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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Bud, while I like what you are saying because you need extremes to balance each other. But the pendulum should come back to center eventually. Creating pictures around how and why things work can lead to understanding and misunderstanding, sometimes over pushing in the other direction can correct misorientation.

If air doesn't have upward pressure, if it is pushed from below, what explains negative pressure in basements and at in basement water heater flues? What explains neutral pressure plane in stack effect? Wouldn't basements be at the same pressure as outdoors, not negative wrt outside?

ted, not to offend,

but your reply is exactly why we need to make this change.  And do understand, I'm not pointing at you, because your current understanding is what everyone is being taught, either through our training or through our reading.  Go back to Dr. Mills post near the beginning of this thread and he quickly agrees that hot air does not rise by its own power, little propellers is how he described it.  My reading so far has been extensive and I have communicated with other technical people in this field, hot air has no magical powers.  Let me add, since your concerns are valid and shared by many.

Stack effect did not get its name from a chimney.  It is named for the stack of light air inside that enables the stack of heavier air outside to create the draft we have come to expect, that being the cold air pushing the warm air up. 

That negative pressure in the basement, it is a result of the stack of air outside our homes being heavier than the stack of air that extends down through our homes, since the air inside our homes is warmer and therefore lighter.  The diagram and description on the link I posted earlier is an attempt to walk through and explain how the positive and negative pressures are created by these two columns of air and the resulting neutral plane in between them.

The creation of pictures and the explanation as to how everything we deal with related to this misnomer is a challenge to me, meaning I may not be the best at this, and it is my hope that through forums like this I can stir up enough interest to start seeing input from others.  Like fanning a fire, I will keep pushing this until the organizations that print this and teach this, feel the heat and adopt the changes.  We have a start and I'm looking for suggestions.  And of course, I will do my best to explain how our atmospheric pressure (gravity) can create the forces behind the air movement we see.  I know, too long, sorry.



I still think that it just as valid to look at this as a symbiotic relationship - I don't think the physics says that the cold air is falling without the hot air rising, they are both moving around because of differing densities.

What is happening here?:  I have a steep slope and I build a fire at the bottom - the fire will spread quickly up the slope.  If I built it at the top, the fire would not spread DOWN the slope, at least not very quickly.

So the fire is heating air, that warmer air is rising up the slope, heating the grass or brush on the slope until it reaches its ignition point.  At the same time, cooler air from the surrounding area is being sucked into the fire down low.  It seems that the initiating force is the heat of the fire, but that cool air must rush in to fill the void.

You can't have one without the other, so how can you say which is more important?

Hi Ed.

Cold air falling due to gravity is real.  Warm air rising by magic is not.  I'm not making fun of your example, it is just that there is no force from a mass of warm air besides down as a result of gravity.  The exception is the expanding air due to heat, but that goes in all directions.  A 10 pound rock on a string suspended into a tub of water weighs 10 pounds, minus the weight of the water it displaced, that's buoyancy.  A mass of warm air that is displacing the cold air around it will be pushed up by the difference in their weights.  The warmer/hotter the air, the lighter it is and the greater that difference.

As for what initiates the sequence, it is again gravity pulling both air masses down.  Since the heavier air will win the battle for the bottom we can say it initiated the process because the direction of the movement was the direction the cold air was pushing.  Since the warm air was not pushing up, it cannot be credited with the upward movement.

The heat was indeed necessary, but heating air in the absence of gravity will result in only expansion.  As I said before, I'm not the greatest in explaining every scenario, but I hope that helps.


Bud, i think there are forces not being considered other than magic.

Fluid dynamics certainly are complex and confusing, but another force we are all grateful for, without which we would likely be crushed, is centrifugal. Maybe this is part of the force driving lighter fluids to rise.

Certainly an explanation of pressures within stack relative to outdoors contradicts the idea that the only force at play is downward. I would really like to get my head around it.

The choice of "magic" was intended as a bit of humor and not sarcasm.

But, I can find no science that can produce the pascals we see in real life other than the difference in the in the air pressures outside our homes as compared to inside due to the warmer/lighter air inside.  Calculating that difference, I can determine the positive pressure outside our basements that makes the inside pressure look negative.  I can locate and explain the neutral plane.  I can then calculate the positive pressure at the top of our homes that is pushing the warm air out.  And all of these numbers are in the range of what we see in the real world. 

As for stack pressures, they are the difference in the weight of a column of air down to our reference point as compared to a column of air down through the stack to our test point.  When that stack is filled with cold air, we get a flow of air we refer to as a back draft.  The air in the chimney is heavier than the air in the house.  When that stack is filled with hot combustion gasses, we get an upward flow inside that stack because the pressure around our combustion appliance is greater than the downward pressure inside the stack.

An example of this weight of air inside our chimneys is the barometric damper and how it controls the draft.  Too much draft and it opens to allow more cool air to mix with the exhaust flow.  Too little draft and it closes to reduce the mix and increase the flow.  The increase and decrease in flow is achieved by controlling the temperature of the air inside the chimney.  That damper is thus named a "barometric" damper because it is responding to the difference in barometric pressure between inside and outside the flue.

No other significant forces are present and none are needed to reproduce the pressures we see in the real world.


Still not completely seeing it, although weight of equivalent column of air might be creating a picture...


My next door neighbor is a retired physicist -  His name is Bruce Rushton.  He worked with 3M and was part of the "Post-Its" development team at one point.  We spent an hour on Sunday talking about this, and yesterday he put something together for you.  It is VERY esoteric and I don't pretend to understand it, but the last paragraph is the salient one:

"Whether or not a fluid will float or sink in the presence of another fluid is not directly related to temperature.  It is related to the relative density of the two fluids.  Archimedes principle says that an object will continue to sink until it displaces its weight in the fluid it which it is emersed.  If the density of the simmering fluid is greater than the average density of the object, the object will float.  If not, the object will sink.  This keeps boats afloat since the average density of the boat with its large air space within its hull is less than that of the volume of water displaced by the hull below the waterline.

But what about gaseous fluids?  Two gases appear to be different than a boat in the water, but they are not.  A large body of air can be assumed to be of uniform composition regardless of temperature... e.g. the molecular makeup of both hot and cold air masses are virtually identical. 
Using Boyle's Law for gases we know that the pressure and volume of a given number of molecules of a gas at two temperature states are related by the formula 
 P1V1 = P2V2, for a given weight and temperature of a gas;
or the pressure times the volume in one state is equal to the pressure times the volume of that same mass and temperature in a 2nd state.
We also know from Avagadro's law that the volume of a gas is proportional to the number of molecules at a given pressure, or
n1 / V1 = n2 / V2;
where the n's are the # of molecules in each of the two volumes of a given gas at constant pressure.
The combination of these laws is the ideal gas law used for design purposes, and is shown as:
Where R is a constant for all ideal gases (only slight variance for mixtures of gases); n is the number of molecules of the gas mixture in a Volume V which is at Pressure P and absoluute temperature (degrees Kelvin) T.
The mass of a gas volume contaning 6.02 x 10^30 atoms of the gas is the average molecular weight of the gas, Mw,  in grams.  6.02 x 10^30 is Avagadro's number and is a constant for virtually all gases. 
If we take a volume, Va,  of gas containing one average molecular weight of the gas in grams at Temperature, Ta, The density of the gas, Da, will be
Da = Mw/Va.         in grams/cm^3                                            
If the same mass of the gas is reduced to a lower temerature, Tb, the density, Db will be stated as    
Db = Mw/Vb.        also in grams/cm^3
Da / Db = Vb/Va
PV = nrT  => Va = nRTa / Pa
Vb = nRTb /  Pb
Vb / Va = Tb * Pa /( Ta * Pb )  => Da / Db = ( Tb / Ta ) * (Pa / Pb )
Since Tb < Ta,
Pa / Pb >Da /Db  or the low pressure air mass at the higher temperature, Ta,  will be less dense than the higher pressure air mass at the lower temperature Tb.
It follows then that the less dense mass of air will rise until it's temperature drops to the point where the the relative densities of the two air masses are equal.
Or by Charles Law:
 Va / Ta = Vb / Tb  for a given weight of a gas
 Va / Vb = Ta / Tb. says that the a molecular weight of a Gas, Mw, divided by its volume, V will follow the following relationship
 ( Va * Mwb ) / ( Vb * Mwa) = Ta / Tb   = ( Density b / Density a ) = Ta / Tb  =>
that since Ta < Tb;  the density at the higher temperature fluid mass is lower and the density will be bouyed, like a boat, by the higher density, lower temperature air mass.
Relative Density determines what will rise and what will sink.  There is no down without up.  Up and down are artifacts of the relativistic concept of gravity. Without gravity there is no relative density effect, since there is no accelleration acting on the masses of the two fluids to create the forces that can then obey Newton's 2nd and 3rd laws.  These are that the force on an object is equal the product of the mass and acceleration, gravity, ( F =  mA = mg in a gravity field ), and that the forces exerted by two bodies upon  each other are at all times equal and opposite.  Any imbalance in these opposing forces will create an accelleration in the direction of net force, thus balancing the equatuib. It is these laws,  which will push the less dense material, and that which is denser, in opposite directions.   To say that the cold air pushes the warm up is to say that only cold air is capable of exerting a force.  The warmer fluid exerts the same force in a downward direction causing it to accellerate in the upward direction. To say that only the denser material exerts the force which causes the shift in position is to ignore Newton's 3rd law."
He and I agree that there is a symbiotic relationship between the warmer air and the cooler air.  One cannot act without the other - Newton's 3rd law as stated by Bruce "the forces exerted by two bodies upon each other are at all times equal and opposite".   So it is true to say that "warm air rises", and it is also true to say that "cold air falls".  But it is correct to say either.  And it is easier for my customer to understand that "warm air rises" than "cold air pushes the warm air up".
Thanks for the thought provoking discussion
Ed Minch
In other words, when clapping does the right hand make noise, the left, or are both party to the result?

Common thought is warm air rises and Bud pushed the other way, so now we can meet in the middle.

These forces exist because they have equal opposites. Nice Ed & Bud, I'm clapping with both hands.

I think you may be right Ted, I seem to be getting that rep.  But I'm impressed with the internet and the exposure it creates.  I'm getting positive feedback from many sources.  Although progress is slow and I'm far from a consensus on any improvements in our wording, there are many minds working on this, willingly or simply because they have heard some of the discussion.

One of my concerns is that even when professionals acknowledge that the current wording is misleading, they give in to the simplicity and don't want to risk confusing our customers.  Although avoiding confusion is certainly a concern, I feel it shouldn't terminate an effort to look for an improved way to describe what we see.  I'm preparing an argument on the often stated "short-circuit" theory involving gable vents being used with ridge and soffit vents, and it requires an understanding of this "hot air rising issue".  It will be interesting to see if this helps or hurts this issue.


Bud, natural venting of attics is about as absurd as natural venting of water heaters.  Assumes a lot, often environmental factors cause failure, can impede deep energy savings, and problems created are often much greater than problems avoided.   I would recommend continuing to focus your efforts on productive discussions.  

Stop "spraying" your roof deck with your bath fan and thermal bypasses, and the need for cutting all these holes in our roofs goes away.  

I've been in a LOT of attics.  I've seen the problems.  

I've seen people spend way too much time obsessing about their attics, as if there is some huge problem up there that needs curing through some ventilation magic.  It reminds me a bit of chicken little - I'll coin a phrase - unquantified catastrofication.  Obsessing about a huge catastrophe where none exists.  

"If we don't solve this immigration disaster it will be the end of life on earth as we know it".

Properly separate the attic from conditioned space via air sealing, redirecting venting bath fans through the roof deck or at gable ends, make sure you cure the all too common continuous moisture supply from below, insulate to a very high level, and MOVE ON.   The disease is known.  Cure the disease rather then pontificating about how to properly control airflow IN THE GREAT OUTDOORS, WITHOUT MECHANICAL MEANS (really, some believe you can do that!) to treat the symptoms.  

Maybe you could start a conversation about ventilating walls.  How much wall ventilation is the correct amount?  Should wall solar orientation be considered when people are considering cutting ventilation into their walls?  

I attic ventilation "science" is not what you are dedicating this next topic to.  Yes, I will participate.  


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