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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I think you may have posted to the wrong topic - we are talking about the science of hot air rising and what the mechanism is that does that.  If attics were mentioned, it is in reference to this, because we are all familiar with how hot an attic gets and how most attics are ventilated (right or wrong).  Go back a few entries to familiarize yourself with the discussion.  We look forward to your constructive criticism and contribution.

Thanks Ed, I think, and do thank Dr Rushton (not sure of his title) for that detailed rendition of this issue.  I will print it off and play with it, but I'm sure he got all of the laws correct.  But, we are far from done with this topic.

One of the primary objectives here is to establish a more user friendly, less confusing explanation for air movement.  The above explanation is most likely correct, but I don't think we can use it to explain this process to anyone outside the academic world.  To defend my contention that what we have is confusing, consider that 90%+ of the energy professionals (not the academics) believe that warm air, by the mere fact that it is warm, is generating an upward force that results in its upward motion.  Even in Bruce's explanation (somewhere in there) he is saying that warm air is pushing against the cold air and it is the denser/heavier cold air that determines the direction of movement or their positions.

From another academic, this one a professor presenting his information to a more general audience.  It's a video and his comments come near the beginning at the end of his candle experiment.  It's short, but a definite statement about warm air.



"Hot air rises" may not be scientifically correct, but what easy, catchy phrase is there that could readily supplant it? "Cold air displaces hot air"? "More dense air displaces less dense air?" We've spent five pages here already batting this around...is the average Joe or Jane on the street going to give it the same level of depth? Not likely.

I do see your point,  Bud, that inaccurate terminology can lead to inaccurate steps to remedy common problems. What I'd like to see is data regarding the things we've discussed here...does a gable end vent REALLY short circuit soffit and ridge venting schemes? If so, explain how. If not, state why. It's like discussions I've had in another forum concerning return air locations for HVAC...some folk there swear that return air location makes all the difference whether a house is comfortably heated or cooled, but can't really explain why, whereas others state it makes no difference and have a bit more data to support their position, but for the opposing side not enough to offset their anecdotal observations about the matter.

I also like what Ted said a few posts back...get down to the bottom line of what is the correct fix to common problems, execute them, and move on . There will always be people out there misinformed, either through lack of knowledge or willfully. The ones lacking knowledge I am more hopeful for; the ones willfully ignorant I don't have much patience for.

Hi Cameron,

I would say you have a good grasp of the issue.  The warm air rises is a simple explanation of what we have seen over and over that has now led to additional conclusions as to what is actually happening.  "The hot air rises" wasn't that bad, until we added, "and then it pulls in its replacement air", which was the logical conclusion if the rising warm air were going up by virtue of some internal force.  But there's no internal force, so the reference to its pulling powers shouldn't have been added.  The unfortunate result of letting this go for so many years is that now, not only will we need to drop the pulling part, but we will need to de-emphasize the rising part as well.

The solutions, and I do believe there are some, will come both in our choice of words and in the presentation of the information.  Stating that the warm air rises and then the cold air falls would be the same to our customers as stating that the cold air falls and then the warm air rises.  But the second approach hints that the cold air is in control and reduces the risk of furthering the rising powers of warm air.  Or the entire wording could be changed to say "the cold air pushes in through the leaks in the lower portion of our homes and forces our warm air up and out the leaks above.  This is just a hint as to what I feel we can do and there will be many subtle changes that will all add up to begin this long slow process of correction.

As for getting on with the job and minimizing the risk of confusing our customers I agree and disagree.  Certainly for now we do as we have done and continue to get the work done.  But, that doesn't mean we should abandon the effort to improve on what we say and how we say it, call it Energy Correct.  One of the unique aspects of the times we are living in is the ready access to new information.  As we talk about this on the internet, untold numbers of those home owners will be picking up on what is being said.  I for one would not want to be unaware of the new thinking and run into a customer that was way ahead of me. 

I will certainly share any new wording I think should be considered and I do hope that as more people get used to what is being said that they contribute as well. 


Often, when I sense someone is ahead of me, I try not to plod on undeterred like I have no ability to sense they're already on my wavelength. It's not an easy skill to learn, but it's a necessary one. When people sense you are aware of their intelligence, there's instant respect that flows both ways.

This discussion strikes me as more beneficial to us building science/energy geeks than John Q. Public, although someone from that strain possessing admirably above average curiosity might benefit. Everyone else just "wants it fixed" and "don't charge me a fortune" to get it fixed. For this crowd I'm inclined to think "leave the technical details to me; I'll deliver you results I think you'll like". The above-average curious person I would feel more at liberty to go a little tech-speak-ish on, but even there one must gauge the depth of curiosity carefully.

Few people have a genuine gift of taking complex matters and condensing them into layman's terms, and simultaneously sparking that same layman's curiosity. Carl Sagan had that gift; I still don't know a lot about astronomy but his ability to explain things about the cosmos I found compelling and wanting to know more. Joe Lstiburek makes learning about "the complex three dimensional airflow patterns through hollow buildings" fun. What both have in common is the ability to evoke that "ah ha!" moment in their respective audiences. That is the true root of learning; when truth, with whatever form of guidance in force, unfolds within one's mind where it then becomes instantly internalized, accompanied by the excitement of discovery.

This is why I enjoy continually learning about building science. My curiosity is often rewarded, with "ah ha!" moments that make the journey worthwhile. And I get opportunities to go put into practice whatever accumulation of epiphany moments I may enjoy. When it dawned on me that what I understand to be the root driving force of building stack effect is the earth's atmosphere, it all seemed to click together. It wasn't "hot air rising" or even expanding because it got warmer. It was because a larger force was already in place to create the dynamic under discussion (stack effect).

Ever notice how foul auto exhaust seems to linger around much more in cold weather than hot weather? There you are...

I received an invitation from John Brooks over at GreenBuildingAdvisor.com to join a conversation about "Hot Air".  John's intro covers how we ended up there, but it was a very positive discussion of this issue about what makes hot air rise and whether we should ditch that phrase or simply increase the education of all so there would no longer be any confusion.  I think the ultimate solution will end up being some of both, but that's just my opinion from where we are today.  I'm sure time will provide its own answer.

Anyway, here's the link over at GBA: http://www.greenbuildingadvisor.com/community/forum/general-questio...

and here's a Building Science link that John posted, one I had not discovered, probably because it is titled "Air Flow Control in Buildings".  I didn't think to search for that :(, but pages 4 through 7 are dead on about stack effect and their pictures are much better than mine. http://www.buildingscience.com/documents/digests/bsd-014-air-flow-c...

I'll let those interested do their own reading, but for me, I now know I'm not alone.  Oh, and don't miss the video by Julius Sumner Miller.  Professor Miller is one of those hands on instructors who will keep you awake, he's fun.


Seems to me language is getting in the way.  Hot air is more buoyant than cold air because the warmer air molecules are more energetic than the colder ones and therefore take up more volume for the same mass as cold air.  The hot air begins to move as a result of differences in density, not pressure. That's bouyancy.

Once the air begins moving, it needs to be replaced by something.  Seems to me the language of buoyancy implies the cold air is pushing up on the hot air.  If that was the case, we would expect the pressure to be higher at the point that the cold air pushes on the hot.

However, if you make a stack effect simulator out of some PVC pipe and light bulb, you'll find that the pressure is lower at the bottom of the stack and higher at the top (WRT to outside the simulator), the exact opposite of what you would think if the cold air at the bottom were "pushing" the hot air up. 

Another piece of evidence in favor of rising air causing negative pressure are thermals.  Because thermals are not bound by a tube or a cavity, it seems that cold air pushing on hot air would be akin to pushing on a string and that if buoyancy was the only thing at work, the hot air would spread out.  In other words, buoyancy explains temperature gradients between large bodies of air, but I don't think it completely explains rising columns of air.

So, how do buoyancy advocates account for negative pressure at the bottom of rising column of hot air?  Where did that negative pressure come from?  Pushing implies high pressure.

Fluid flow dynamics is complex.  I suspect both buoyancy and slight, local negative pressures caused by moving air are at play.  Buoyancy to get things moving, slight negative pressure to replace it.

Hi Joel and welcome to the discussion.  In addition to learning much about air movement, the one failing I have uncovered is how poorly this seemingly simple topic has been defined.  But discussion has been brisk and there does seem to be light at the end of all of this.  Note, this is just one of three discussions currently ongoing.

You said " The hot air begins to move as a result of differences in density, not pressure."  It is those differences in density that create the differences in pressure.

The cold air does push the hot air, but, as for measuring at the boundary, not possible.  But we can see before and after the boundary and the direction of motion goes with the direction of the greater pressure.

The pressures in a pvc stack simulator should follow the density of the gasses and their attempt to equalize that total pressure difference between top and bottom.  In my link: http://myenergyworkshop.homestead.com/hot-air.html I walked through this equalizing process for stack effect where we end up with a neutral plane in the middle of our homes.  With the pvc pipe having an exit and entrance the same size, then it too would have a neutral plane somewhere in the middle, with a positive pressure above and a negative pressure below. 

And last "So, how do buoyancy advocates account for negative pressure at the bottom of rising column of hot air? " It is the reduced barometric pressure directly up through that warm air.

To be fair, my opinions are certainly not in the majority, but I account for that by my earlier statement that this topic has been poorly defined.  Where the academics seem to agree with the science, they disagree that the term "hot air rises" has been the cause of any confusion.  Where the that phrase may not have caused the confusion, it certainly did little to prevent it.  Whether it is a foot note or a few extra words, we need to start telling everyone why hot air rises so our audience can stop guessing.


Maybe it's "air expands as it absorbs heat".  As it expands it applies pressure to cold air.  

Seriously, I agree "warm air rises" can lead to misunderstandings as we often find oversimplification does when it comes to building science.  Further, coming to parity often requires excessive force on the pendulum to clear preconceptions and open minds to alternative viewpoints, which Bud's OP seems to have accomplished remarkably well.  (I wish I could get as much traction on my "setback does not save money" essay, but that unmeasured preconception is really firmly entrenched).  

Maybe the best way to think about this is "warm air pushes up and cold air pushes down, these are opposing forces just like two magnets.  You can't have one without the other."

Bud, the illustration found at your link is excellent! In just one spot, several variables I've wrestled with when contemplating our topic at hand seem to merge beautifully. Well done!

In your example, you gave a measurement of 101,325 pascals at the base of the structure. This converts to a barometric pressure, for Imperial measurement holdouts, of 29.92 inches of mercury. This ties into something I began realizing last year, in that as atmospheric pressure and temperature vary, so does the behavior of stack effect within a building. This is well known to some degree in that many in our line of work understand winter "stack effect" and summer "reverse (or inverse) stack effect". But, as you continually point out, the focus is traditionally on the air mass within the structure, not so much the atmosphere without.

I've pulled up trend data for barometric pressure over a year's time in my area. While daily and weekly swings are evident, looking at it all over a year's time shows generally higher barometric pressure conditions exist in winter vs. summer. Since we modern humans prefer a fairly narrow range of indoor air temperatures, it becomes easier, when puzzling this whole scenario under discussion through, to regard indoor air temperatures as relatively fixed and the outdoor temps and barometric pressure in continual flux. 

Currently our outdoor barometric pressure is 30.32 inches of mercury, which converts to 102,664 pascals. Referring to your illustration, with the indoor air fixed at 70 degrees F, the difference between the indoor and outdoor air masses at my present barometric pressure would be even greater than in your illustration, with outdoor air warmer than in your example (41 vs. 35 degrees F). Therefore the pressure difference at the bottom of the structure would be 664-317 vs. 325-317. Meaning, how I'm seeing it, that as barometric pressure fluctuates, so does the intensity of indoor stack effect.

"...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"

I don't think there is such a belief because I think people mostly stop short of that due to lack of interest.  I think people are imprecise when they speak and write for a variety of reasons, including work to be done for a fee.  Bud, do you believe a new clarity about this will help home energy pros do more business?

For what it's worth, my understanding is that as long as there are pressure and temperature differences indoors and outdoors and throughout the enclosed volume, air will be moving and mixing.  Units of less dense air (warmer) and units of more dense air (cooler) will be complementing a lot like yin and yang, one diving under, the other floating over, mixing and partially conforming (averaging constantly to form new units) in that process.  What doesn't conform, all the survivors, so to speak, keeps diving and floating, respectively, encountering fewer and fewer units that are different, until stopped by or escaped from the envelope.

I mentioned diving first in honor of gravity, which drives it.

Hi David, have you ever heard the phrase that I find frequently following the "hot air rises", that it pulls in its replacement air.  It doesn't.  Stack effect doesn't suck the air from under our basement slabs.  Chimney draft isn't created by the hot air pulling in fresh air.  The list of incorrect statements is long and guess what, many people out there do know better.  When we make incorrect statements, we expose our lack of understanding to the well informed among us.  I, personally, would rather find a better explanation that is easily understood by my customers yet states the process correctly so I can look more professional.  Should we risk confusing our customers with a proper explanation, as some have suggested?  I think the confusion is already out there and it's time to begin the correction. 

One more point on keeping pace with our customers' knowledge base.  They are already reading over our shoulders and will be waiting to see how the energy professional they hire one day handles some of this new wording.  It's the new age of communications and I'm getting a lot of thumbs up from the people watching.



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