The Short-Circuit Theory, Keep or Remove Those Gable Vents?

When a ridge vent is added to a house that already has gable and soffit vents, you may hear advice that goes something like this.

"You must remove or close off those gable vents to avoid short circuiting the air flow from the soffit to the ridge."  The assumption, I'm assuming, is that the warm air exiting the top will pull its replacement air from the gables instead of the desired location, the soffits.  But now we know that warm air cannot rise on its own and thus is not pulling in its replacement air from anywhere.  So what's happening?

Attics are a somewhat isolated space, much like the house below, with one temperature air inside and another outside.  With a space and a temperature difference you have a stack effect.  In an attic, that gives us (warm in and colder out) a high pressure at the top and a lower pressure at the bottom.  The key here is that the attic is filled with warmer lighter air that has been compressed a pascal or two by the cold air pushing in through the soffits.  That cold air pushing in has reduced the lower (-) pressure and increased the (+) upper pressure, thus pushing the warm air out the top, or gables if still there and forming a neutral pressure zone somewhere in between.  The actual determination as to where the air exits is based upon the vent areas available.

The bottom line is, cold air will continue to enter via the soffits regardless of where it exits, no short-circuit.  There are reasons to remove the old gable vents, prevailing wind, rain, snow, or other moisture sources, but short-circuiting the air flow from the soffits is not one of them.

Bud

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Bud, You ask a lot of good questions.....

I think your conclusions about "what happens" depends.....

It depends on the House

Is it an Extremely Tight and highly insulated House? or an average Loose House?

or a Really Loose House?

 

It seems to me that a "Loose House" that is well connected to the attic can have a significant influence on the location of the NPP

You are correct, pressures and the location of that NPP can vary widely, BUT, the air exiting the top isn't pulling in its replacement air, thus the air flow in through the soffits will be based upon the difference in the barometric pressures.  With the gable vents open, that should encourage the NPP to be located higher, thus more ▲p for the soffits. 

If the house is extremely well connected to the attic, that also needs to be fixed.  Usually it would be some rather large bypasses.

Bud, I agree with you...

Ridge vent plus Gable vent will result in a higher NPP than with Ridge Vent alone

The NPP will move toward new or additional openings

Bud,  moving the NPP higher is not necessarily a good thing.

especially if the house is not-so-tight.

 

have you seen Joe Lstiburek's Rules for Venting Roofs?

http://www.greenbuildingadvisor.com/blogs/dept/building-science/lst...

Joe:"3) Put more vents down low than up high.
This is where the code tends to have it wrong. You want more entry points at the perimeter than exit points at the top.

People say you want to balance the lower down ventilation with the upper ventilation, and a lot of people interpret the codes to say that if you get it unbalanced you want more ventilation up high. That is absolutely wrong; you don’t want more places for the air to get out than to get in. The reason is, if you construct a house with a leaky attic ceiling and you have lots of ridge vents or you have lots of vents up high, the makeup air is going to be pulled from the house rather than being pulled from the outside. That scenario is a disaster."

Bud,

After just quoting Joe Lstiburek....I admit that I believe your viewpoint of Dense Air Pushing provides a more realistic image of what is happening and your words are more consistent with Professor Julius Sumner Miller and Professor Walter Lewin (MIT)

Hi John, Joe is correct, but I'm certain he doesn't need my approval :).  But there are trade offs in many places in a home and improving attic venting may indeed increase the ▲p between the house and attic.  However, the solution is not to make the attic venting less, but to better seal between the two.

If we take a balanced soffit and gable in an attic where the NPP is right in the middle, and add a ridge vent, the NPP will move up, but only to where the gable vents begin to act as both intake and exhaust.  In a small attic that might be less than a foot or in a larger attic a couple of feet.  For most I would guess we are looking at an average of less than a pascal for the shift in ceiling to attic pressure.  Something to be considered, but not an automatic reason to remove the gable vents.

Bud

Shaking head slowly...

When it is needed most, the driver is heat. On a very cold winter day the heat escaping from a house needs to be drawn up & out to eliminate possible ice dam issues - with proper venting it will escape via the ridge. When you have both you can create unwanted currents, etc... letting that radiant heat & moisture get to the sheathing. I know you can't figure out how to express that mathematically / it doesn't compute with what the numbers say but reality has an interesting way of taking "this will work" & laughing at you 

During the summer, your attic is going to be hot - period (unless you are way up north) The sun beating down on your roof heats the air (radiant) inside making it more buoyant / whatever term you wish to use & it will flow out through the ridge. When you introduce the gable vents you can disrupt that flow (which should be even - i.e. every flipping bay should be vented not every other one, or every third one) causing additional currents, heat to build up &/or hot spots in the attic

As I said reality has a way of turning this is how it should be & laughing at you. In many cases having both won't matter, but in others it can matter thus the best practice is to have one or the other. Personally I side with hot roofs or a combination gable & turtle vent system as I have seen to many "roof leaks" boil down to the ridge allowing wind driven rain & snow in. I got to say the manufacturers are getting a lot better at preventing that

Hi Sean,

The bulk of what you stated boils down to understanding each specific home and its needs.  Part of that understanding is the force behind that air flow.  Not just the number, but the source.

As for cooling each rafter bay, recall that often pictured and described air flow, where the incoming air is supposed to be "washing" the underside of the roof deck.  Not happening, beyond a foot or so, where the cooler air spills out onto the attic floor.  There is no soffit to deck air flow under 90% of the roof, just a gradual filling of the attic that pushes the warmer air up and out.

The change will be slow.  But eventually contractors will learn when the gables should stay and when they must go.  The alternative has been, they must go at all times and that adds expense and reduces performance in some situations.

Bud

This conversation is great.  Seems we are considering attic ventilation as the passive result of isolated-attic stack action.

I'm becoming more and more of the opinion that wind effect likely has a much greater impact on issues of air exchange, whether looking at an entire building or looking at an isolated zone such as an attic, than I previously understood.

"Stack effect and fan pressurization usually produce air pressure differences of between 5 and 10 Pa in houses and from 50 to 150 Pa in tall buildings....Hourly wind pressure values for selected locations in Canada are listed in the Supplement to the National Building Code of Canada (NBC), in the climatic design data table. Values for individual locations range from 170 to 1500 Pa, but when adjusted for building shape and height, these may result in local pressures or suctions on the building two to three times greater. Wind gusts lasting 3 to 5 seconds can exert forces on a building in excess of 2500 Pa."

(Ganguli, 2001)


2500 Pa?  Now THAT's an air-moving Delta-P!  

In the real world, my thought is that wind-induced suction/pressurization of the attic probably drives considerably more air change than stack action.  Thoughts?

Hi Grant.  Here is the link you are referencing: http://www.nrc-cnrc.gc.ca/eng/ibp/irc/bsi/86-air-pressures.html

I skimmed the article and would conclude that those huge pressures are more related to tall structures than low rise residential buildings.  But wind is definitely a factor and one of those elements that needs to be considered in attic venting.  Although we are trying to move air through our attics, what it brings along with it can be a problem.

With our modern world wanting to "seal them tight and ventilate right" it will be important to understand the results of all air movement.

Bud

Bud,

No doubt a typical single family home isn't realizing 2500 Pa wind-induced pressures....unless the breeze in question happens to have a first name (e.g., Andrew, Hugo, etc).

But 50 Pa pressures?  You bet.  I have a buddy that lives on top of a hill in the Kittatinny Mtns, northwest NJ (about 2.5 hours from Turnpike Exit 18 for those who like their NJ jokes...God's country up there).  He probably has 10 mph breeze, minimum, on his home 8,000 hours per year.  

I should throw a few data loggers up in his attic and see what happens....

Here's a related question for you.  A 1,500 sq ft home, 12,000 cubic feet volume.  How much air needs to enter or exit to change the pressure 5pa.

Shifting the Neutral Pressure Plane in a house requires adding or removing air.  So I played with the numbers just to see how much air has to move to accomplish this change.  How's your physics, I used Boyle's law.  It's not perfect, but worked.

If I'm close, the answer is interesting.

Bud

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