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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Thank you all for the Great Insight into Venting the Attic.  

I owe you an email, I think.  Glad you found this as it has been some time.  The issue of hot air has taken time to raise my confidence level to where I could proceed.  There will be more discussion on static venting, wind, and powered venting.  It's nice to have the actual numbers to consider.

Bud

>>But now we know that warm air cannot rise on its own and thus is not pulling in its replacement air from anywhere.

When did we get to know this? Am I still lost in the dark ages where warm air rises when there is an air temp differential within an enclosed space? Please fill me in on this.

John Leeke

Hi John,

This has been a long and difficult battle and unfortunately, even with the great exposure on some of the forums I was pleased to reach, few have had a chance to weigh in.  The main thread here was this: http://homeenergypros.lbl.gov/forum/topics/the-science-of-hot-air-r...

There are several other discussions relating to stack effect and hot air, lots of hot air, and I will provide links if you need more reading.

The problem with the reading is one would definitely conclude I was in the minority, but the science prevailed despite the unpopularity of considering change.  It is the depth to which we have all been lead to believe in hot air "just rises" that concerns me.  I will exclude the academics as they have known this all along, they just didn't want to tell the rest of us.  I'm being cynical and I shouldn't, as this has simply been a convenient oversight that got carried away.  Now that we actually need to know why hot air goes up, we need that science.

Here is my simple example.  Hold a cork under water and release it.  It races to the top.  Now examine it, it's just a cork, with no internal powers to move upwards.  Buoyancy was the science, and our hot air is pushed up by the denser cold surrounding air just like our cork was pushed up by the heavier (denser) water.

Now the focus is turning to explaining all of those simple "hot air rises" events without the hot air being the source of the force behind the movement.

Bud

This is one reason why we are seeing more mold in attics from homes that increase insulation. insulators are blowing R-50 in older homes that typically had R-15- to maybe R-21. they are not careful about installing baffles even if soffit vents are existing, or they blow insulaiton over the baffles limiting air flow, and they never add soffit vents to homes that need them or did not have them. in older homes with large gables and no soffit ventilation and little to no insulation there usually is not any mold until they have the attics insulated, this might have a tie in with re-roofs as well and the roofer adding jack vents to the roof, but again not ventilating at the soffit? Usually we find a source of humidity, usually in the crawl space, maybe this existed before and the attic with little to no ventilation at the soffit, and little to no insulaiton in the attic had enough ventilation to remove the humidity? or maybe the moisure occured after the re-insulation? hard to tell for sure? any thoughts on why we are finding more mold in attics with added insulatio?

The attic would be colder for one.  Combine that with some overlooked house to attic leakage and the moisture potential increases. 

Static venting is very marginal in terms of the pressures available to move the air.  Think in terms of 1/4 Pascal per foot per ▲t of 35°.  That is just a number I worked out for my illustrations, but the changes are linear so halve the temperature and halve the pressure, or double/double.

Then, once you decide on the total pressure, you have to divide that above and below some Neutral Pressure Plane.  If the NPP is centered, then half above and half below.  Think one or two pascals.

Here's the kicker.  In cold country, as we remove the unwanted heat, we are reducing our ability to remove heat.  Like trying to pick yourself up by the shoe laces so to speak, static venting can never remove all of the heat.  Pray for wind:).

There was also a good article (for me at least) on how moisture can move through materials until it finds a moisture resistant barrier.  Basically, it can move right through most insulation.  So those old home with no or poor VB's are still subject to moisture moving to the attic even though air sealing and insulation have been improved.  I can find the link if needed.

Bud

The truth is most ridge vents are JUNK. Really, do a smoke test and see how long it takes the air to clear out. Bring something to read because it's gonna be awhile.

There are a few basic types of ridge vents:

1: The "fiberglass mat" type, which looks like a hoghair filter. You can image what the filter looks like after a few months if any air was going through it to begin with.

2: The plastic shingle over type with a "filter" over the ridge, same issue with "filter" (Most common type installed)

3: Corrugated plastic "cor-a-vent" . Better due to no filter, but still restrictive

4: Metal, probably the most effective of the bunch, but not many are installed due to looks.

With that being said the "heat rising/NPP" theory only applies when there is little wind. WIND speed/direction the number one influence of how much air exchange in the attics. It isn't uncommon around here to have hot air coming OUT of soffit vents on the "back side" of the building, and a 5MPH wind is about all it takes.. With that being said I say leave the gables OPEN for maximum cooling.

Do the same "smoke test" on an attic with "whirlybirds" and you will find a MUCH faster evacuation rate, especially if any wind is blowing. Electric fans are also effective at cooling attics, but have their own set off issues. Passive "turtle vents" work decent if you have enough of them.

http://www.youtube.com/watch?v=SyIw2GTiKvQ

Bob, easy to tell you've been out in the real world :). Good post.

To put some numbers on static venting to illustrate why it does so poorly:

Attic stack pressure = .007 x ▲h x ▲t  Pressure will be in pascals, h in feet, t in °F.

For a 6' attic with a ▲t of 35° it comes out to 1.5 pascals total attic pressure.  Divide that in half, assuming a balanced high/low vent area, and you get 0.75pa pushing air in and 0.75pa pushing air out.  Considering how modest 50 pascals is, 3/4 pa would be barely a breath of air.  Any wind at all would quickly blow the static venting away.

As for whirlybirds and power vents, we need to balance the negative pressures created or put a number on what the extra house to attic leakage is costing.  Is it $5, $50, $500 per year? 

As for ridge vents I want to share one of my personal conclusions as to why we find or hear about so many being left covered during installation.  I think the roofers leave them covered to avoid a callback due to rain blowing in.  Few people would ever notice that their attic isn't as cool as it should be, but everyone will notice water coming in and someone will be responsible for the damage.  2 years later someone else opens the ridge to where it should be and it leaks, not the roofers fault, Hmmm!

I think we can call the short-circuit myth "Busted" and use our new understanding to decide when those gables should stay or go.

Bud

IMHO the amount of negative pressure generated by whirlybirds is minimal but flow is high due to their "open" design. Without intake vents, whirlybirds simply spin and don't remove much air. When no wind is present, whirlybirds act like a 12" hole as far as airflow is concerned, the .75pa you mentioned is actually enough to spin the whirlybirds. As if you couldn't tell, whirlybirds are my favorite attic venting solution solution. Attics with whirlybirds or working electric fans are the coolest.

Electric fans DO pull high static pressures when presented with resistance, and will "suck" from conditioned space if intake venting is inadequate. Thermostats must be set relatively high to become effective for heat gain/power used, about 120f on/110f off works well. Electric fans can work well for attics over cathedral ceilings when nothing else seems to help. If used be sure to get the type with 1100 RPM PSC motors, they seem to offer the highest CFM per watt in the real world.

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