Concrete, Thermal Mass, and Stable Ground Temps
I’m deeply curious if there’s a calculator or theory of calculation that can determine the regulating impact of concrete walls (x” thick) that are connected to footings of a given depth.
I’m working on an ICF house that is the following exterior -> interior.
Black brick facade -> 2″ gap -> 2.5″ EPS -> 8″-10″ concrete -> 2.5″ EPS -> Drywall
What doesn’t seem to be accounted for in any of the discussions of R-Value and ICF is the impact that connection to regulating ground temperatures has on efficiency of the wall system.
In the summer, as heat attempts to get from the outside (100 degrees, say) to the inside of the wall system, it will be slowed by the air gap, and then the 2.5″ of EPS. I’m curious if an ICF wall connected to footings that are 24″ below ground level will experience a cooling effect as heat passing into the wall attempts to find equilibrium with the cooler temps below, similar to how a geothermal system works. And vice versa in winter.
Or I could be completely out of my mind.
Any thoughts on how a mass of concrete running underground would impact the temperature of the wall and thus the movement of heat/cold to the interior?
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I don’t have a scientific answer for this. I probably don’t even have a correct answer for this. But we are in the design phase of an ICCF shop/living space and have asked ourselves this same question. My assumption is that at that depth (48” where we are), the temperature delta T isn’t significant enough, and the footers don’t have enough uninsulated surface area for it to matter one way or the other. I do plan on asking our engineer if there is any insulation with enough compressive strength to work in that scenario under the footers (our climate is heating dominant). If nothing else, it’d be easy enough to use eps on the sides of the footers placed before concrete was poured. I think that with a good water/vapor barrier would be plenty adequate. That is nothing more than a hunch, so take it for what it’s worth.
Thanks Jackson. I'm thinking in a slightly different way -- my footers are uninsulated and I don't have a basement, so I'm hoping that 24" deep concrete surrounded by 60-70 degree soil will help to cool the wall in the summer and warm the wall in the winter.
You're probably right that it isn't significant enough, but I'm sure curious where the math lands on it. Even cooling/warming the temp of that concrete wall a few degrees would have an impact.
A certain someone is so going to post that thermal mass is not a thing
The proper term would be thermal capacitance...
There is no significant affect that you seek.
If you look at calculations for heat loss, they all revolve around Delta T, or the difference in temperature
The difference in temperature from the ground to the foundation, and from the foundation to the room is too small to create any kind of geothermal effect.
If you have a deep uninsulated basement it will try to be the temperature of the surrounding soil. If you live in zone 5 like I do, that will feel nice in the summer, but cost you money in the winter if you try to heat it
Likewise your footings will be the temp of the soil at that depth. They will transfer that temp through the small thermal resistance of the concrete wall and if left uninsulated will make the edges of your floor slightly cooler in the summer. If you have a heating season, they will also be cooler in the winter.
There are areas of the country where uninsulated floors can be an advantage, primarily I would think where there is no heating season
I believe the term you're looking for is "heat capacity."
Heat loss through the foundation is a complicated equation, even ASHRAE seemed uncertain about it years ago. Has to do with depth, soil type, outdoor temperature and foundation above and below grade. They also had a slab calculation that was based on slab perimeter as I recall. If you live in a heating dominated climate insulate the foundation and isolate it from the surrounding soil.
Doug
If you're thinking the ground is an infinite heat source/sink, then you're mistaken. The ground has a limited capacity to either suck away or provide thermal energy. This is one reason why closed loop ground source heat pumps aren't perfect: the ground eventually warms up (or cools off) enough in the immediate area of the loops that the "ground temperature" is no longer what it was. The problem is that earth insulates too, so it doesn't just conduct heat. You can't consider the earth to be a constant temperature that is unaffected by whatever you are doing on "your" side of the wall.
I've always just considered the earth to add insulation to a wall, and to be a bit of a heat sink for a slab. I've never considered it to "regulate" the indoor temperature any, and I've never really seen it behave much differently than insulation: slowing the rate of change of the indoor air temperature a bit compared to what you'd get with a wall with open air on both sides. I think the reason you don't see the caclulations you're looking for is because you can't rely on the earth to do what you're hoping for here.
Bill
I've never considered it to "regulate" the indoor temperature any.
One of the reasons I come across as a jihadist when it comes to the term "thermal mass" is it's usually a gateway to a rabbit-hole of magical thinking about "self-regulation" and the like. Similarly when people talk about "thermal batteries" or "thermal flywheels" or "thermal capacitance." They're all trying to draw analogies that mislead more than they illuminate.
That opens the door to magical snake-oil products too, where people can waste lots of money for little if any actual benefit. I have done one thing useful with "thermal mass", and that is to make a sort of "thermal battery" for large (1,000+ ton) chilled water cooling plants. We have an issue where the big chillers can't immediately restart after a power interruption, they need about a 15 minute delay before they can restart. This is an issue in datacenters where the heat load (thousands of computers) keeps going on the UPS system and generators while the cooling system is offline waiting to restart. Running the chillers on UPS means a lot of extra equipment, losses, and maintenance. My solution has been a large insulated tank on the "cold" side of the chilled water loops. Since the circulation pumps can restart immediately, this big tank of cold water can keep the system temperature low enough for long enough that the temperature in the data rooms doesn't rise enough to be a problem during the restart delay for the chillers. All of this can be easily calculated. The temperature risk is slowed down by the cold water in the tank, since all that water needs to warm up before the rooms "lose" cooling. The water is a sort of "thermal battery" (I usually use the term "buffer"), and it doesn't require any maintenance, and it has pretty low thermal losses.
That's the only place I've made this kind of system work. I know the wood boiler people use 1,000 gallon tanks of water as "thermal storage", and they heat that water up to near boiling with the boilers than use the stored heat to heat their homes between firings of the boiler. In both cases, the thermal differential is fairly large (assuming a 70 degree room, my cooling system has about a 25F buffer, the boiler guys have around a 120F buffer). Systems that only run a few degrees of thermal differential just can't store enough energy to do anything useful.
I like to say that there is "one true law of the universe", and that is "physics is a b****". That's what happens here for residential systems: physics just won't let this kind of thing work in a passive system.
Bill
"All of this can be easily calculated."
Bingo.
When the discussion centers around "thermal mass," it's extremely rare for there to be any sort of quantitative analysis. In fact, it's almost unheard of for "thermal mass" proponents to even know what the units would be for such a calculation should be.
Perfect! That's what my question pertains to. How do we calculate it? Is there *any* impact on interior heating/cooling with a thick stone/brick/concrete wall in place of a 2x4 of 2x6 wall, assuming the same r-value? Is it a positive or negative impact?
The thing about "thermal mass" or "heat capacity" is that there are proven strategies (earthships, etc) that use thick walls to maintain a comfortable indoor temperature without active heating/cooling. I always understood these systems to be, basically, the following:
During the day, the wall starts heating up because the ambient outside temp is higher than the wall temp, which is accelerated by the direct radiation of the sun. Because the walls are so thick, they heat up slowly. By the time they start transferring an uncomfortable amount of heat to the interior, it's evening and the outdoor temps drop below the temp of the wall, so the wall starts cooling. In the morning, the cycle starts again.
Seems to me the physics there would only work in the Southwest and similar very swingy climates. Where I am in Oklahoma, we're often more like 100-80-100-80-100-80, so a thick wall would never be able to shed that heat and the interior would just stay hot.
That explanation likely contains a lot of ignorance that I'd love to better understand, but what I'm really after is a better grasp of what happens if I take a double 2x4 wall that is, say, r-22, and put a 10" slab of concrete right in the middle of it. The r-value is basically the same, but what happens to the way heat transfers to the interior? How does it impact interior temps? Are they more regulated? Does heat move differently? What impact does it have?
Would love to be enlightened!
Earthships have all kinds of other problems. I'd never build like that.
What heavy masonry does is to average out the temperature over the day, that's all it does. There is no magic to it. The "thermal mass" (sorry DC) slows the rate of heat rise, and slows the rate of heat drop too, due to the need to "warm up" or "cool off" the massive mass of the material in these structures. Stone castles, earthen structures, all act basically the same way here.
All also tend to suffer from the same problem too: constant dampness. They are also very hard to "force" to something other than that average temperature too, because any heating (or cooling) you do has to deal with the massive mass of the structure too. This can actually end up being somewhat self defeating.
With insulation, you aren't fighting the massive mass of the structure, so you have more control. The insulation works by slowing the loss (or gain) of thermal energy, which slows down the rate of change of the temperature in the living spaces. Insulation ends up doing essentially the same thing as "thermal mass", but in a more controllable way. I'm not a fan of "thermal mass" in structures outside of the limited examples I've mentioned earlier, and in those situations the thermal storage is used as part of an active system where you have a bigger thermal delta to work with, meaning you get more bank for the buck for the system (ability to store more BTUs), and you maintain controllability of the system.
Bill
The thing about "thermal mass" or "heat capacity" is that there are proven strategies (earthships, etc) that use thick walls to maintain a comfortable indoor temperature without active heating/cooling.
Proven? By who?
Earlier this year I was in Taos and visited the Earthships. I had two thoughts on the tour: First, these things are beautiful, they really did some clever things with the architecture. Second, the science -- or "science" -- is load of hooey. I probably ask more probing questions than the average tour visitor, but I got them to concede that they aren't as self-sufficient as they try to claim.
I also visited Taos Pueblo. Adobe houses are often held up as an example of successful application of the principles (or "principles") of "thermal mass." It was a pretty raw day, not super cold but mid-30's, sunless and a stiff breeze. Just about every building in the place was downright miserable to be in. The ones without fires were bone-chilling, even the ones with fires were cold and smoky. The only building in the place that was bearable was the church, which had a propane heater.
The landscape is littered with failed attempts at utilizing "thermal mass." Almost all of them were never properly engineered, if they had been they never would have been built. I have a neighbor who bought a house built in the 1970's with a Trombe Wall (See https://en.wikipedia.org/wiki/Trombe_wall ). Now my friend is a smart guy and knowledgeable, he worked in the Department of Energy in a policy position, he's written books and taught college courses about energy.
After studying the house extensively, he had it torn down and built another one. He did the analysis, and realized the house that not only was the house never going to work as intended, it was never going to be energy-efficient or even comfortable.
Is there *any* impact on interior heating/cooling with a thick stone/brick/concrete wall in place of a 2x4 of 2x6 wall, assuming the same r-value?
Let me ask this: what is magic about stone, brick or concrete that you would expect it to work differently than other building materials?
Using clarifying terms should always be applauded! I'm not a Whorfian (see Sapir-Whorf hypothesis) generally, but that's not to say there isn't wisdom to inapt language. And we should eschew qualitative analogies which result in magical thinking.
Thermal capacitance in buildings is an important attribute to characterizing energy efficiency in reality. But it's complex to analyze and especially hard to predict without a LOT of effort to correct one's model with reality. If you can achieve your efficiency goal with just thermal resistance, then adding some capacitance probably won't hurt you (and I'm trying to imagine a situation where it would).
DC-- I think what you're arguing is a 'trust god, all others bring data' kind of thing. And I think that's exactly what's needed when adding thermal capacitance into physics modeling. One needs a finite element analysis at least to get an 'idea' of this. Beopt does it with Energyplus-- BeOpt produces garbage like any other building physics modeling, but very useful garbage if you know how to use it.
“How do we calculate it? Is there *any* impact on interior heating/cooling with a thick stone/brick/concrete wall in place of a 2x4 of 2x6 wall, assuming the same R-value?”
The mass is not a factor in the equation for heat loss!
BTU/(hr·ft²·°F)
When it gets cooler outside than you want your house to be for weeks or months at a time the mass does nothing but make it take longer for the indoor temp to fall and equally makes it take longer to warm up.
The number of BTUs it is practical to store is tiny compared to the number of BTUs that will escape thru the thermal envelope over the weeks or months that the outdoor temp is lower than the desired indoor temp.
Thermal mass is a good plan in someplace like the desert when it is too cold each night and too warm every day but even there it is only true in the spring and the fall.
You can’t really count the mass of the concrete inside the ICF walls as it is insulated from the conditioned space and the outdoors.
Walta
Walta, I don't want you to feel like I'm picking on your choice of words, but in the above you say "mass" when what I think you really mean is "heat capacity."
The reason that I get so squirrelly at the term "thermal mass" as a substitute for "heat capacity" is that it easily leads to the misconception that mass is the same thing as heat capacity. So people go around looking to make their houses as heavy as possible, and since apparently many of them never solved the brain-teaser, "which weighs more, a ton of bricks or a ton of feathers," they look for the densest materials they can find, usually stone, brick and concrete.
Or as I like to call it, "magic masonry."
Normally I'm content to let people build the house they want, but larding up a house with a bunch of unnecessary masonry does nothing for the energy consumption and has significant negative environmental impacts.
Deleted
>"since apparently many of them never solved the brain-teaser,"which weighs more, a ton of bricks or a ton of feathers,"
That's why I said "massive mass" in my post, to emphasize the "massiveness" of the "mass". A ton of concrete will seem like more massive mass than a ton of feathers was my thinking. That extra "massive" added to the term was for you :-)
BTW, I agree with you that people seem to think "thermal mass", or "thermal storage" is some kind of magic that defies physics when it's not. The thermal storage the outdoor boiler guys use is sound -- they heat up a big tank of water to near boiling, then gradually cool it off throughout the day through the radiators in their home, which heats their home. The big tanks act like "heat capacitors". This works pretty well since the tanks get up near 200*F or so, and the houses are usually kept in the 70ish *F range, so there is a large thermal delta, allowing for a good amount of BTUs to be stored in the water. My own example on the chilled water plant is similar, but with a smaller thermal delta (about 25*F or so), but I also usually only need about 15-20 minutes of reserve time, and I can tolerate a bigger thermal swing in my application than people would want to see in their house.
Doing something like building your house with a foot thick living room slab isn't going to do much to keep your house comfy. You just can't store enough BTUs in there to accomplish much, and you have little control, if any, over how those BTUs get stored and released.
Bill
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So then here is the ultimate question. I've got a bet going with an architect friend of mine who is, as you guys would describe, fallen into the "thermal mass" trap.
Which of the following wall configurations will be more efficient for heating/cooling in climate zone 3a?
#1 -- brick facade -> 2" gap -> 10" concrete -> r-10 EPS -> r-20 cellulose in 2x6 wall
#2 -- brick facade -> 2" gap -> r-15 EPS -> 10" concrete -> r-15 EPS
Both are technically ~r-28 walls, but one puts the concrete on the exterior and one puts the concrete in the middle (ICF). Based on what I think I've learned here, they will actually perform exactly the same, as the concrete has no effect on efficiency aside from air-sealing in both cases. Is that accurate?
Concrete doesn't help "efficiency", it just tends to even out temperatures over the short/medium term (typicall day/night). It does that by storing and releasing thermal energy. It does NOT make or remove energy! It ONLY stores and releases it! It does NOTHING to aid efficiency, aside from the somewhat low R value provided by the thickness of the concrete.
Both walls will perform about the same in terms of overall BTU loss for a given temperature (assuming heating season here, but cooling season is the same, just in reverse). The wall with the concrete towards the exterior will be a bit easier to control, and more responsive in terms of time to adjust the indoor temperature, but that would be mitigated by the R15 on the interior in the second assembly.
I'd skip the 10" concrete unless you need it for load bearing purposes or have some kind of unusual requirement (tornado proofing, stuff like that). The concrete isn't really gaining you anything here.
Bill
I like how you pose this question swooley.
Here's the thing: all buildings have some heat capacity. The question you should be asking is do buildings ordinarily have insufficient heat capacity, such that deliberately increasing it improves the performance of the building? Because that's the implicit assumption that advocates of "thermal mass" are making.
Heat capacity is not ordinarily calculated for buildings. It is implicitly assumed to exist, the standard -- and code-mandated in 49 states -- way of sizing HVAC equipment is Manual J. While Manual J does not take heat capacity as an input, it does contain the assumption that sizing equipment for the first and 99th percentile temperatures is sufficient, which implies that the building has enough heat capacity to maintain comfort during brief excursions beyond the capacity of the HVAC system.
Where I live, Washington, DC, the IRC and the IECC are strictly enforced. To build a new house you need to have a soils engineer determine the bearing capacity of your soils, you need to have a structural engineer determine the bearing capacity of your foundation, your walls, your floors and your roof, and everything in between. You need to have a mechanical engineer determine the heating capacity, cooling capacity, electrical capacity, plumbing capacity and sewer capacity, and a civil engineer determine the rainwater capacity of your gutters and the dispersal capacity of your soil. So you're calculating all these capacities: bearing capacity, heating capacity, cooling capacity, electrical capacity, plumbing capacity, sewer capacity, rainwater capacity and dispersal capacity. But for some reason you don't have to calculate heat capacity.
I would posit that the reason for that is that ordinary construction provides sufficient heat capacity.
Deleted
For the love of God, I hope that 10" of concrete is necessary for structural reasons.
I would put the question differently: for a given amount of money and a given amount of space for wall thickness, what gives the most energy-efficient assembly? I would argue that getting rid of the concrete altogether is the best solution, if you're going to have a wall that is 15"+ thick a double stud wall with a brick veneer will give better performance than either of the walls you described.
Maybe they're building a parking structure above their house? C'mon DC! That's right up your alley with maximizing useable square footage on a tiny lot :-)
I think the money spent on that concrete would make a much bigger improvement in energy efficiency if it was spent on insulation instead, or even on a solar system.
BTW, my experience working out your way is that all the extra engineering steps had a lot more to do with CYOA for the building dept people and less about getting things done correctly. The building dept people have no problem spending YOUR money after all.
Bill
I wouldn’t factor this into anything. If may slightly reduce peak loads but it’s nothing special.
Deleted
Here's the way to think about heat capacity:
Imagine a brick in a styrofoam cooler, left outdoors. When the temperature outside the cooler is warmer than indoors, heat flows into the cooler and the brick warms up. Conversely, when the temperature outside the cooler is lower, heat flows out of the cooler and the brick cools. The insulation and the heat capacity of the brick slow the response of the brick to temperature swings, and the brick will stay closer to the average temperature than the ambient air does. This is called damping.
The amount of damping you get is determined by the heat capacity of the brick and also by the insulation level of the cooler. In fact, it's determined by the proportion of them to each other. If you want to increase the amount of damping, you have two choices, either increase the size of the brick or increase the thickness of the styrofoam in the cooler.
Note also that if you want to keep the brick at a steady temperature -- say you want to use the cooler as a cooler -- and the ambient temperature is always either warmer or cooler than the brick, then the size of the brick doesn't matter. The only thing that matters is the insulation level.
Applying this analogy to a house, the cooler is the insulation of the house and the brick is the heat capacity. If you're not in shoulder season -- you're heating all day or cooling all day -- the heat capacity of the house doesn't matter. If you are in shoulder season, you can reduce day/night swings in temperature either by increasing the heat capacity, or by increasing the insulation level. And the same percentage increase in either gives the same reduction in swings.
But here's the thing: while increasing heat capacity helps not one bit during pure heating or pure cooling season, increasing the insulation level helps a great deal.
Now, as I noted above, we don't usually measure heat capacity of buildings. But just about every place in the US has some shoulder weather where it's hot during the day and cool at night. Empirically, you just don't hear people complain that their houses don't perform well in that weather. To the contrary, that's the kind of weather where people who have leaky old houses say, "this is the only time of year my old house is comfortable!" So it seems empirically that customary construction provides at least enough heat capacity. Effort should be directed not at increasing heat capacity but at increasing insulation.
As a footnote, I'll add that shoulder weather is when energy usage is the lowest, and even if heat capacity were insufficient the impact on efficiency would be low. Whereas insulation helps you when energy usage is at its highest.
Well described DC.
As for efficiency, heat capacitance can be used to effectively change the amount of energy used during shoulder 'weather' (I'll nitpick annoyingly that our buildings live in weather, not seasons, nor climates, which are more conceptually statistical).
In my work, thermal mass is VERY important for the pure cooling season. The difference between 2pm cooling and 5pm cooling is roughly $0.40/kWh, so having a battery of some sort for price arbitrage is very helpful. What's more expensive-- a LiFePo chemical battery, or some dumb heavy stuff with lots of exposure to the interior environment that can make the interior 'brick' in the cooler marginally heavier (not a rhetorical question)? Being able to turn off the AC from 4pm-9pm is with a lot by itself. Enabling a chemical battery to export to the grid is itself worth up to ~$3/kWh, so that's icing on the cake.
This is to say that increasing heat capacity during the pure cooling is worth a LOT. That's not to say that increasing resistance isn't also worth a lot too, but it's not an either/or proposition here. Buildings are RC circuits, so let's continue to talk about them as such.
In my work, thermal mass is VERY important for the pure cooling season.
Presumably then you are measuring the "thermal mass" of buildings? Tell us about the measurement techniques you use. What are the units?
Where are you with a 40 cent/kwh price difference between 2pm and 5pm? In my usual commercial work, the price difference between on and off peak has gotten so small that ice chillers are rarely cost effective anymore (which is very dissapointing, because I like those). The usual on/off peak times are basically day and night too, not just a few hours apart in the late afternoon.
If you want to economize on cooling, you can set your thermostat to "overcool" by a few degrees prior to the rate change, so that you do most of the cooling with the cheaper power. You can't hold a target temperature setpoint as closely this way, but you don't need to any anything extra to the building to do it, either.
Batteries store too little energy to be of any use to me commercially. I haven't really thought of them for residential load leveling the way you describe (shifting loads to off-peak periods), but I suppose they could make sense. I'd be concerned with the cycle life of the battery system in that case to see if the total cost of ownership of the battery system offset enough utility cost to justify the system over a reasonable period of time (3-5 years or so usually).
Bill
I had assumed the setpoint was changing, that's the only way you could possibly take advantage of the heat capacity of the building. You can't just say to the assembly, "OK, it's 2pm, Thermal Mass, Do your thing!"
Yeah, we use the water storage to slow the rate of rise of the temperature in the data rooms during the restart interval for the chillers. We try to keep the data rooms around 70*F, but can allow for up to 72-74*F or so for short periods (the fans just ramp up in all the equipment to move more air to compensate). The water storage lets us keep within the allowable range until the chillers restart, without the storage, we'd see the temperature in the datarooms climb too much during the chiller's restart interval. The air handlers in the data rooms are just radiators with chilled water in them and a big blower, so they can't do any active cooling -- only the chillers do that.
Note that the chillers in the chilled water plant are always in an N+1 configuration, so one is typically offline at any given time. The one that wasn't running can usually be started up more quickly, so we start that while we wait for the others to restart. That also helps keep temperatures under control since we end up with reduced cooling capacity instead of no cooling capacity during the restart interval that way.
It sure would be nice to be able to slab a thermostat on big block of thermal storage material and have control! That must be in the world of rainbows and unicorns though :-D
Bill
So let's back-of-the-envelope this out.
If you add up all the dead loads in typical residential construction it comes to around 40 PSF, so a 2500 SF house weighs 100,000 lbs all-in, give or take. Almost everything that goes into a house has a specific heat around 0.5, so a house like that is going to have a heat capacity in the neighborhood of 50,000 BTU/F deg.
Let's say our hypothetical house has a peak cooling load of 25,000 BTU/hr. So if you turn the AC off in the middle of the day for four hours you'd expect the interior temperature to rise by 2F.
Let's say you don't find that acceptable, you want to limit it to 1F. You need to either increase the heat capacity or reduce the heating load. To do it by increasing the heat capacity you need to add another 50,000 BTU/F deg, or another 100,000 pounds of concrete, or 40 pounds per square foot. That's about a five inch layer of concrete throughout the entire house.
Alternately, you could double the insulation layer. This means spending twice as much on insulation, and making your walls and ceilings twice as thick. It also means upgrading the glass.
But here's the thing: when you turn the AC back on again at the end of the afternoon you need to cool the house back down, all the heat that went into the house has to be removed. If you doubled the insulation level, it's half as much heat, and you end up cutting your power bill in half. If you increased your heat capacity you use the same amount of electricity*, the only savings is from time-shifting the rates. In fact, with the higher insulation you save energy every day of the year, it may not even be worth your while to time-shift.
*(There's probably some savings from running the AC at a higher COP when the outdoor temperature is cooler. But you get that in either case.)
I like your analysis DC.
In non-extreme weather scenarios where there's a good diurnal variation around the comfort temperature, then there's a lot of infinite or high COP cooling that can be derived from the ambient environment through mass. The classic high-COP cooling tech is a ventilation cooling fan. Point being, in many parts of the cooling season, it's not a zero-sum game for time-shifting. And the efficiency of time-shifting CAN be much higher than just a slight benefit to AC-condenser COP you described.
While I appreciate the back-of-the-envelope approach, I'm wary of this kind of approach unless the building is highly insulated-- e.g. Passive House. It matters how and where the thermal mass is integrated, and its physical attributes i.e. thermal diffusivity. In your back of the envelope of 50 kBTU/degF, how beneficial is that mass in reality to the interior comfort of the space? I *think this is arguing for a position that you've held for why claims of the benefits of mass are a bit dubious...
But ok-- let's accept the magic mass hypothetical you describe .
In my work, efficiency has a highly variable time value. And it can be fairly extreme-- i.e. 3 orders of magnitude. A kWh at 4pm in the summer can be worth 1000x more than a kWh at 10pm that same day for the purposes of building code. It follows that it's possible that a building that uses 10,000 kWh per year can be more efficient than a building that only uses 1000 kWh per year, purely based on the time when that building is drawing electricity. I've never seen it practically that extreme in my work, but it's typical that my more efficient buildings use more energy than less efficient buildings, all due to this time-variable distortion. I can respect that this might seem facially contradictory-- the energy transition is weird.
So is it more efficient shift 10 kWh of cooling from 5pm to 10pm? Yes. Indeed, it more efficient to shift 10 kWh of cooling at 5pm to an off-peak time, even if it means that would take 20 kWh to achieve the same result at that other time? Yes, more efficient, and probably cheaper.
"A kWh at 4pm in the summer can be worth 1000x more than a kWh at 10pm that same day for the purposes of building code. "
Could you explain what that means?
Here you go:
https://efiling.energy.ca.gov/getdocument.aspx?tn=239439
TDV (now LSC) has been softening in recent years, but it's been a policy tool to make sure that efficiency incorporates a really strong signal to avoid energy consumption during peak grid-weather conditions. Such as heat waves when a combination of weather and grid constraints presents a possible brownout/blackout scenario which would then compel expensive infrastructure upgrades.
Looking at the raw data now, it looks like the signal-to-noise ratio is more in the 100x realm. The 1000x factor is more distant in my memory from several code cycles ago.
Looks like I was wrong. They just reduced things to $0.35 delta between on peak and super off peak.
https://tariffsprd.sdge.com/view/tariff/?utilId=SDGE&bookId=ELEC&tarfKey=898
Those rates are crazy high. Not much different from the rates I've seen in the Virgin Islands, where the power is all sourced from diesel generators (not a cheap way to make power)!
Bill
For us here, it's a lot of things, but main culprits are wildfire hardening, efficiency, and onsite generation. Costs for the grid have increased substantially, but consumption hasnt, and has probably decreased, so rates go up. Theres also a bit of profit taking too.
https://voiceofsandiego.org/2024/03/01/san-diego-gas-and-electric-posts-record-profits-again/#:~:text=The%20company%20made%20%24936%20million,infrastructure%20along%20the%20Gulf%20Coast.
I doubt load has decreased. Load has been increasing everywhere for some time. All the efficiency improvements have slowed the rate of load increase, but not reversed it.
Utility profits are usually set by the regulatory body, so the utility might always make a 1.7% profit (just making up numbers here). If the rate increases for any reason, the profit increases as an absolute dollar amount, but the percentage stays the same. This is one of the reasons utility companies are considered to be pretty safe investments in the investing world.
I can't say I know all the details about the CA utility enviornment, since I don't work in that region, but I can say a lot of things have been done out there that drastrically increase operating costs for the utility, and the utility will pass those through in their rate structure. This means record costs can also mean record profits, without the utility changing their actual profit margin (which typically requires regulatory approval anyway). Utilities have to present their costs to the regulatory body of the state when requestion a rate increase. The utilities don't generally have a lot of ability to make money on their own unless they've managed to increase their operating efficiency, which is unlikely.
Bill
@Bill
Fair enough. Bring data.
Here you go:
https://www.energy.ca.gov/data-reports/energy-almanac/california-electricity-data/2022-total-system-electric-generation
(Scroll down for the chart, or download the spreadsheet if you want to see the numbers)
2022 total electric generation (including imports) is down 7% from its peak in 2012. Load is rebounding a little bit in recent years, but I think it's fair to say that it's not generally 'up and to the right' that most other utilities experience.
Yup
Specific heat, density, conductivity, and thickness of all materials, which are then assembled into constructions. Both internal and to ambient and ground.
So what are the units of "thermal mass" itself?
I think you're asking a rhetorical question. Lol..
But I'll bite: BTU/F-sq.ft. is a partial characterization of 'thermal mass'
What did I miss?
I'm not being rhetorical, maybe a "probing question" is a better description. The reason I ask is that most people who say they are "designing" thermal mass into their structures can't even say what the units of said "thermal mass" would be. And how can you design something you can't quantify?
So why a "partial characterization"? I'd think if it's "VERY important" it would be something you could quantify and measure.
In your weather, what would be an ideal quantity of "thermal mass" for a typical house? Let's say 2500 square feet, 3 bedrooms.
No ideal quantity. Custom design means customs problems. Typically I recommend as lightweight of construction as possible and then targetted deployment of thermal mass, as lightweight as possible (do I contradict myself?) in targetted locations. Either lightweight or really cheap thermal mass.
I think it's very important, but my architects don't, so ideal, or even optimized thermal mass is just a fairy tale. And as much as I can game for peak pricing, that pricing is going to change dramatically in the next 15 years. That Wayne Gretzky quote comes to mind.
In short, I try to negotiate more on the margin. I usually lose.
OK, let me ask this: if you had a project where the architect was enthusiastic, and the client said, "for this I'm willing to make room in the budget," how much "thermal mass" would you use?
0.01 ton-hrs per sq.ft.
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[reply to #46]
"In your back of the envelope of 50 kBTU/degF, how beneficial is that mass in reality to the interior comfort of the space? "
Let's go back to the brick-in-a-cooler analogy. Let's now imagine that the cooler is surrounded by a layer of concrete. To make the math easy, let's say the cooler is an inch of EPS (r3.6) and the concrete is four inches thick (R-0.1 per inch or R-0.4). So the assembly has an r-value of 4.0.
Let's say it's the middle of the day, it's 90F outside and 70F inside. If we're at equilibrium there is a thermal gradient across the wall assembly. Since 90% of the insulation is in the foam, 90% of the gradient is in the foam, so the boundary of the foam is 18F above room temperature, or 88F. One side of the concrete is at 88F, the other side is at 90F, the average temperature is at 89F.
That's at equilibrium. Let's also say that at sunrise it was only 70F, and the box had cooled down to ambient at overnight. So between sunrise and midday the outer layer of concrete warmed up by an average of 19F. While it was warming, the outer layer of the foam was cooler than it would have been had the concrete not been there, and flow into the interior of the box was reduced.
Let's say over the course of the day the brick inside the box warms by 1F. Let's also say it's the same weight as the concrete shell. The concrete shell is gaining and losing 19 times as much heat as the brick inside. It's doing much more to buffer the temperature inside the box than the brick is.
Note that this buffering is passive, you can't control it. There's no way to tell the concrete that you really need it to do its thing between 2pm and 6pm.
Sure you can control it...
But only young hippies have the energy to do it.
I have uninsulated walls and 100 year old stucco home. A previous owner painted it dark green, so it can be a solar oven in the summer, but reasonably comfortable in the winter afternoons on a sunny day. I've been trying to control that a bit, so I've installed a few exterior shades on the southwest wall (most exposed) in order to control the solar/direct gain on that exterior mass. It's time to expose that mass, at least until the weather turns warm again. Winter heat waves happen...
So hypothetically I put a cubic yard of concrete in my living room. Maybe it’s in a sculpture form so it’s genuinely nice to look at. It’s 3000 lbs and has .2 BTUs/lb/degree F storage capacity. If I can cool it 10 degrees in the morning, I’ll have stored .6kwh-ish. Which is worth, under the most favorable demand response program my utility offers 0-3 times a year, about $.75 + the weekday $.2/kwh TOU rate which adds about $8. Surely the cement’s carbon footprint cancels this out?
Not only that, but in order to cool the chunk of concrete you have to cool the surrounding room. And to get the heat out again you have to allow the room to warm up again. So to get that 10F swing in the block temperature you have to have a more than 10F swing in room temperature.
This is why I intensely dislike terms like "thermal flywheel," "thermal battery," or "thermal capacitance." They imply that there is some magical property of concrete that allows heat to flow in and out of it without the surrounding environment changing temperature. It's pseudoscience.
Note that you are also INCREASING your losses while you're overcooling the room to cool down that concrete, since the higher delta T you create between the room and outdoors leads to more losses through your walls.
Bill
Indeed! If only there were materials with greater specific heat, lower gwp, nontoxic, and cheap too.
That would be magical in its own way
Water! But why would someone do this?
The problem is not in our concrete, but in ourselves.
I said maybeeeeee
You're gonna be the one that saves meeeee
Cuz after alllllll
You're my water{.....}
I'm sure you know this, but phase-change materials such as paraffin store and release more heat as they go from solid to liquid than materials of similar density and specific heat capacity that don't change phase near room temperature.
Indeed!
And now for an eyeroll
I just wanted an excuse to wax poetic with some help from the gallagher bros.
I've been wondering if P&G would pivot the Crisco brand (and all their hydrogenated oil products) away from food and towards building products
And unlike "magic masonry," phase change materials actually have the "magic" property, in that they hold a specific temperature. So if you had a phase change material with a melting point of 72F it would tend to hold your space at that temperature.
But unless you have enough for seasonal storage -- where you can store all the heat you extract during the summer and let it out during the winter -- you're still stuck with it only being useful in shoulder weather*. And the practical matter is that conventional construction doesn't particularly struggle with shoulder weather. I log the run time of my HVAC system, and we turned off the AC on September 3 and didn't turn the heat on until November 22. So for over a month and a half we were comfortable with no cooling or heating, and no "thermal battery" either. It's just a solution in search of a problem.
*(For an in-depth discussion see this thread: https://www.greenbuildingadvisor.com/question/calcium-chloride-hexahydrate-for-phase-change-heat-storage, particularly posts #11 and #15.)
A final point about phase change materials: if you had a phase change material that maintained, say, a steady 73F, you'd still have to figure out how to get heat in and out of it. Because heat flow requires a temperature gradient, in order to get significant amounts of heat out of the store you'd have to let the house temperature fall quite a bit below 73F, and to get significant amounts of cooling you'd have to let the temperature rise.
If that's not acceptable, you want to use a heat pump to create a temperature delta in order to move heat in and out of the store. Which doesn't give the no-moving-parts, organic, self-regulating system that people imagine when they think about systems like this.
I've definitely learned a lot through this thread which I appreciate. It's been super enlightening!
But is it really accurate to compare a big chunk of concrete in the conditioned space to a layer of concrete outside of the insulation? As DC explained above, the concrete outside of the insulation does a lot more than the "brick" in the middle of the ice chest.
"The concrete shell is gaining and losing 19 times as much heat as the brick inside. It's doing much more to buffer the temperature inside the box than the brick is."
So did I miss the purpose of this analogy or have I missed that there's a difference between material outside of the insulation and material inside of the envelope when it comes to calculating heat loss/efficiency?
The insulation also reduces the the amount of "thermal battery effect" you get, by slowing BOTH the "charging" and "discharging" of that battery.
I doubt very much that thick layer of concrete will ever save enough money to cover the materials cost alone of the concrete installed.
Bill
If you double the insulation level, you halve the energy usage on every day of the year.
Usually.
I've said that before in projects.
It hasn't always worked out.
Kinda like thermal mass
Luke,
But the two are not at all analogous. That doubling the insulation halves energy usage is a useful predictive formula. So far the benefits of thermal mass, even after a discussion spanning some 70 posts, remains at best speculative and elusive.
Thank you Malcolm!
You've concisely hit upon what I've been trying to get at. Predictive power! I've been focused on the lack of units of measurement, but that's just a symptom of the larger lack of any kind of quantitative model.
I'm not suggesting that they're analogous. DC has made a great case for the use and usefulness of mass.
I know exactly what you're talking about with the UA algebra.
I've just happened to find that in my work, many of my super efficient homes with super low UA often use more energy than the poorly home that it replaced. Let's just assume for the time being that we can chalk this all up to my incompetence. It just looks so easy though... how hard is UA and a blower door? Where is my math wrong?
"DC has made a great case for the use and usefulness of mass."
If that's what you've taken away from what I've written then I really haven't explained my opinions properly.
To wit, if you can read nothing else, read the first paragraph of post #66, which conveniently right now appears directly below this one.
Excellent!
That settles it then. And so you have the comfort of knowing that you're heeded, I will attempt to rephrase in your own words what I think you've said in a sharper, declarative tone so that I can demonstrate my appreciation of your insights:
Supplementing mass elements in a conventionally built home is never beneficial to the energy efficiency of that home. Furthermore, those mass elements themselves are deleterious to the environment and are always more harm than good if deployed solely for the purposes of energy efficiency design (portland cement... ech). In general, thermal mass as a concept is itself vague and ineffectual because it has no clear metric to describe it (unlike insulation), and thus it's a kind of underpants gnomes technology (i.e. Step 1: Thermal Mass, Step 2: ???, Step 3: NO MORE ENERGY BILLS!) rife with colorful and fantastic analogies (thermal flywheeeelll!) that teach destructively false models of physics that continue to rear their heads in practice and forums such as these so people like DC have to play friggin' whack-a-mole every day with jackarses like me to deprogram this thermal mass inculcation. It's sisyphian!!
If I got this anywhere close to the mark, then I owe you a beer DC (or your preferred beverage)
:-)
What you should really be taking away is that there is no reason to believe that a house of conventional construction is lacking in heat capacity, and that deliberately adding material to the house in an attempt to increase its heat capacity at best is ineffectual.
A few years ago we had a similar discussion here on GBA, I wish I could find it but searching the archives is a treasure hunt. For that discussion, I did some modeling of heat flows over time with material inside the insulation, outside the insulation and even between two layers of insulation. I made some lovely graphs. I was quite surprised at the time to discover that if you want to dampen interior fluctuations, you get the most effect by having your material outside the insulation.
But again, don't get too excited about this. Dampening is only a factor if you are switching between heating and cooling, if you're heating or cooling all day dampening does nothing for you. And most houses have enough natural dampening anyway that they are comfortable in shoulder season weather without any special interventions.
I would say the takeaways are to be suspicious of analogies, and also be suspicious of synedoche. We've gone down some rabbit trails here, and those trails have avoided the trench in your OP.
I know for my world, we generally recommend strong ground coupling, not only for seismic purposes but also for thermal 'grounding' in our climate. But there's large space for disagreement there.
Indeed. The concrete in my particular case is because I'm in Oklahoma and have been dangerously close to a couple of tornadoes and thunderstorms with wind over 100MPH. We're looking at concrete as a way to feel safe in our home—but that brings with it the questions I asked throughout; namely, where should the concrete be and do I gain any kind of thermal benefit by having a wall that effectively runs a couple of feet underground.
The answer to both seems to be that, basically, neither matters. The concrete will not have any real impact on interior conditions, though I would still argue that (especially with standard builders) such an assembly is a lot easier to make "airtight" in comparison to all of the seam taping, staggering of panels, etc. (especially over 50+ years). And that airtightness is a pretty big benefit. Again, doable with standard framing and excellent technique, but I bet most here would understand what I mean when I say that "excellent technique" is hard to find anywhere, let alone the rural parts of the state.
All of this has been wonderful to read. Anything additional is always welcome!
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My suggestion is to start a new thread for your project, you'll get more responses and better ones.
Lol old retired
I feel like Nietzsche in his last lucid moments (something about a horse)
But if you insist after reading this thread and this whole forum of threads, then perhaps you can start with Duffie and Beckman chapter 8.5
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Suggest you follow DCs advice
New thread