Heat Stratification in a Tight House
Here’s a building science/theory question. During heating season, houses are often much warmer at the ceiling than at the floor. You can notice this going up the stairs. In order for this to happen there has to be a temperature difference in the air, and I would think that would primarily be due to infiltration, with very cold outside air entering the house.
In a very tight house there is less infiltration, which I think would mean that overall the air within the house would be more uniform in temperature. Absent infiltration the tendency of air in a contained space will be toward uniformity in temperature as any warmer air loses heat to cooler air.
Am I right? Does anyone have real-world experience to support or contradict my theory?
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Replies
DC,
No, you are bang on. That's one of the big advantages of well air-sealed and insulated houses - but you need both to avoid stratification.
I'd sort of agree with Malcolm that a well-sealed and insulated building would have far less problem with heat stratification. However some stratification would still occur. The main difference is that with heat rising (that's always true), the heat at the top of the building is more likely to escape from a leaky building, and cold air is most likely to get sucked into the building at the bottom. Hence, heat stratification is magnified by infiltration of cold air.
The reverse is also true in summer. If you air condition the upper floor of a building, cool air is likely to fall down stairways or other open spaces to floors below. So an air conditioner located on the upper floor of a home could help cool the lower floor (cool air falls as heated air rises), but cooling air on the lower floor would not effectively cool the upper floor. In winter, the reverse: The heat generated on the first floor would rise to the upper floor more effectively than heat on the upper floor moving to the lower floor.
Another factor: Heat moves not only from convection (air currents, described above), but also from conduction (heat moving through objects in contact). So heat can move through walls, ceilings and floors to other rooms, but those do have resistance to heat movement (measured by R-value).
So this issue involves insulation of the building. If a building is both airtight and very well insulated, then the internal heat will stabilize throughout the building. Typically, interior partitions, ceilings and floors (not facing the exterior) have fairly low R-value. Exterior walls, ceilings/roofs facing the exterior, and lowest flooring of the building have higher R-values. If the insulation of the building's exterior shell is very high, interior heat during wintertime will move between rooms much faster than it moves out of the building to the exterior. So heat becomes better distributed inside the building if the building shell is both air-tight and well-insulated.
Bottom line: For a stable and well-distributed indoor temperature, seal the building very well, and insulate it very well, to reduce cold air entering (more at the bottom) and allowing heat to dissipate throughout the building evenly.
For a real-world example, here's one involving air-conditioning. My elderly mother was insistent on not allowing any window air conditioner in her small Cape style home in New England. She claimed it dries out her skin. (However she sat under ceiling fans, which cool people by moving air rapidly across the skin, increasing evaporation of sweat, and drying the skin!)
On one occasion, I visited during our hottest summer weather, and found her miserable with interior temp of 88F and high interior humidity. (A medically risky situation for an elderly person.) Since she was insistent on no window air conditioner, I installed one upstairs in a bedroom, which was connected to a 10' hallway to a steep stairway to the living room on the main floor of the home. Over a few hours, the humidity of the first floor decreased significantly, and the temperature of the living room dropped. Eventually, the temperature of the kitchen and bedroom dropped, as they were connected to the living room by a small central hallway. And eventually she realized that the house was more comfortable with an air conditioner operating, so (with some nagging), allowed the air conditioner to operate in her main floor bedroom during the day when she was not in the bedroom. (It would gradually dehumidify and cool the main floor.) However, the air conditioner in the main floor bedroom failed to cool the upper floor bedrooms and hallway.
During the winter, the upper floors had minimal heating from the central heating system (no return air duct, and ineffective uninsulated supply ducts with long duct runs, definitely not a balanced system!) Opening the door at the bottom of the stairway would allow heat to rise up the stairway, heating the upstairs modestly. You could feel the cool air breeze exiting the door at the bottom of the stairs, despite no fans operating. So heat would rise to the upper floor moderately effectively during winter, but cooler air in summer would not.
"The main difference is that with heat rising (that's always true)"
That's not actually true--heat goes in whatever direction there is lower energy. Warm air, however, is more buoyant than cooler air that is otherwise equivalent. In a tight house there is very little stratification.
I don't think you need outside air to create stratification
But in a tight house you have less volume of less hot air from the heating system, meaning I would think less chance of stratification
A poorly designed heating system, IE heat upstairs, might still have some stratification.
I happen to measure this yesterday. I had 0.5C between floor and ceiling peak (14'). So not much.
In the summer with AC you do see much more though. Because there is no cooling up high, you get much more stratification. I remember measuring around 3C.
Thank you for providing hard numbers!
So here's the context of the question: I'm considering ceiling radiant heat for a new build. First question from the architect was "won't all the heat stay at the ceiling?"
I think in a well-sealed, well-insulated house it won't because there will be little convection. Air-to-air conduction will tend to even out the heat in the room. To the extent there is radiation from the ceiling to objects below that will tend to warm the air from below which again will lead to evening.
Radiant heat is a third type of heat transfer (other two are conduction and convection). Examples are your proposed ceiling radiant heat units, the sun shining on you, and a hot wood stove. Heat jumps through space or air, from the hot object to cooler objects. Radiant heat panels would heat objects in the room; and those objects would then distribute heat to the surrounding air by conduction and convection. So the ceiling radiant panel would not just heat the air at the ceiling as the architect believes, radiant heat would heat objects below the ceiling panel.
Radiant heating tends to be perceived as very pleasant when the environment is cool or cold (e.g., heat from the sun on a cold day, or heat from a wood stove in a cold room). Central heating that distributes hot air to a cold room are perceived as less pleasant; Objects are still cool to the touch until the air finally transfers heat to the objects over time. Air doesn't have much thermal mass (heat capacity) so it is typically hotter than the temperature desired in the room, to move enough heat to heat all the objects in the room more quickly. At first that hot air might feel good in a cold room. But when the room is warmer, the hotter than ideal air temperature is not perceived as so pleasant. So radiant heating units or radiators are more pleasant than hot air distribution systems.
Even radiators -- despite their name -- transmit heat through a mixture of radiation and conduction to the surrounding air. The heat transmitted through conduction is then distributed through convection depending on what the air in the room is doing.
One thing I'm trying to figure out is the relative share of radiation vs conduction. Ideally there would be 100% radiation. If it's mostly conduction I would expect a temperature gradient from ceiling to floor. Very simplistically, imagine a room with a heat source at the ceiling and an insulated floor exposed below to the outside. For the purposes of simplification assume the walls have infinite insulation. There will be a temperature gradient from the ceiling to the bottom of the floor. At any given point the temperature will be determined by the amount of insulation above and below. So the temperature above the floor is determined by how much insulation is below the floor and how much insulation the air between the floor and ceiling provides (which is itself a tricky question).
However, if radiation is a significant method of heat transfer the radiation is hitting the floor itself and warming it directly. So the relative contribution of conduction and radiation matter.
One of my concerns is that radiant panels are typically run at lower temperatures that radiators -- maybe 110F as opposed to 180F. The formula for radiation depends on the fourth power of the absolute temperatures of the two surfaces; plugging in those temperatures and a room temperature of 70F I get about 3.3 times as much radiation at 180 as at 110. But ceiling panels could easily be triple the size of a regular radiator so I think you could get the same net radiation.
As others have said in various ways, you will, have stratification whenever there is a source of heat (heat air duct, radiator), or a source of cold (infiltration, walls, windows), and some convection, or a source of heat at the ceiling. It's an interesting question, surely someone has done some testing? There are probably some academic papers, but they are hard for laymen to find. 110F does seem kind of low to get a lot of radiation. Since it's at the ceiling you won't have natural convection. You will have conduction through the air and if that will give you a temperature gradient. You could make a rough calc with the radiative heat transfer equation, and the heat transfer parameters for the floor & furniture. My guess is that it would be similar to having a heat register high on the wall.
We have radiant cove heaters and never noticed it being hotter near the ceiling. In fact, if anything I would say the effect is greater using the high wall mounted minisplit.
You have to first figure out why the ceiling heat makes sense.
For hydronic, the part and labour cost is about the same but you need almost 1.5 to 2 times the surface area. I guess if you have a high temp heat source (ie outdoor wood boiler) it might be better but than you start running into the temperature limits of drywall.
Electric would be similar (floor heat vs radiant ceiling panel), maybe cove heaters VS baseboard might make sense for interior space.
Ceiling appeals to me for a number of reasons. Performance doesn't depend on the placement of furniture or floor coverings. You can run it at a hotter temperature than a floor because you don't touch it.
Radiant floors have to operate inside a narrow window, you can't really have the surface temperature above 100F or it becomes uncomfortable to walk on. But that's not much of a delta from room temperature. If the heat loss of the room is more than calculated or the heat contribution of the floor is less than calculated, you don't have a lot of room to make adjustments once the floor is installed. With a ceiling you have more leeway.
Most well insulated houses need relatively small amount of floor heat. Even with the stricter temperature limits, if you cover the entire area, the floor will never get hot enough to feel it. For comfort, you actually only want to put the floor heat in the higher traffic areas that way you get that warm toes feel when it is cold outside.
For example, I'm easily heating a 4ACH50 balloon framed house with just blown cellulose walls and 20 year old double pane windows in zone 5 with floor heat. You need a ridiculously lossy house to not be able to heat it within the floor temp limits.
Furniture is not a problem. You do have to watch for carpeting though. Smaller carpets are fine, larger shaggy ones can be an issue.
DC, here is some real-world experience. I live in an extremely tight SIP house in CZ 5 that's taller than it is wide. I have three floors on a 28x18 footprint, and the blower door result was something under 0.5 ACH50, below the limit of the tester's equipment. Unfortunately, I have serious air stratification issues. There's an open stairwell between the floors, so air movement is quite free to occur. In hindsight, I would have designed things differently. I should have installed more insulation in the finished basement (currently have R-15 continuous interior on walls, R-10 under slab) and I would locate my HVAC differently.
I have two 9K minisplits: a ceiling cassette on the first floor which handles all wintertime heating, and a mini-ducted in the peak of the second-floor cathedral ceiling which is used for cooling in summer.
On these cold-ish winter days, the basement can be up to 10 degrees colder than the upper floors, and that gradient seems noticeable in our feet as we walk on the first floor. Of course, the only active heat source for the ENTIRE house is in the ceiling of the first floor, a full 16' above the basement floor. It just can't push warm air down that far. In hindsight, I would substitute the ceiling cassette for a mini-ducted in the floor (basement ceiling) to serve both the first floor AND the basement.
Another contributing factor (the main culprit?) is the HRV, which in my case is a relatively inefficient unit dumping barely conditioned fresh air into the basement and upstairs bedrooms. It's only 30 cfm spread over three rooms, but I'm not willing to damper off the basement, so perhaps spending up on a better HRV would have been prudent.
Andy, I would say that a 10°F temperature difference between an unheated basement and the second floor is actually quite good. In leaky homes the difference could be like 20 or 30 degrees, or more. Without a heat source in the basement, even with decent levels of insulation, it is a heat sink--it wants to be close to the ground temperature, roughly 50-55° where you are. If it's warmer than that, and I bet it is, it's from heat coming from upstairs and miscellaneous heat loss from water pipes, water heater, etc..
Have you considered adding inexpensive electric resistance heat in the basement to take the edge off? I'm currently designing a basement fit-out in Maine with specs not unlike yours; the heat load for the 600 sq.ft. space is only 1100 watts, and that's to keep it at 70°.
Why are you supplying air to the basement? Even efficient HRVs supply colder air than ambient indoor conditions. I usually extract air from the basement unless it includes living space.
Didn't even occur to me that 10 degrees was reasonable! #tighthousesnob. The basement gets an HRV supply because it is a bedroom (has a casement egress window.) And indeed, I have a 1500W electric baseboard heater in there, which is just fine for keeping the room comfortable when/if occupied. Its just that the COP of 1 feels like a design fail, and if the bedroom door is ajar, that "dirty" heat escapes continually up the stairs to mingle with my COP 3-4 minisplits!
Andy, if it makes you feel better about your "dirty" heat, consider that if used intermittently, it's probably costing you on the order of $100 per year (maybe $20, maybe $200) to operate--the upcharge to go to a nice mini-split would cut that by 2/3 but would cost a few grand--hard to justify the return on investment for part-time use. If the upstairs is at the same temperature as the basement, or higher, then the bedroom heat isn't really escaping unless it's being driven by air leaks.
Andy makes a really important point. Even in a very airtight and well insulated home the distribution of heat is critical. In my house with gas forced air, the furnace runs 3 times per hour distributing heat evenly to both levels. My current temperature difference between the floor and ceiling on the main level is less than 1F. This house was built in 1978 by others but has been upgraded thermally, especially in the attic with air sealing and copious blown insulation. The temperature in the lower level is always cooler because the basement slab has no insulation, sub slab insulation in a cold climate is a must.
I have been monitoring my neighbors house this winter for gas usage after a ceiling insulation and air sealing retrofit. Some ceilings (60%) were cathedral with the balance conventional flat ceilings over the bedrooms. The cathedral ceilings were a compromised R-22 and the conventional ceiling was about R-30, both leaked enough heat and air to cause ice dams each winter. Both ceiling areas were brought up to R-50 and made as airtight as possible. The cathedral ceilings have a dedicated airspace below the roof decking, the flat ceilings have eave and ridge venting and have been air sealed with blown cellulose added to R-50.
Gas usage so far for the neighbor's house is 24% less this winter than before the retrofit using normalized weather data. We did a blower door test prior to the retrofit and will do another post retrofit to see what improvement was made to the ACH50/CFM50. I will be interested in the final blower door test to put some numbers on energy saved due to infiltration reduction and add this to the calculation for the thermal improvement for the ceilings. I may find the reduced gas usage is more than the the 2 improvements made would indicate using standard calculations but we shall see. Trying to understand the value of making an airtight (as possible) ceiling plane in an existing home.
I am encouraged by this project as ice dams have been eliminated and snow now lays on the roof where before it melted due to heat loss. I firmly believe energy efficiency for a building must include an airtight and highly insulated ceiling area. The stack effect is held in check and interior temperatures will be more even throughout.
It is nice after completing an air seal / insulation project to go out on a cold morning and see the improvement in the frost pattern on the roof :-)
I’ve seen similar results to you when I spray foamed a VERY leaky cathedral sealing in my own home. Other projects have been smaller, so less of an impact on energy use, but still noticeable. A recent renovation on my home office has made it MUCH more comfortable just because of air sealing work! It’s really worth the effort!
Bill
Great thread and something I've been battling for over 2 years. I have a LEED certified home with an ACH of 0.8 (close to passivHaus). I have the misubishi Hyper heat mini split system through out the home as the main source of HVAC. What I've noticed is a significant (and annoying) heat differential between the floor and ceiling. Granted we're dealing with 14-16' Ceilings through-out the home but it's a pain seeing the evaporators sensing 80 degrees in the ceiling while the coffee table is 68. This means there's a lot of stagnant hot air sitting up in the ceiling area (I have R-70 in the roof). No matter what is said about entropy, diffusion, etc. Hot air (not heat itself) rises because when you heat air (or any other gas for that matter) it expands. When the air expands, it becomes less dense than the air around it thus move upwards.
As I've spoken to HVAC experts and Mitsubishi product engineers in length about this issue with the units the best way to address is to use the OEM thermostat and change the ceiling height settings and put in ceiling fans.
As of now I've added 3 celling fans that can rotate in reverse for the winter and it's truly night and day. Will be adding ceiling fans for each room to resolve the issue. Going forward I would recommend every builder and home using wall mounted units in a tight home to have a ceiling fan for the winter (summer it purrs perfectly) or simply go with the floor mounted configuration but you're no longer going duct-free and that has additional thermal loss (moving air through vents). Pros and Cons to think through.
Applied Build Science, what levels of insulation do you have? The ideal of very little stratification depends on both extremely good insulation and excellent air sealing.
R-49 in walls, R-70 in roof. And 0.8ACh in house.
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Thanks. That's really interesting. I would not have expected that much stratification with such good insulation and such good air sealing.
From a really simple point of view, if the ceiling is at 80 and the furniture is at 68, you should have radiation heat transfer of something like (80-68)/1.2 = 10 BTU/hr per sq foot. (The radiation heat transfer resistance should be about R-1.2.) If the outside wall area in a given space is equal to 2X the floor area, and it's 0 F outside, that loss is about 4 BTU/h per sq foot of floor area. So where is the rest of that heat going? Maybe into the floor if there's a cold basement under the floor? Or maybe the 0.8 ACH of leakage, though small, happens to be in spots that cause trouble?
From a practical point of view, mounting heat sources low make a lot of sense, with a ceiling fan in summer if needed. From a physics point of view, it's surprising to me that it's as bad as you say in the house that's that well insulated. Thanks for the interesting and useful information.
Basement is fully enclosed and part of the thermal envelope--We're looking at 5400 SQ/ft total. It's simple--heat rises and some level of stratification occurs to a degree. It's worthwhile having heat sources lower and some form of mechanical destratification during the winter (aka. Ceiling fans).
On average it's more like 75 floor-80 celling but on -15 degree days outside the difference is larger.
I just realized the piece that I was missing: heat loss from windows. Given your excellent insulation in the walls and ceiling, and very good air sealing, that's a major part of the heat loss. So cold air falling off the windows pools on the floor. The radiation from the ceiling to the floor balances with that cooling effect (plus the smaller heat loss through the walls) when the temperature difference is 22 degrees between the ceiling and the floor, unless the fans mix it up.
If you had a windowless room with the same level of insulation you'd have much less stratification and probably wouldn't need the fan. But fan plus windows sounds much nicer than a windowless room!
I realize most of this thread is old now, but it's interesting nonetheless.
I've often struggled with temperature stratification vs stack effect. I will offer this: (it's much along the lines of what Lucas Durand offers in this oldie thread: https://www.greenbuildingadvisor.com/question/what-must-we-say
see his comments #43)
If there is no engine to move air, there is no driver for stratification or stack effect alike. By engine I don't mean 'fan' but source of energy.
The implication is that the greater the energy input to maintain a steady interior state, the larger the engine to drive air movement/stratification.
Temperature, even of air, will seek thermal equilibrium, not stratification. It is only by adding heat that we create conditions for stratification and stack effect. If we eliminate all air exchange with the outside, we eliminate stack effect, but there can still be stratification due to energy inputs needed to maintain steady state.
This is inevitable, so some stratification is inevitable (but can be mitigated by mixing currents)
But a well sealed and insulated structure, as Malcolm states in #1, reduces the engine of stratification. Likewise, a smaller house will reduce this engine.
Stratification is the manifestation of thermal equilibrium just at the wrong altitude :-)
Perhaps... I have to admit I'm not sure I really understand what you're saying. I think I might be saying the opposite though: that hot air will not 'trend towards' the ceiling without an engine, but rather will seek equilibrium. Equilibrium implies diffuse temperature throughout the system.
Remove the engine, remove the stratification. Obviously that's not possible if we want warm houses, but if we insulate and airseal well, the engine will be smaller, and slight mixing currents will be more likely to even out temperature stratification.
Right, if you have a building where all of the heat loss is at the bottom and all of the heat gain is at the top the equilibrium would be a continuum from hot at the top to cold at the bottom, even with no air flow. The floor temperature would be determined by the r-value of the air between floor and ceiling compared to the r-value of the insulation.
A system with heat gain 'at the top' and heat loss 'at the bottom' is not in equilibrium. So I'm not sure what's being said in this case.
To be fair, equilibrium won't happen until a house's internal temp is the same as outdoors. My reference to it was a sort of hypothetical closed system—an internal equilibrium.
In this hypothetical closed system (picture an air-tight container in space, super-insulated and shielded from radiation): if a heat source were placed in this container, temperature differentials and stratifications* would occur (depending on specific turbulence/currents). Once the heat source is removed, this closed system (now at a higher energy state) will seek it's own internal thermal equilibrium—it will not seek a state of thermal stratification. Perhaps this is obvious, but it wasn't to me until I thought about it.
*or would it, since it's in space? ;)
In the real-world, what we're mostly dealing with are steady states. See this cartoon of a video: https://www.youtube.com/watch?v=j--5dZ_zHs8
And since a house system will lose heat to the surrounding environment, stratification can occur due to that, even if all it's doing is cooling down and approaching equilibrium with the environment.
This has me thinking that the concentration (temperature) of the heat source, not just the total amount of energy, would affect stratification. It seems if the energy is concentrated (such as a small woodstove) and the air temp reaches a higher delta T with the overall average set-temp, that air will have more buoyancy than a diffuse, cooler heat input. (Keith said this is #4 it seems).
DC, I read your comment again, and your #14 where you originally state this gradient notion. I think I get what you're saying, sort of. Basically you can have a hot ceiling and a cooler floor, and it won't necessarily increase total energy loss other than from a marginal delta T increase at the ceiling. The steady-state will simply be a gradient, which is true of other heating systems as well, like wood stoves.
It's just a matter of whether you are fine with this temperature gradient. (Will your head be warmer than your feet, and do you care?).
> Will your head be warmer than your feet
Or will the upstairs bedrooms be warmer than downstairs? This is definitely a comfort problem.
Yes, more emphasis should be put on making sure there is good mixing as air leaves the heat source. And zoning is often needed. Both even with lots of insulation and perfect air sealing.
Interesting topic, there is a strong correlation between dew point and energy usage in a residential distribution system. So if you know temperature and the dew point/humidity trends you can forecast energy usage. This is best illustrated by visiting New Mexico at 90F and Houston at 90F. Similar to Minnesota at 20F vs Philadelphia at 30F, Minnesota “feels” warmer, because humid air conducts heat better. Again air sealing and control of humidity is important to comfort.
The engine for rising warm air is the cold air pushing it up in the same way that water pushes up a boat. Water is heavier than air, so it acts to displace or fill the air space created in the water by the boat.
Assuming that a house is completely air tight, but can lose heat conducting out through the walls; the interior air in contact with the walls will cool by the heat loss through the walls.
The cooler air near the outside walls is denser than the warmer air toward the center of the interior space. So the cooler, denser air near the walls falls down to the floor along the walls. That cooler air falling down along the walls, then moves to the center or the interior and lifts the warmer air to the ceiling. The warm air then is forced to spread out over the ceiling, and to then come in contact with the cooler walls where it begins the cycle over again.
It is a classic convection cycle of heat transfer. It does not involve heat being lost by being carried by air through air leaks.
If this effect is objectionable, there is too much heat loss through the exterior walls.
Nice description of the convective loop Ron.
I might disagree slightly that the cold air is the 'engine,' but it's mostly a matter of language and intent. Perhaps most accurately, the engine is the entirely of the system: the outside environment, the walls, the inside air, and the energy input via the heating source. Differential pressures certainly drive motion, but the creation of those differential pressures is arguably integral to 'the engine'.
In your boat analogy, there has to be a leak to represent energy loss through the envelope. The boat is slowly filling with water. To maintain a floating boat, we must pump this water out. If cold air (or water in the case of the boat) is the 'engine,' as you say, then our engine is really nothing more than a sea in which our boat is sinking, trending towards equilibrium. Not what I typically associate with an 'engine.'
A hot-air balloon is the same idea. One could list a large number of reasons 'why' it is rising, and many would be correct. Cause and effect may be the befuddling concept here. The only scientific principle privy to it is entropy. And so perhaps the best ascription of 'cause' to anything is that which increases entropy. (what the heck am I talking about?)
To further obfuscate with another analogy: take the case of a vacuum water pump. What is the 'engine' in that case? The atmospheric pressure (?caused? by... gravity?), the vacuum itself (absence of air), or the physical pump powered by some fuel. If I had to choose, I'd say the pump is the 'engine' in strict mechanical terms, but the phenomena of water rising (up to 33.9 ft.) is described by the system conditions: pressure differentials, defined at the limits by the state of a perfect vacuum, and the state of atmospheric pressure.
In this case— and in the case of "does hot air rise, or does cold air push it up"— I believe the DELTA should be the focus, not the 'cold air' nor the 'hot air'.
Perhaps similar to how energy alone is useless if it is too diffuse: if entropy is too high. If the sun surrounded the earth, evenly on all sides, we would have no engine to sustain life. Plenty of energy, but no engine. We rely on the sun, but we also rely on the coldness of space. How's that for a race to the bottom of pedanticism?
Stratification tends to be a reality and, in a "tight" house, one may want to look at each and every heat source and take those into account.
Recent modeling on a superspreader event accounted for people (5 kW) but not lighting or powered JBL speakers mounted at the ceiling (maybe 3kW or more). It may be that stratification (as in an inversion) "trapped" virus in a lower layer of the room such that the entire volume of the high ceiling room may not have been the effective volume to use in calculating concentration of the virus where the people were seated (and infected). Some analogy to inversions in valleys, which are notorious for trapping air and increasing pollution levels.
Relatively decent peer-reviewed studies show that energy from ceiling lights helps to stabilize thermal stratification. Vertical height, of course, is very important too, many high ceiling structures have two distinct gradients (slopes), for example, one from the floor to about 8' and then one above. Many years ago worked on an NREL stairway stack cooler (one doesn't need 20' or more in vertical rise to get substantial stratification, 10' will do).
"Study the Thermal Effect of Ceiling-mounted Light Fixtures for a Displacement Ventilation System": "ceiling-mounted lighting fixtures are considered to stabilize vertical temperature profiles".
https://www.aivc.org/sites/default/files/members_area/medias/pdf/Conf/2008/Paper_141.pdf
At #22, don't forget that the ceiling fans generate heat too (maybe even phantom draw heat when they're not spinning). If you search around, you can find numerous studies that look at computers (servers, workstations, etc.), where thermal stratification has been elaborated in detail (through both CFD simulations and reality). Watch out bitcoin miners.
My house has similarities to #22 and I have thermal gradients that I have mitigated to some extent with flow via horizontal and vertical louvers in my supply/return ducts that I adjust, along with fan speed of the Mitsubisihi SEZ-KD15 (a MERV 8 filter for lesser pressure drop, higher CFM and more FPM throw). I have an anemometer and an IR thermometer that I've used for tweaking to reduce stratification.
In a "tight" house, it may be that each and every electronic device (e.g., set top box, lights, etc.) and person (large dogs too?) will add heat that will heat air and that air will rise. So, a "tight" house may help to avoid flow (cold leak at floor to hot leak at attic) but it is likely to have other thermal stratification related concerns.
Some light reading: "Determining thermal stratification in rooms under mixing and displacement ventilation", PhD Dissertation, MIT, 331 pages: https://dspace.mit.edu/handle/1721.1/104255
"CFD study on the effect of Archimedes number and heating rate on the thermal stratification of a ventilated office"
https://www.researchgate.net/publication/329183459_CFD_study_on_the_effect_of_Archimedes_number_and_heating_rate_on_the_thermal_stratification_of_a_ventilated_office/link/5bfbc7c0458515a69e3bdc47/download
A bit lost in all of this discussion of the "driving engine" of convective stratifcation is the temperature at which heat is delivered.
The rising column of air coming off a 220F steam radiator or a 300F wood stove has a MUCH higher temperature than the room or house average. The much lighter very hot air will tend to pool near the ceiling even if the house is tight and the heat load is low.
At the other end of the spectrum, a radiant floor is but a few degrees warmer than the room /house average. With no concentrations of superheated air to rise quickly, convection drives are minimal. Even in fairly lossy rooms/houses with radiant floor heating the stratification is small.
In between, low temp (<120F) panel radiators create some amount of stratification, with the more buoyant air warmer (but not scorching hot) air rising, but that is counteracted by the cool (but not frigid) cascade of air coming down the face of windows. If the rads are near or below windows the mixing can be quite good, and there won't be bubbles of 90-100F air heading toward the ceiling or puddles of cool air near the floor.
With reasonably designed/specified low temp air systems (ductless or ducted) the mixing within a room is good in a tight home, but in a leaky home there can still be significant stratification if the basement is being cooled by entering infiltration and the attic is leaking warm air. A cold first floor combined with a warm second floor is still a likely leakage symptom.
Yes, and the low temp panel radiators also deliver a decent fraction of their heat via radiation, and thus deliver it around the room more evenly. (Depending of course on the depth of the radiators.)
If you have excellent air sealing in the ceiling, and excellent insulation above it, stratification isn't necessarily a bad thing. You effectively get a radiant ceiling heating system without the expense of installing hydronic tubing. But if your air leakage is out the ceiling (as is common), 100 CFM of 80 F air leakage is 1080 BTU/h worse than the same leakage of 70 F air. And if means minisplits have 80 F air in their intake, that lowers COP.
One more note: in houses that have the problem you note at the end--a warm second floor and a cold first floor--people often try to solve the problem by cracking open windows on the second floor, hoping to let in cool air, but in fact they just boost the stack effect flow, making the problem worse.
In our 1910's bungalow, which is not super leaky but could stand to be tightened, we are able to maintain a temp of around 50 degrees F upstairs (only used for sleeping) and 65 +/- downstairs for two main reasons:
1) There is a door to the stair well, so levels are mostly separated, and
2) There's essentially no heating unit upstairs. (A tiny run of hydronic baseboard upstairs compared to plenty of it downstairs and a pellet stove in the one room with a thermostat so the baseboard basically isn't even on when the pellet stove is).
This is certainly not a set-up one would emulate (and we'll probably make some changes someday) but hey... the upstairs sure ain't hotter :)
>"A bit lost in all of this discussion of the "driving engine" of convective stratifcation is the temperature at which heat is delivered."
Lost perhaps, but there if you look Dana. In a handful of comments. Appreciate your clarity on the matter.