Measuring Impact of Shading on Solar Gain
How do we measure the impact of direct solar gain? Intuitively this seems to be a significant variable that is ignored. Not long ago, I did a video in which we took white and black bowls and filled them with water. First, we set them in the shade and measured the temperature in each. Not a huge delta. Next, we set them in direct sun, and the delta went way up. Our original intention was to show the impact of color, but we were also demonstrating the impact of direct solar gain. R values are easy to calculate. How do we calculate the impact of improved shading in cooling climates and the reverse in heating climates?
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Good questions. I would think modern energy modeling programs can give a breakdown. Modest glazing, appropriately oriented will daylight a building and earn some of it's keep in the form of solar gain.
Is modeling done that specifically? We do solar shading studies on all windows, but we are designing by the seat of our pants on this. We say we want no more than 10% direct solar contact at 10:00 am on the East side of the house and no more than 10% at 6:00 on the West. There is zero real data connected to that formula. Also, what about the rest of the wall? Material is a significant variable in this consideration. I'm in a hot, humid climate. How different is the impact of solar gain hitting a thermal mass such as brick or stone instead of siding with a rain screen? I sense that this is a significant issue but that it is too multivariable to get easily captured.
I've given a lot of thought to this too.
I keep my thermostat set to 68 degrees in the winter. On bright sunny days, it can be 45 degrees (probably even a bit colder) and the furnace stops running at about 10am because the solar gain is enough to keep the house at 68+ even when its 45-ish outside. On a cloudy day, the furnace would still be running occasionally even when it's in the 50s-low 60s outside.
So the solar gain on bright sunny days seems to be equal to at least 10-20 degrees worth of ambient temperature?
I imagine somebody could probably quantify it a lot better and more scientifically than I can.
Borst has calculators for this. Not super trivial. Especially if you have a lot of windows and a low energy house, definitely something you don't want to overlook. I haven't used PHPP or WUFI, but I believe this is taken into account by them as well.
EDIT: I realize you are talking about the whole building, not just the windows. Not sure on that one, seems like it's worth taking into account though.
When I did my Manual J, I measured the area and orientation of every window, input the SHGC and calculated the solar gain. This came out to nearly 50% of the total cooling load.
When calculating conduction through the walls, I applied an average correction for the fact that the surface might be hotter than ambient. My design temperature is 92F, my interior design temperature was 75F. I figured exterior walls would have an average temperature of 109F on the design day, and the top of unvented roofs would have an average temperature of 150F.
In modern construction walls are so well-insulated that it's not a big adjustment.
If the sun shines on a wall all day, it will be hotter than ambient. If that wall has a high thermal mass, such as brick or stone. It will act like a heat battery pushing heat to the house until the outside temp is lower than the material. In a hot humid climate, that may not happen even through the night. This could have implications not only for overall heat loads but also for the type of material we consider.
"I sense that this is a significant issue but that it is too multivariable to get easily captured."
I think the question might be whether beyond broad stroke strategies, trying to manipulate it is worth the effort?
It's not like this is something new. The relationship of solar and thermal mass was one of the dominant concerns of high performance builders right through the 70s and 80s. That the approach has been marginalized says something about the difficulties of employing it effectively, and the demonstrated effectiveness of strategies that largely ignore it.
I would go further and say that the early days of passive solar was a time dominated by a belief based upon intuition, and that intuition turned out not to be backed by the science.
If the walls have heat capacity, that capacity only comes into play on days when the building fluctuates between heating and cooling. On days when the building needs all cooling or all heating, the heat capacity slows the flow of heat but does not change the net heat flow in the building.
In the shoulder season -- days when you cool during the day but heat at night -- the amount of energy used tends to be very low, and conventionally constructed buildings tend to have enough heat capacity to remain comfortable.
>"If the walls have heat capacity, that capacity only comes into play on days when the building fluctuates between heating and cooling. On days when the building needs all cooling or all heating, the heat capacity slows the flow of heat but does not change the net heat flow in the building."
Isn't the purpose of thermal mass (heat capacity) in passive solar design partly just to shave (dampen) the peak temperature spikes due to irradiance? Like a 'mass damper.' Even if energy budget is more or less the same, it reduces those spikes in homes with lots of glazing-- for comfort reasons, no? (Note I'm not advocating for passive solar design, just saying why it utilizes extra heat capacity).
I would say that's a hypothetical question, I've never heard of a passive solar house that was designed in the sense that a modern engineer would accept -- using computer modeling to predict energy flows. If you're not doing that you're just guessing, so using words like "purpose" isn't really appropriate.
Let me ask you this: have you ever lived in a house with so little heat capacity that you had to use both heating and air conditioning in the same day? Because I haven't. One of the flawed assumptions of passive solar design is that somehow houses normally are lacking in heat capacity, so it needs to be artificially added. That just isn't true.
"have you ever lived in a house with so little heat capacity that you had to use both heating and air conditioning in the same day?"
I'm not sure I understand how your response is in reply to my point. I'm saying thermal capacity is not only relevant when heating and cooling are required in the same day. Whether there is 'enough' is another question. I am merely saying your assertion that it 'only comes into play' when one heats and cools in the same day does not appear correct.
"using computer modeling to predict energy flows. If you're not doing that you're just guessing, so using words like "purpose" isn't really appropriate."
Oh freaking please!
Sure one should design and model as necessary, but that doesn't mean the word 'purpose' is inapplicable. Whether or not something 'works' is a different discussion, and depends on the specifics. I'm pretty sure there are plenty of houses/systems designed with computer models, etc. and still fail to 'work' as intended.
The basic logic of passive solar isn't entirely flawed in principle, and I am pretty sure there are a few example of success. It's just hard to do because the parameter's are much less predictable than on-demand heating/cooling systems. The band of conditions that it 'works' in is quite narrow. It is also that comfort is harder to achieve. I'm not here to defend passive solar principles.
But... one PURPOSE of thermal capacity when significant solar gain is designed into a structure is to dampen the inevitable influx of heating energy beyond it's present need. Capacitance. It doesn't always work perfectly (whether 'properly designed' or not), but that is one theoretical purpose.
"In the shoulder season -- days when you cool during the day but heat at night -- the amount of energy used tends to be very low, and conventionally constructed buildings tend to have enough heat capacity to remain comfortable."
If only that were true! All across the south and southwest of the USA I've been in conventionally built homes that were so at the mercy of external fluctuations that they needed heating and cooling in different parts of the house at the same time of day! Hopefully new construction meeting current insulation requirements will improve on that. But given the requirements for Zones 1 and 2, I'm not confident your typical spec builder is going to be making homes much more comfortable than in the past.
"When I did my Manual J, I measured the area and orientation of every window, input the SHGC and calculated the solar gain. This came out to nearly 50% of the total cooling load."
Interesting. DC, suppose you put some decent quality (e.g., double-cell honeycomb) window treatments on all windows and closed the window treatments on the hottest days. Any guess as to how much keeping all of the window treatments closed (versus keeping them all open) would save on the cooling load?
Why does this calculation matter?
You need to size the equipment for the worst case.
You need to buy heating equipment large enough to do the job on a cloudy day.
You need to buy cooling equipment large enough to do the job if the shading object happens to get removed.
This is about more than sizing equipment, though that too is important. I am also thinking about materials and built-in shading, such as overhangs.
If this is about sizing the equipment I think it would be a mistake to deviate from a proper manual J calculation.
Simply you can’t depend on the suns energy to heat your house until you figure out how to control the clouds in the sky over your home.
As I recall shading is one of the input used in manual J for the cooling calculation.
Yeah this was also my thought. For heating days, gain should be ignored for sizing equipment. For cooling, it should NOT be ignored since it can be a significant contributor to peak load.
And so in the same vein, designing overhangs for shading may desirably affect not only energy use, but peak power use. The reverse (for heating) would not likely be true, though the question may then be can it reduce external *energy* inputs (and as Malcolm suggests, can it do it effectively and predictably).
[Replying to Tyler #14]
"I'm not sure I understand how your response is in reply to my point. I'm saying thermal capacity is not only relevant when heating and cooling are required in the same day. Whether there is 'enough' is another question. I am merely saying your assertion that it 'only comes into play' when one heats and cools in the same day does not appear correct."
The assumption built into the Manual J process is that the house has some heat capacity, and that it's sufficient. The process sizes heating and cooling equipment so that it meets the needs of the house 99% of the time, the other 1% of the time -- 80 hours a year -- the expectation is that the inside temperature will be somewhat slow to react -- because of the heat capacity of the house -- and that while the HVAC is not actually able to keep up with demand, the interior temperature still stays acceptable.
I know I'm quibbling over semantics, but I would accept "intent" over "purpose."
There were a lot of houses built in the 70's and 80's where a designer -- not an engineer -- said "we need 'thermal mass!'" and they added a bunch of poured concrete. That's what I'm trying to scare people away from. Concrete is a wonderful material for some purposes, but it's terrible for the interior of houses -- it's hard and unforgiving, and has enormous negative environmental consequences, it's hard to insulate properly. More fundamentally, the notion behind using it -- that houses somehow lacked sufficient "thermal mass" -- was unsupported by science and just wrong.
Here's what I mean when I say that heat capacity only comes into play when you heat and cool in the same day. Let's say a house requires cooling all day. Over the course of the day it needs 240,000 BTU of cooling. In terms of energy used it doesn't really matter if that's 10,000 BTU/hr for the whole day or 20,000 BTU/hr during daylight or even six peak hours of 40,000 BTU/hr.
Where heat capacity does help is in preventing days from requiring both heating and cooling -- let's say it's 50F at night and 80F and sunny during the day.
Well, I guess I agree and disagree.
If you low have heat capacity in the winter the sun in your south facing glass drives the room temp above 70 degrees, you might theoretically say you need 'cooling' but most people don't mind it being warm on a very cold day so they do not call for cooling.
If you had additional heat capacity/thermal mass[IE interior brick exposed to direct sunlight] you would prevent some of that temperature rise by 'storing' that heat in the masonry[or concrete or water or...] and releasing it over time
As I mentioned in my other post we don't find it useful to have that much glass in an efficient house anymore, so it is kind of moot
Again, we simply not do that today because there just better ways to accomplish the goal, but some level of 'solar heat' will always remain popular if elusive because 'free heat'
I'm going to be a bit pedantic here: "thermal mass" is not really a term that is used in science or engineering. What property of matter do you think you're describing when you use that word? What are the units of "thermal mass"?
Well, to not put to sharp a point on it, yes you are being a bit pedantic
A tent and a concrete building have very similar R values, which would you rather try to heat with a limited heat source?
I have heated a tent [GP medium] with a wood stove so I know the answer
While thermal mass is not a scientific term per se, people know what you are talking about when you say it, and frankly many are puzzled about the vehemence of its rejection
In an introductory physics course, students learn about concepts that they usually already have an intuitive sense of -- gravity, friction, inertia, momentum. A big part of what they learn is that their intuition is almost always wrong, and they have to re-learn how to approach those subjects.
And so it is with "thermal mass." It's an intuitive concept, and the intuition is wrong. Part of what people are trying to capture is heat capacity, which is a principle in physics that raising the temperature of an object requires an inflow of heat, and the amount of heat tends to be proportional to the temperature rise. So if you were to express the heat capacity of a house you would express it in BTU/degree F -- for example, 25,000 BTU/degree. Or in metric units, joules per centigrade degree.
Part of the intuitive sense of "thermal mass" seems to be that the material needs to be cold to the touch to have thermal mass. So wood or drywall, which have pretty good heat capacity, are viewed as not having much thermal mass. The physical quality there is heat conduction -- which really has nothing to do with heat capacity.
The other persistent belief seems to be that materials that have "thermal mass" need to be dense. So aluminum, which has excellent conduction and decent heat capacity, is viewed as not having "thermal mass." I don't know where this belief comes from.
reply to #28
Well I'll jump on the pedantry train to point out that aluminum cannot have 'good or bad' heat capacity because heat capacity is an *extensive* property. So you either mean specific heat capacity (based on per-unit mass (oh look mass!)) or volumetric heat capacity (or I suppose molar heat capacity or some other such intensive measure).
I have argued before that volumetric heat capacity is probably the most relevant given limited space inside a house. Along those same lines, if two materials had the same specific heat capacity, the denser one would have better volumetric heat capacity. That, I think, is where the notion that dense objects have better heat capacity comes from. I do not know for certain, but I would wager that if you graphed density and volumetric heat capacity of common objects on a graph, you would see a correlation between density and .volumetric heat capacity-- even though the specific heat capacities of those objects may not be correlated with density.
The broad point that masonry/concrete is perhaps not as desirable in this realm as many believe is a good one and probably the most important one, imo.
The challenge with these types of discussions/pedantry is that whether it's effective and useful or annoying and trite depends on who's ear you have. If someone is clueless or slightly misinformed, perhaps they will learn something enlightening from such obsessions as this. If someone already knows what they are talking about, and casually uses the word 'thermal mass' in a discussion, then dumping loads of pedantry on them does nothing but to make the pedant look like they're trying to prove something.
To be fair, we never know who's ear we're dumping stuff onto. And therein lies the challenge, especially in online forum. I too have a affinity for detail and pedantry (as do lots of GBA regulars I dare say), so pots kettle black on this one. It's usually about balance.
I'm just trying to keep people from putting unnecessary concrete into their houses. And to focus on what's really important, insulation and air sealing.
Aluminum is a trick question, it's actually more dense than concrete, 2.7 g/cc vs 2.4 g/cc.
Volumetric heat capacity (kJ/m3.K)
overly concerned with minute details or formalisms, especially in teaching.
I've only ever heard people argue for volumetric heat capacity as an important metric when they've already decided that their house needs more concrete and they need a justification.
That Australian government page is just terrible. In the US, there's a law called the Data Quality Act that prohibits government agencies from publishing information that is not supported by science. I wonder if Australia has a similar law.
>"-- that houses somehow lacked sufficient "thermal mass" -- was unsupported by science and just wrong."
But we're talking specifically here about housing with excessive glazing oriented to have significant solar gain. That practice itself may be inadvisable, but IF one does utilize that practice, having thermal capacitance helps to keep the sunny days from causing overheating (which would imply those people *would* have needed cooling if they wanted steady temps. It's more a 'power' thing than an energy thing. I'm NOT suggesting people pour extra concrete or that houses generally 'need more thermal capacity' than is otherwise present by default. It seems like you are really arguing with me on the latter points which I am not even making.
As far as your example in post #16: I think the issue is you are talking about total energy usage (which technically could be affected because delta T's would change slightly, but it would be negligible I agree), whereas I'm pointing out that there *may* be other reasons where the dampening effect of thermal capacity is desirable. Namely to keep peak temperatures down in houses with certain features (whether those features are smart or not).
Martin has written (I believe several) pieces about how 70's passive solar houses often ended up over heated on sunny days, and too cool on cloudy days. Thermal capacity won't fix the latter problem, and it may not fix the former either, BUT it *could help* to some degree with the former. I agree that designing a system that relies on excessive solar gain and perhaps some extra thermal capacity to reliably deliver comfort is perhaps a fools errand (assuming reliably delivering comfort is the goal), but if one designed said houses with the same solar glazing and with even *less* thermal capacity, I think the situation would only get worse.
One way to think about the 'need' for thermal capacity with a passive solar house is that a passive solar house is 'designed' with a heating system that occasionally delivers far too much heating capacity for the load. That heating system being the sun. So it's basically pouring in extra energy (hey its' free!) and the thermal capacity is (to some degree, perhaps too small a degree) storing that thermal energy in a way that keeps peak temps down somewhat, and so when the sun stops loading up the house (heating system shuts off) there is more thermal energy inside the envelop than there would be without extra thermal capacity, which buys you time (for over night for example) before needing to potentially fire up a separate heating source.
Again: I'm not advocating for this, and it should be clear it's ripe with complexity, uncertainty, challenges etc. I'm just discussing the scientific principle.
In a well insulated wall assembly the cooling load increase is going to be in the hundreds of btus
If you have a 1000 sq ft of wall, 50 percent of it is not going to be affected by the sun. The half that remains is only affected half at a time. With an R40 wall, the numbers are just small
It is about the glass.
Current efficient design doesn't put the amount of glass that used to be fashionable, but it still ought to be grouped to the south if possible. A nice 2 foot overhang does wonders.
In my 'pretty stupid house' the glass can add quite a bit of heat and if there were more 'thermal mass' where the sun hits it directly it would probably be helpful, but at the glass levels in a PGH or PH, I cannot see it mattering.
Agreed. And more to the point, in a house built to modern codes the cooling loads through a wall are marginal.
To me the interesting question is whether glass can ever be energy-positive or at least energy-neutral. Because the real categorical error in those 70's passive solar houses was the belief that the glass was energy-positive.
Clearly it depends on the climate, there are tropical climates where houses protect from rain and mosquitos and that's enough. But what about a climate where you have a real heating season? My suspicion is no, but I haven't done the work to prove it.
Something I have thought about. What if you have a south facing window, let's say an operational casement. The day during the heating season is calm and sunny and near solar noon. The window is located where the approximate neutral pressure plane is in the wall. With the window open or the window closed what happens to the net gain/loss at that window opening? I suppose the type of glazing for the window would be a factor if a calculation could be made.
By all means, build a windowless house......
Wait, not going to happen?
Ah, so let us make the windows as energy positive as possible.
So it is a false question[especially as related to green or efficient building] to ask if windows are an energy positive, more relevant is how do we make them as little of a negative as possible.
First is group them facing south when possible
Does not mean we are building a passive solar house.
I generally agree, but the answer depends on where you are. If you're in a cooling-dominated climate, you don't want to face them south.
"To me the interesting question is whether glass can ever be energy-positive or at least energy-neutral. Because the real categorical error in those 70's passive solar houses was the belief that the glass was energy-positive"
I did the modeling on my house in Colorado, and south facing glass is energy positive. I suspect that is the case for most of the west, but not true where it isn't as sunny as it is here. The problems with 70's passive solar homes could be too much glass & not enough insulation.
The energy modeling program I use most of the time, BEopt, includes inputs for the roofing and wall cladding color of light, medium or dark. I don't know what their underlying formulas are but doubt they would ask those questions if they weren't including them somehow in calculations. I don't recall running cases with different exterior colors but I'll try that sometime to see how much difference it makes.
I occasionally use the PHPP Passivhaus software, which is very accurate in predicting energy use for projects that are close to Passivhaus performance, assuming the inputs are correct. It goes into detail on how windows are shaded and for walls and roofs, requires inputs for the "exterior absorptivity" and "exterior emissivity."
" wall cladding color of light, medium or dark"
Interesting. Does it allow an input for albedo as well?
Yes, or more accurately, they include inputs for solar absorptivity and emissivity, as well as the variables used to calculate "thermal mass:"
I tend to follow Dr. Joe Lstiburek's advice on passive solar as stated in his article "Zeroing In": Don't bother with passive solar (you can jump to paragraph nine for that quote).
Another great piece of advice is "The very best window makes a really lousy wall" which means why replace an R30 to 40 wall with an R4 to 8 window?
Also his advice: to "use just enough windows for aesthetics and ventilation and no more"
Back in the 70s I was as intrigued as the next person with regard to passive solar design, natural ventilation and similar strategies. The "best practice" design approach has gone in a different direction since then, focusing primarily on blocking out local environmental conditions, i.e. high-insulation, low air infiltration... and then controlling the interior through mechanical means.
But 2 experiences remain forever at the forefront of my mind. One was in southern California on a summer day. My car's thermometer measured 96 degrees. I stopped to visit one of the old adobe Spanish Missions, Mission San Antonio de Padua. The adjacent residential building was a long single story rectangle, a string of rooms oriented east-west, so that it had a long southern face with a continuous covered patio and a long north face looking out over gardens. The walls were adobe. The roof was clay barrel tiles set over lath. As I stepped from the sunshine into the shaded patio it felt as if the ambient temperature dropped 10 degrees. Stepping from there into the rooms, it dropped another 10 degrees and was comfortably cool. I looked up and saw the rafters, lath and the underside of the barrel tiles. Pin pricks of light could be seen between the tiles.
The south wall of the patio, a good 2-3' thick adobe wall never received the direct rays of the sun throughout most of the year and consequently remained at roughly the mean daily temperature. It acted as a giant radiating surface, radiating "coolth" towards the patio during the day. The rooms north of this had high ceilings with a very gentle updraft due to the hot air rising and exhausting through the roof, bringing in cooler air from the shaded north side of the building.
The ensemble was an object lesson in how to mitigate the environmental extremes using the most minimal of resources. No energy was used to run mechanical systems and the only materials used were unfired adobe for the walls and fired clay for the roof.
I had a similar experience when visiting Pompeii one summer, although that house no longer even had its roof. The shear mass of the walls and their height compared to the courtyard space were enough to shade and cool the courtyard down to a comfortable temperature.
There is an awful lot we could learn from all this. But it's not the sort of thing that can be easily, quickly and economically applied to design projects in our normal commercial environment. So we have to take a different approach.
The California house you visited on a 96 degree day may have had walls that radiated "coolth," but such "coolth radiation" (of course, a better description would note that heat is radiating from your skin to the adjacent adobe wall) is only possible if the location had nighttime temperatures that dropped significantly below 60 degrees. A classic case of a climate where thick adobe or stone walls work.
But in much of the country, including where I live in Vermont, we have other problems when it comes to space conditioning -- many months when the daily high is about 25 or 30 degrees, and the nightly low is 10 degrees. Uninsulated adobe walls help during some seasons in California, but not in most climates.
Of course VT has different environmental conditions than southern California. I didn't think it needed to be stated. My point was that "There is an awful lot we could learn from all this."
And many of us, including most of the people frequenting the GBA forum, do understand these lessons. One of the challenges is spreading that knowledge. Another is finding ways to apply it "easily, quickly and economically applied to design projects in our normal commercial environment."
Our building codes tell us we don't need to use as much insulation in zone 2 as we need in zone 7. That's a very modest acknowledgement of the differences. But the average new home in zone 2 still wastes plenty of energy due to a poor envelop that does not take advantage of passive solutions and relies wholly on mechanical means to control the interior conditions.
Incidentally most of the new housing being built in the USA is in zones 1, 2, and 3.
This is what I meant in post #11 about "shoulder" weather. Although I guess in California it's shoulder climate.
JGSG, the concept of "thermal mass" has been discussed regularly on GBA since its inception. https://www.greenbuildingadvisor.com/tag/thermal-mass. (I use quotation marks so DC Contrarian does not develop a tic; it's not a precise term and he is on a mission to correct that.)
I have to agree with Martin that it's a poorly-understood topic, evidenced by the regular questions here assuming that thermal mass matters. In the early days of GBA I argued in favor of thermal mass but have revised my stance. Doing the math helps--it's a combination of specific heat, volume, density and--importantly--temperature change. If the temperature isn't changing outdoors, indoors or both, or if the mass isn't being "charged" by the sun or mechanical means, thermal mass does nothing.
Words matter. If we want building science to be taken seriously we have to think like scientists, part of that is speaking like scientists.
I know, and mostly agree. Part of having words matter, though, is finding terms that are precise and memorable if we are to expect mass adoption. The term "thermal mass" is so deeply embedded in the building community that it's going to be difficult to change. If there was another term that concisely conveyed what people mean by thermal mass it would help a lot. But when the substitute term is along the lines of, "some mix of specific heat or specific heat capacity, volume, density, time and temperature change" it's not going to replace the evocative term that everyone knows (and thinks they understand).
OK, I get "thermal mass" as a shorthand for heat capacity, something that is well understood and modeled. It's when you throw in all of those other things that I have a problem. Yes, those things are also well understood and modeled, and each can be modeled independently. The implication is that somehow in combination there is some synergy that causes them to act differently from how they would act when modeled independently. I don't get that, and I don't see any reason to believe why that would be true.
On these very pages I have been criticized for call "thermal mass" pseudo-science. The idea that there is synergy is exactly what I mean by pseudo-science.
" If there was another term that concisely conveyed what people mean by thermal mass it would help a lot. "
Part of the problem is that "thermal mass" can mean a lot of different things to different people. Just off the top of my head I can think of three different usages of "thermal mass" that actually describe completely phenomena:
* People here have claimed "thermal mass" as what keeps basements cool in the summer and warmer than outside in the winter.
* People here have claimed "thermal mass" as what makes the heat from radiant floors, cast iron radiators and masonry stoves comfortable.
* People here have claimed "thermal mass" as a property of houses that limits the temperature swing as the heating and cooling load varies.
Those are all different phenomena.
If you ask me, thermal mass is just heat capacity and there is little confusion as to that. The main confusion is really just how useful it is. Which may or may not be changed by calling it heat capacity! There's countless examples in industry where scientifically imprecise terms are used. Confusion among the general public is not grounds for such vehement nay saying, imo.
In my opinion, volumetric heat capacity is the most relevant intrinsic property to discuss materials, but the available thermal mass in a building is synonymous with heat capacity (extrinsic).
>"Part of the problem is that "thermal mass" can mean a lot of different things to different people."
I think it's usually a misunderstanding of the phenomena, not the term.
"I think it's usually a misunderstanding of the phenomena, not the term."
On that I agree.
I recommend posts 17, 18 and 22 in this thread:
I did some modeling of the relative value of insulation and heat capacity in shoulder weather.
I also recommend posts 17 and 22 in this thread: