Evan, I'm hoping you will comment on this.

Since energy myths are front and center at the moment I would like to discuss a single myth, the science of hot air rising.  Actually, I have for my own purposes upgraded this one to an energy legend, as it has proved extremely difficult to correct.  I'm assuming it is wrong but I'm certainly open to all opinions.

There are several areas in our energy business where this is important, attic venting, stack effect, convection, and chimney draft to name the obvious.  The legend as I see it is that people have observed warm air moving up for so long that there is a belief that warm air has some inert power of its own.  Statements like "the warm air will rise and exit the upper vents and pull the cold air in the lower vents", implies that the warm air initiated that process and as a result not only pulled the cold air in through the soffits, but additionally air from the house as well.  My belief is, the opposite is true.  The cold air pushes its way into the attic and forces the warm air up and out the upper vents based upon the principles of buoyancy.  Here is a simple article by April Holladay that explains this invading cold air process very well is:http://www.usatoday.com/tech/columnist/aprilholladay/2005-02-18-won...

At first glance this appears to be just a simple statement of what everyone sees in the real world.  But the concept that cold air is the driving force becomes important in properly explaining the other, above, modes of air movement.  As energy professionals I believe it is important that we determine the truth about this legend and learn to state it correctly so future generations will not be led to believe that there is magic in warm air.


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There are three kinds of people in the world:  those who say hot air pulls cold air, those who say cold air pushes warm air and those who say an air mass is complex interaction subject to gravity that trends to vertical gradation according to density.  Because the saying is usually a casual attempt to explain (unlike the gifts in this discussion), what those talking believe remains a mystery to me, if not to you.  It's conceivable that they hold the same belief.

It's wonderful that you are seeing thumbs up.  Enjoy!

David, I appreciate the input and hope you will follow over time.  I am creating a web site where I can maintain the information I collect or generate to help improve the understanding of hot air. 

I just finished reading an unrelated article published in building science by Dr. Lstiburek and he comes to a point where there is no practical way to obtain the input he needs.  He rhetorically asks himself, what do I do now.  His answer is, guess, but make it an educated guess.  The easy audits we do are easy.  It is the most difficult ones that will show our true abilities and yes at times, we will need to guess.  It is my curiosity into the details of the numbers and methods we use that will result, I hope, in a better understanding, so that when I or others have to guess it will be better than it otherwise might have been.


David, some of your writing strikes me.  Nice. 

How about this.  Imagine a balance scale, one side has a 1 lb weight and the other a 2 lb weight.  Both are effected by gravity, but some force in the middle causes the 1 lb weight to rise and the 2 lb weight to fall.  Is either weight causal, or is the pivot point? 

I think that lever point is the same for stack.  Hot air is the 1 lb weight and cold air is the 2 lb weight.  The motion of the weights is interdependent upon both each other, and the pivot point.   

Good way to put it, Ted. Well done.

The air columns in this case are the weights, and the pivot point is the building envelope. The air columns are at different weights due to their respective temperature/density continuums. As Bud's illustration shows at the link he posted earlier, there is no difference in weight or density when air inside the envelope is at the same condition as air outside the envelope. We are the ones who impose the imbalance by preferring our indoor temperatures to be what they are. Atmosphere is what it is regardless of what we do indoors.

"The air columns in this case are the weights, and the pivot point is the building envelope"

Which case is this case?  Bud's illustration?

Causality in our case at hand?

Whatever upsets equilbrium. In Bud's illustration, Figure 1 has the indoor and outdoor conditions of the building at equilibrium. Only when the indoor air was heated above the outdoor air temperature was this equilibrium upset. Even if the envelope was airtight, inequality exists due to the difference in air mass densities. True, the airtight envelope would have little to no stack flow. But an inquality would remain between the two air masses in question.

In short, we humans are the cause. We like to be indoors at air temperatures higher than 35 degrees F.

Hi Folks .. Nice discussion

Stack Happens

Many/Most of us maintain the Fluid inside our enclosures within a narrow comfort range.

I don't mind taking that for granted.

Many of John Straube's Illustrations at BSD-014 (like Bud's) assume that an interior temperature will be maintained.....as in a heated or cooled cylinder or building

Our enclosure are not perfectly airtight pressure vessels.

I don't mind taking that for granted either.

I recently read an interesting comment from Colin Genge(Retrotec)

Colin:"Atmospheric pressure adjusts inside and out almost instantaneously even for a tight house. It would be in the order of a few seconds at most."

Hi John,

One of the reasons I created my work page was the realization as to just how much variation there is in atmospheric (barometric) pressure just between the top and bottom of our homes.  One of those details we just take for granted, the 14.7 psi, but when we look at its Pascal equivalent the pressure can change 60 pa in 20 feet.  So why doesn't that pressure difference create an air flow? Here's where my physics falls apart, but my thinking is, that pressure difference is what is supporting that air column.  The higher we go the less air we need to support.  The delta p that moves the air in and out for our stack effect comes from different atmospheric pressures inside and outside our homes.  Now you know why I had to draw the pictures :).

Another interesting detail, we cannot see these up and down atmospheric pressure differences with our fancy manometers.  Toss a long reference tube out the front door and drag our manometer up to the third floor and we will see almost no change, yet the numbers would suggest almost 100 pa from bottom floor to top ceiling.  Turns out it is the weight of the air in the tubes that cancels out all of that stair climbing.  A minor detail, but it turns out that understanding how the weight of air affects our stack effect also helps us understand the limits of our test equipment.


I am not going to pretend that I can visualize this FLUID that we live in.

My goal is to learn to visualize it more better.

I'm trying.... but I am still confused.


Allison Bailes' recent Illustrations are a step in the right direction.

I think Bud is on a good learning path ...

I think there is more to know than just "Warm air Rises (sometimes)"


I think a better understanding will be useful to Home Improvement Professionals

and Enclosure Designers

What's  you're thoughts on buoyancy and thermals?  Is it the same as stack effect, even though stack effects exists in a bounded area?

I can see how could, denser air pouring into a building, or a valley, displaces hot air.  It'll displace all the hot air in the building over a given area.

I'm having trouble visualizing how this works with unbounded thermals, which are columns.

Hi Joel,

There is a lot more involved when you start looking at elevations that are miles high, but I believe the basic principles are still in effect, heavier air pushing down will dominate.  For those huge cumulus clouds we see billowing into the sky, picture a blanket of cold air covering the landscape with a layer of warm air trapped under it.  Now create a hole in that blanket and the warm moist air below will be pushed up through that hole forming those clouds.  I'm sure the meteorologists can explain it better and include all of the extra details, but what we are learning is that warm air needs some help from somewhere before it can move up against the forces of gravity.  I would have to add that the warm air below didn't just pop out of the ground, it is undoubtedly created by solar energy heating the ground and thus heating the air, which does expand and contribute to the process.  But expanding air and air moving up miles into the sky are two different issues.  But explaining that would take a bunch of reading.

For now, we have defrocked our hot air and recognize that it moves at the will of other forces.  Within our energy profession, explaining that change in thinking will be difficult enough, I'll leave the clouds for another time.


Hi Bud

Since I am confused ...

I thought I would try to follow along with your "worksheet" and with John Straube's BSD-014 examples

I will attempt to attach an Illustration

(My Illustration is a variation of yours and Straube's)

If I am doing the math right... using your example temperatures 70f inside and 35f outside

and Using John Straube's formula ... I also get an incremental difference of 0.25 pascal per ft (1 pascal per 4 ft)

for a total of 8 pascals over 32 feet

please tell me if my illustration and math makes sense so far



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