I'm opening a separate discussion under a new thread as this one: http://homeenergypros.lbl.gov/forum/topics/locating-the-neutral-pre... was getting overly congested and this topic deserves its own space.
Homes need to be tight to be energy efficient, but then comes the question of how to provide the necessary fresh air that we and our houses need. Robert Riversong (above thread and several others) has posted some great information on passive vents and the economics of simple exhaust venting that I think offers a good alternative to expensive H/ERV installations. However, I feel there needs to be a better understanding of just how static venting works.
Most of us in the energy business have read about, used, or advised on using some form of passive venting for replacement air that involves a form of air trap. Robert posted his version and mentioned the "Saskatoon Loop" as methods of restricting the unwanted air flow while still providing a path for the desired air flow. I have looked at the "duct ending in a bucket" and the "loop up at the bottom" cold air traps in the past and concluded they are not exactly what they appear to be. Essentially they modify the height and resistance of the flow path, but otherwise do not act as an air block.
Since the explanation of the above can be long, I have put together a simple statement that I feel conveys the guidance we need when designing and installing passive vents, at least some of the guidance.
"For any fresh air vent duct passing from inside a home to the outside (under natural pressures), the effective pressure from end to end of that duct is the stack effect pressure (wrto) at the height of:
1. the outside opening when the duct is filled with inside temperature air.
2. the inside opening when the duct is filled with outside temperature air.
3. the penetration through the envelope when outside is filled with outside air and the inside is filled with inside air."
I haven't reviewed this for summer conditions, but I believe the statement will hold.
When any kind of winding path is filled with the same air as is around it, it might as well be a straight shot, if the structure allows. Alternatively, if a straight shot is not possible, a winding path will not alter the effective air flow, other than adding a bit more resistance.
The bottom line is, passive venting should follow and use the internal pressures within a home, positive, negative, and that somewhat elusive NPP.
John is very good at challenging or explaining many of my statement and he creates great artwork, so I'll post this and see what we get for input from all.
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.
I think Temperature, Moisture and Density are Likely to Stratify
... Pressure WILL Stratify
... Pressure WILL decrease vertically
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".
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)"
How about this for water vapor?
The less dense (moisture laden) air will tend to rise, causing water vapor stratification.
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)
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
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.
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.
"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)."
" 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.
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".]
My comment was about Straube's formula  in BSD-014
the coefficient is based on standard temperature and pressure