Controlling Passive Air Flow

Replies to This Discussion

Permalink Reply by Robert Riversong on September 22, 2012 at 4:32pm

Not at all. It's the same principle: buoyancy.

The leakier the house, the more air temperature stratification you'll get, because lots of cold air is entering at the bottom to displace the warm air to the ceiling (or attic). A tight house has very little stratification, regardless of heat source.

It's like the old myth that wood heat is dry heat. A woodstove makes a house dry only because it's a 24/7 exhaust fan sucking in the same volume of replacement (cold) air which gets warmed and drops in relative humidity. Similarly, a woodstove increases infiltration and hence increases air temperature stratification.

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Permalink Reply by John Brooks on September 23, 2012 at 7:15am

I think Temperature, Moisture and Density are Likely to Stratify

... Pressure WILL Stratify

... Pressure WILL decrease vertically

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Permalink Reply by Robert Riversong on September 23, 2012 at 9:23am

That's an odd way to put it. The less dense (warmer) air will tend to rise, causing air temperature stratification. The air pressure is always stratified because of the diminishing weight of the air column with height (or altitude), but the pressure doesn't "stratify" (verb) any more than warm air rises because of "Hindu levitation".

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Permalink Reply by John Brooks on September 23, 2012 at 11:48am

I appreciate the Julius Sumner Miller reference.

Robert said: "The less dense (warmer) air will tend to rise, causing air temperature stratification. The air pressure is always stratified because of the diminishing weight of the air column with height (or altitude)"

Well said.

How about this for water vapor?

The less dense (moisture laden) air will tend to rise, causing water vapor stratification.

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Permalink Reply by Robert Riversong on September 23, 2012 at 12:35pm

While moist air is less dense than dry air at the same temperature, the difference is inconsequential compared to the density difference between warm and cold air at the same relative humidity.

Air at 70° and 50%RH = 0.0745 pcf

Air at 70° and 100%RH = 0.0742 pcf (or 0.4% less)

Air at 0° and 50%RH = 0.0863 pcf (or 16% heavier)

Air at 35° and 50%RH = 0.0801 pcf (or 7% heavier)

Permalink Reply by John Brooks on September 23, 2012 at 1:53pm

sorry if this post is out of sequence

 

Robert said: "While moist air is less dense than dry air at the same temperature, the difference is inconsequential compared to the density difference between warm and cold air at the same relative humidity."

My source for "moisture laden air will tend to rise" was

Lstiburek BSI-036

I have also read similar comments by Bill Rose.

Permalink Reply by Robert Riversong on September 23, 2012 at 1:59pm

Dr. Joe likes to toss out "counter-intuitive but true things that amaze people in bars late at night" but which are insignificant once you're sober.

The reason that moisture damage is often at the highest point of the structure is because of thermal buoyancy and the fact that warm air at a given relative humidity contains a lot more absolute humidity than cooler air at the same RH.

70° air at 50% RH contains 3.6 times as many water molecules per cubic foot as 35° air at 50% RH, and 8.2 times as many water molecules as 0° air at 100% RH.

Permalink Reply by tedkidd on September 23, 2012 at 7:45am

stratify, to separate or become separated into layers.

You see things as having a natural tendency to separate? That is completely opposite of my understanding. I thought things had a tendency to come together. To equalize.

I think of stratification as referring to air inside. In other words, cool air falling as well as warm air rising. The respective weights being more powerful than the natural tendency to mix, creating layers. Air of different temperatures separating, moving in opposite directions within a container.

I think of stack as introducing air from outside. Cool air coming in and moving up. to me stack means cool and warm air moving in the same direction, following the same path in and through a structure.

To me these are not even close to the same thing. I simply don't believe you can define an air column going in the same direction as stratifying. The exterior forces overcome forces within the column.

For air to stratify the natural tendency for air to mix, the inclination of air temperature to reach equilibrium, needs to be overcome which requires significant energy. Significant delta t.

I don't believe stratification exists in occupied structures unless there is high delta. Just the mixing effect of occupant movement dramatically overcomes any potential for stratification.

Homes are dynamic. If they were static, with no external forces at work (stack, motion, mechanical air movement, etc) you might see stratification but I doubt it. Not without a high temperature difference. Now, if you define stratification as stack, I guess by that definition stratification is common

Maybe moisture goes toward moisture, heat goes toward heat, but I don't think that's correct.

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Permalink Reply by Robert Riversong on September 23, 2012 at 12:58pm

Warm and cold air masses have both a natural tendency to stratify and to mix.

The natural tendency to stratify is called buoyancy induced convection, and the natural tendency to mix is called thermal diffusion. If the mixing tendency dominated, we would have no weather, which is due to air mass stratification.

When you say "stack" I assume you're not talking about a stack of cards but rather about "stack effect" (pressure differentials and consequent air flow due to container height and delta-T).

Air movements due to stack effect convection are hardly just in one direction (in one season). Cold air infiltrating due to negative pressure at leaky first floor windows will drop directly to the floor (you can feel the downdraft under the window just by placing your hand there). The cold air will pool at the floor, lifting the less dense warm air to the ceiling and eventually to the second floor and the attic.

Various forces, including mechanical convection, the motion of bodies, and thermal diffusion will attempt to equilibrate the air temperature, but as long as additional cold air is being drawn in (or pushed in, depending on your perspective), the air will continue to stratify by temperature. Surface resistances and the pinch point of the stairwell will restrict warm air uplift to the attic and create a "pool" of warm air at the ceiling on each level of the house.

This is, in fact, such a common problem in existing (leaky) houses and office buildings, that ISO has a human comfort standard (ISO 7730) that limits vertical air temperature differences from ankle to head to no more than 3°C (5.4°F). People are more comfortable with cool heads and warm feet, and complain of discomfort with the opposite (which is why radiant ceilings are generally a bad idea).

Yes, homes are dynamic, and more so with high delta-T to the outside, which increases stack effect infiltration. While air has a very high thermal diffusion rate (even higher than cast iron), it still takes time for a static stratified air mass to equilibrate. If cold air is constantly infiltrating, then the air temperature will never equilibrate.

This is why a very tight house will exhibit little to no air temperature stratification - because the rate of thermal diffusion exceeds the rate of cold air infiltration.

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Permalink Reply by tedkidd on September 23, 2012 at 2:42pm

"This is, in fact, such a common problem in existing (leaky) houses and office buildings, that ISO has a human comfort standard (ISO 7730) that limits vertical air temperature differences from ankle to head to no more than 3°C (5.4°F). People are more comfortable with cool heads and warm feet, and complain of discomfort with the opposite (which is why radiant ceilings are generally a bad idea)."

 

Nice.  Lightbulbs.  

 

" If cold air is constantly infiltrating, then the air temperature will never equilibrate.

This is why a very tight house will exhibit little to no air temperature stratification - because the rate of thermal diffusion exceeds the rate of cold air infiltration."

I guess that's another way of saying it, although I suspect a very tight home with a wood burning stove WILL experience significant stratification.  

Intuitively it feels like stack and stratification shouldn't be muddled.  Stratification, to me, implies air within a structure experiencing a force that separates it from itself, whereas stack is introduced air that had never previously been conjoined. Density is the common force keeping the two apart in both circumstances.  

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Permalink Reply by Robert Riversong on September 23, 2012 at 3:52pm

Every superinsulated house I've built has had a woodstove as a primary (or only) heating system, and there is little to no air temperature stratification because the houses are very tight (about 2 ACH50).

First, the tighter and more thermally efficient the house, the smaller the BTU/hr output of the woodstove or any heater (a 2,000 SF house I built had a design heat load of only 16,000 BTU/hr at -10°F).

Second, because of the very high thermal diffusivity of air, it thermally equilibrates very quickly in a confined space, if there is little cold air infiltration.

Air stratification is not air "separating from itself", but warm air lifted up by denser cold air, in the same way that a cold weather front lifts a warm, moist air mass and causes a thunderstorm. While there is some mixing at the junction of the two air masses, the dominant effect is uplift and stratification because of the speed or rate of flow of the cold air front.

The same forces operate in a leaky house, with the flow of incoming cold air sufficiently rapid and large as to lift the warm interior air to the ceiling.

[P.S. The term "stack" is meaningless. "Stack effect" is the term professionals use, often more specifically as "stack effect pressure" or "stack effect infiltration".]

Permalink Reply by John Brooks on September 22, 2012 at 3:06pm

My comment was about Straube's formula [3] in BSD-014

the coefficient is based on standard temperature and pressure

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