. If I were King, I'd sign a decree forbidding anyone from getting into this business who hadn't attended at least a year of college with some building science woven into the curriculum (That would, by the way, result in my getting myself kicked out of the business as well.).

Without an understanding of the way building science works, I don't really see how anyone can do a competent inspection on anyone's home. Then again, maybe that's just me. After all, I have been called a commie dictator on more than one occasion.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Chad Fabry

Tell me what you'd say if confronted with the following statement:

Gable vents were once the primary form of attic venting, but have proven to be ineffective or counter-productive in creating the necessary airflow

I'd say, "I think a home inspector wrote that. It has all the marks: May or may not be true, no source cited, no plausible explanation, no logical connection between what gable vents 'once were,' and their current 'proven' status of being 'ineffective.' And, there's the usual awkward staccato syntax."

Could it have come from (cue Dana Carvey) home inspector school?

WJ

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It was whether a statement like the one I quoted could be proven true

If there is no significant prevailing wind reaching the house or sun shining on it then no passive system will work. How can it?

If the prevailing wind is perpendicular to the eaves then putting in gable vents will probably work. I know, I've seen it work in that senario.

If the eave vents are in the prevailing winds and there's problematic moisture in the attic then sticking in gable vents is probably going to do nothing since no venting is working to solve the problem anyway.

The way the statement is worded it implies that if you have a working conventional eave and roof vent system that's working and you stick in gable vents then you can foul up a working system.

The logic is messed up. Why would you put gable vents in when the other system is working?

Someones talking theory not real world problems.

Chris, Oregon

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The best method for attic ventilation is a continuous ridge vent in combination with continuous soffit vents.

Gable vents, while they do provide some air flow through the attic (depending on wind direction) do not provide the required turbulance to exchange the air within the rafter cavities. Passive ventilation is based on the effect of thermal siphoning. Anything hot will rise when in it's own element. Hot air in the rafter cavity will rise and be trapped within the rafter cavity. A slight breeze through gable vents may cause the air to flow through the attic, but it will not disturb the air in the rafter cavity.

If a gable vent is used in combination with a ridge vent, the 5-8 feet on the end will be ventilated, but that is about it. The air will enter the end and go right out the ridge.

Roof vents do nothing more than vent that one rafter cavity. We have all seen the pristeen cavity with a roof vent next to darkened cavities on either side.

Continuous soffit and ridge vents will allow the thermosiphon effect to work and carry the hot humid air out of the rafter cavity through the ridge while pulling cooler air in through the soffits.

If you want to get picky, very few homes are properly ventilated. Even with a continuous vinyl soffit vents, the actual open vent area is minimal. If there are no issues related to moisture, I would leave it alone but recommend additional ventilation be added in the future when re-roofing.

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  • 3 weeks later...
  • 4 months later...

The best way to vent an attic is to not vent it at all. The current trend in building science is partially conditioned attic space with an insulated roof deck. This makes perfect sense if you think about it, it is virtually impossible to keep conditioned air out of the attic and similarly difficult to keep the built up heat in the attic from migrating to the conditioned space, so insulate the roof deck to prevent the heat buildup in the first place and use the lost conditioned air as a buffer - in effect using the total attic space as insulation.

The problem with venting is that it falls on the roofing/siding crews to design the system when it should be designed by the architect/home designer. HVAC systems are designed for the building with consideration given to duct size and location, plenum dimentions, etc. The roof vents should be designed in a similar fashion. Gable vents do not work on every house. Soffit/ridge vents do not work on every house, and in fact do not work well at all on roofs over 30 square. Through in a couple of hips and valleys and you cannot adequetly vent without mixing systems.

I say damn the vents, partially conditioned space is the way to go.

Tom

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  • 4 months later...

Hi,

New in here,

Each home environment is in an environment and unique to its own environment. What I mean is each house is in a symbiotic relationship with its environment, just as we are in a symbiotic relationship with our homes. We can't make blanket statement about venting a home. We can recommend workable systems, if we look at our homes and there environments, and this site proves that.

Quote: I'd plant my feet, square up, feint left, & hit the statement w/a solid right hook.

I can tell you guys are serious about venting. And I would hold that statement down, so we can kick it around.

Just want to let you guys know what side I am on here.

The health of the nation is in our hands. An old plumber told me that once as we were wrestling a snake out a sewer line.

John,

http://web.mac.com/howlandwoodworks1/iWeb

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Originally posted by Chad Fabry

Tell me what you'd say if confronted with the following statement:

Gable vents were once the primary form of attic venting, but have proven to be ineffective or counter-productive in creating the necessary airflow

Chad, are you questioning the statement condeming gable vents or the wording of the sentence. The way it is worded makes nonsense.

The sentence as worded tells me nothing about the house.

Were the gable vents sealed? Were they replaced with something else. Are they still there and something else has been added?

Also sounds like the author was trying to sound very smart and use "big words"

I would rather:

The house was originally built with gable vents, they have been sealed and replaced with eave/ridge vents.

If you were looking at a car for me to purchase, would you tell me...

"When the car was built it had a carburator, but more efficient ways of supplying gasoline to the engine have been developed?"

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Originally posted by J Howland

Hi,

New in here,

Each home environment is in an environment and unique to its own environment. What I mean is each house is in a symbiotic relationship with its environment, just as we are in a symbiotic relationship with our homes.

Pardon my butting in, but let me gently suggest that you look up the definition of the word, "symbiotic." It doesn't apply to inanimate objects (houses). Humans don't have symbiotic relationships with houses. Hint: the "bio" part is a giveaway.

Just doing my little part to remind HIs that we need to be as careful with words as we are with, say, ladders.

WJid="blue">

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Chad, are you questioning the statement condeming gable vents or the wording of the sentence. The way it is worded makes nonsense.

I don't like the wording or the intended message: that gable vent systems are ineffective. Ineffective? That's not true. Counter productive? only when some schmuck adds ridge vents or box vents because he/ she thinks more venting is always better.

With one broad sweep, the statement covers every climatic (I almost wrote climactic just to drive Walter crazy) condition and every style of home. I feel that in many cases gable venting is an adequate form of ventilation.

Greek revivals look stupid with ridge vent and perforated vinyl soffit.

My point is, if a system has been functioning well for, say, 150 years then it's just dumb to say it doesn't work.

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OK, so you are talking about the system. I agree, to make a blanket statement that Gable vents are ineffective does not sit well with me either. But, a particular gable vent system could be ineffective and may need enhancement.

What about the house with a big open attic and gable vents, that has been converted to 3 separate rooms with heating, partitions, insulation and gypsum board covering the roof rafters?

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Originally posted by SonOfSwamp

Originally posted by J Howland

Hi,

New in here,

Each home environment is in an environment and unique to its own environment. What I mean is each house is in a symbiotic relationship with its environment, just as we are in a symbiotic relationship with our homes.

Pardon my butting in, but let me gently suggest that you look up the definition of the word, "symbiotic." It doesn't apply to inanimate objects (houses). Humans don't have symbiotic relationships with houses. Hint: the "bio" part is a giveaway.

Just doing my little part to remind HIs that we need to be as careful with words as we are with, say, ladders.

WJid="blue">

Parts of my house are made from biomass.

My point was, that we can make a difference in peoples health if we look at our clients in a symbiotic relationship with their environment, and not use blanketing statement about their homes and there parts.

I do appreciate the use of this forum and the kindness of those who use it. I have learned much and hope to have a good relationship with all.

John,

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