Cool Temperature of Slab on Grade
After my central air quit in my split level, I realized I don’t have the contacts I used to, to buy new equipment anymore.. So I decided to rough out the summer in the basement.. To my surprise, its always at least 10 degrees cooler in my basement because the slab is about 4′ lower than the outside surface.. Even the smallest 5000 btu window AC chills the 400 sqaure foot area down to 68 on a 95 degree day if I want.. But what I don’t understand, is how a shop I used to work in, always stayed cool when the slab isn’t in a basement but close to the surface..
Don’t you need at least 4′ of depth before you get the the constant 55 degree soil, so how did this shop slab stay so cool? And more of a statement than a question.. I’m glad my builder didn’t insulate the basement, because I don’t think the slab would be cool if it was… I close the registers in the winter and use it for storage, so no real heat lose..
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One reason: concrete has a high heat capacity and is thermally conductive, so even if it's at room temperature it will feel cool to the touch because it sucks heat from your body more quickly than most other materials do.
Another reason: there is a lot of thermal mass in and below the slab, so it takes a while for the temperature to change. Especially if you live in a place with hot days but cool nights, or with a few hot days followed by a few cool days, the slab and soil below will be closer to the average temperature than the highest temperatures.
Concrete has a lower heat capacity than most common materials. It does have good conductivity.
It depends whether you look at it from the point of view of per unit mass or per unit volume. It's got a lot of mass per unit volume, so even though the heat capacity per unit mass isn't that great, the heat capacity per unit volume is pretty high, though not as high as water.
Heat capacity per unit volume isn't appreciably better than wood, tile, or drywall.
The reason I harp on this is I'm really trying to counter the belief that putting a lot of concrete into a house does something to make it more energy efficient. It doesn't, and the production of concrete is quite damaging to the environment. It's a great building material and it has its place, but it shouldn't be overdone.
Most of what people are observing when they think they are seeing the effects of "thermal mass" with concrete is actually due to its high heat conductivity.
DC,
What source/numbers are you using for volumetric heat capacity of wood vs concrete? My searches say that concrete is significantly higher than wood. Something like an order of magnitude. Drywall is closer but still less. I agree though that people shouldn't put concrete in a building simply for 'thermal mass.'
Check out the table here:
https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
Specific heat of concrete is 880. For wood it varies by species from 1300 to 2500, so about 1-1/2 to 3 times that of concrete. Density of concrete is about 120 lbs/ft3. Wood varies too, but averages around 50-60. So a cubic foot of concrete weighs about twice what a cubic foot of wood does, but wood holds more heat per pound so it evens out.
https://energyeducation.ca/encyclopedia/Thermal_mass#Thermal_mass_materials
I don't have time at the moment to check your numbers and math, but the above says your way off. Also, I'm dubious of the claim that concrete weighs only twice as much as wood per unit volume.
Unless I am missing something, concrete has about half of heat capacity of water if measured by volume: https://www.greenspec.co.uk/building-design/thermal-mass/
dry pine might be 30 pounds per cubic foot
concrete weighs 150 pounds per cubic foot
not twice, 5 times
Thanks Michael Maines.. But that shop in Iowa I worked in saw only 70 degree lows in the morning during august, yet we always felt cool and comfortable. Unless we had to open outside doors, then we would lose the cool air. Granted, I didn't measure the slab temperature, but it had to be cooler than 70, which to me says, the ground cooling effect begins at a more shallow depth than 4'.. Ive never read a study on the benefits in a warm climate scenario. Trick now would be to somehow insulate a concrete basement with the flick of a switch for winter, and un-insulate it for summer. But not everyone has the ability or space to move into their basements during the summer..
One benifit would be if you were to lose power in the summer.. The insulated basement would more closely follow the temperature of the hot house imo.. Yet a un-insulated basement, you would be comfortable.. It may actually keep you warmer in the winter during a power lose.
Soil below the surface stays near the year-round average temperature because the soil between it and the surface acts as an insulator and buffers the heat flows. As a rule of thumb a foot of soil has an r-value of about 4. If you think of the R-value of an entire building, its going to be the same as at least several feet of soil. So the soil under an at-grade building is going to stay at the temperature of at least a few feet underground. If the building is conditioned the interior of the building is going to stay at about the same temperature year-round so there won't be nearly as much temperature fluctuation as there would be with soil that is exposed to outside air.
This is true in the middle of the building. However, around the perimeter there is a horizontal pathway to outside air, and since soil has a relatively low r-value that soil is effectively exposed to the exterior. The bigger a building the larger the middle relative to the edges, so bigger buildings will have more stable temperatures beneath them.
[I'm replying to Tyler's post #9 here because we've gotten as far nested as can be.]
I find that whole page is pushing an agenda that I disagree with.
They write:
"When incorporated into passive solar heating and cooling technologies, thermal mass can play a large role in reducing a buildings energy use."
This is simply not true. And the rest of the page -- emphasizing that thermal storage material has to be extremely dense, cherry-picking unfavorable numbers for wood -- points toward a conclusion that it's a good idea to put lots of concrete into your house. It's not. Use as little concrete as possible. And it leaves the impression that a wood house alone doesn't have enough heat capacity to stay warm. A typical single-family home weighs on the order of 100,000 pounds. For the most part the materials have as high a specific heat as concrete, or higher. There is plenty of heat capacity without having to do anything else.
As long-time readers know I'm a pedant when it comes to the use of the term "thermal mass." It's not a term that comes out of engineering or science, and people who throw it around are usually practicing pseudo-science. I would put this article into that category. The correct scientific term is "heat capacity." Note that this article also tries to define "heat capacity," and gets that wrong too, confusing it with specific heat.
Use as little concrete as possible. That's the take-away.
I agree and often repeat that the assumed benefits of thermal mass are almost always exaggerated, but an uninsulated slab coupled with the soil will absorb lot of heat before changing temperature and can be effective at moderating diurnal temperature swings.
I avoided getting into a discussion on specific heat capacity because it's unnecessarily scientific for this discussion, in my opinion. I used the term "heat capacity" and not "specific heat capacity" because I was factoring in the mass. Concrete is usually estimated at about 150 pcf, softwood framing lumber is 20-30 pcf and hardwood is 40-50 pcf.
Heat capacity equals specific heat times mass.
I understand. So if you take the mass of the concrete and the first few inches of soil below, and multiply that by the specific heat capacity of both, you have a decent amount of short-term heat storage.
OK, I feel like we've been down this road before, but one more time.
Let's assume the footprint of the house is 1000 square feet and the slab is 4" thick. So that's 333 cubic feet of concrete. That's roughly 40,000 pounds of concrete. Let's say you can tolerate a 10F swing in temperature, from say 68F to 78F. Concrete has a specific heat of about 0.2 BTU/lb/F. So the heat required to create a 10F temperature swing in that slab is 40,000 lbs * 10F * 0.2= 80,000 BTU.
To put that number in perspective, heating oil has 138,000 BTU/gallon. So the slab contributes as much heat as burning about five pints of heating oil. Heating oil is about $4 a gallon right now so about $2.50 worth of heat.
That slab contributes that heat once a year in the fall, when it cools off from its summer high. It also provides cooling in the spring when it absorbs a similar amount of heat.
It's hard to account for the soil below, but it's going to be a similar order of magnitude.
OK, but what about days when it gets hot in the day and cool at night? Yes, having heat capacity does help to buffer temperature swings. You know what else helps? Insulation. And here's the thing about insulation: it works for you every day of the year, not just once in the spring and once in the fall, and not just on days that are hot in the day and cold at night. In most climates insulating that basement is a much better investment than trying to use it for thermal storage.
I'm a pedant on this subject because many people have an intuitive idea about how "thermal mass" works, and like a lot of intuitive ideas about science it just isn't correct. When people want their house to have more heat capacity or "thermal mass" invariably what they really want is better insulation. Occasionally we get a poster here who's spent a lot of money building a house to "passive solar" principles, only to find it doesn't save any energy. Invariably we find that no engineering was done before the construction of that house, and that had engineering been done it would have predicted exactly what happened.
While I'm being pedantic: it's "specific heat," not "specific heat capacity."
Someone should tell the authors of ASHRAE Fundamentals that specific heat capacity is not a thing.
DC,
I think the regulars here are aware of your feelings towards 'thermal mass,' both as a concept and as a phrase. I can appreciate a pedantic rant as much as the next chap, but in this case I was simply pointing out an issue with your statement "Heat capacity per unit volume [of concrete] isn't appreciably better than wood, tile, or drywall." If the link I provided had an agenda, I apologize: I was just using it to show a citation of the volumetric heat capacity numbers. That said, those numbers are perhaps not the most reliable; fair enough point.
Looking at other sources and doing some math as you have done, I still contend your statement is incorrect, at least by my standards of your use of language 'isn't appreciably better'. The specific heat capacities you cite for wood and concrete look fair enough (wood being about double). The densities looks off. For common framing species, I see densities more in the 30 lb/ft3 range, and for concrete more like 150 lb/ft3. So maybe 4-5 times as dense as wood.
I will agree that many people probably erroneously think concrete is even more favorable to wood than it is on a volumetric heat capacity basis due to the higher thermal conductivity of concrete vs wood, hence the 'cold concrete, warm wood' feel.
Another angle to consider is that it's not uncommon to have significant mass of concrete in a building or single room (i.e. a slab), whereas wood is used a bit more sparingly in construction, save perhaps for mass timber.
Not at all ironically, the reason concrete has better volumetric heat capacity than wood is because there is more MASS in a smaller space!! haha. I contend the term 'thermal mass' isn't quite the scientific abomination you make it out to be. I understand why you would loathe the term in regards to how it gets applied, given that it does get overused, mis-played, and misunderstood. Mass, as a property, adds thermal capacity. Obviously. Of course so does volume. Neither fact negates the other or makes any extensive property scientifically invalid. 'Heat capacity' — an extensive property — is divided by mass to yield the intensive property of 'specific heat capacity.' Likewise, HC is dived by volume for volumetric heat capacity, or moles for molar heat capacity. The point being, one can choose to use a variety of metrics and not be wrong in doing so. Thermal mass is essentially a synonym for 'heat capacity'— an extensive property. Not pure unaltered science but what is? No one said all mass is equal, nor is all volume equal (in terms of countless scientific properties).
I'd like the volume in which I live to be filled with air please and the mass that is me to be mostly water.
Here's the thing: the implicit assumption is that you need to add heat capacity to your house, and that you need to add so much that where to put all that material becomes an issue, so you need to use a material that's really dense.
My contention is that there's no need to add heat capacity to begin with, a typical house is plenty heavy and made of materials that have good heat capacity anyway. Many of those materials have better heat capacity by volume than concrete, but that's beside the point because they're already there anyway. Just for fun, the next time you see a house being built ask if you can carry the drywall in for them. I just finished a house where we brought in 40,000 lbs of drywall -- twenty tons! Just the drywall compound was probably three tons, we brought it in on a pallet with a forklift.
My broader point is that if you're even thinking about heat capacity, you're doing it wrong. There's an intuitive notion that a house needs to be able to "hold its heat," and the way to do that is to somehow increase the mass. And like many intuitive notions, it's just wrong. If you want your house to have a stable temperature, insulate it and air seal it to the greatest extent possible.
If building science wants to be treated seriously as a science, practitioners have to use the language of science. Words matter. In science, when used as a measurable quantity, "mass" has a very specific meaning, assigned by Isaac Newton who coined this usage. It refers to the property of matter that causes it to resist acceleration when a force is applied. Most commonly this is measured by weighing an object in the gravitational field on the surface of the earth but it can be measured other ways. Scientists may talk of different types of mass -- sprung mass, resting mass, relativistic mass -- but they are all referring to the same property of matter.
"Thermal mass" does not refer to that property of matter. It harkens back to earlier meanings of the word. When Isaac Newton first used the word "mass," he didn't invent the word. It existed already and was used to describe a gathering or accretion -- "a mass of insects" or "a benign sub-cutaneous mass" -- and also as a synonym for enormity. In the second sense it is still used by architects. When an architect talks about the "mass" of a building, he's not saying we should put it on a scale and weigh it, he's describing whether it looks like it was made from fairy wings or cannon balls. And it's that sense that the usage of "thermal mass" comes from -- think of it as a synonym for "thermal enormity."
There's two reasons I start to sweat when people start talking about "thermal mass." First, it isn't the correct scientific term for the property they're trying to describe, that's "heat capacity." But people don't use scientific terms all the time, I don't get excised when people use mass and weight interchangeably, or speed and velocity. Really, it's that "thermal mass" has a history of being linked to pseudo-science. It's a scientific-sounding word that isn't a scientific term. And when I hear it used my radar goes up that what's about to follow is not going to be science-based advice.
While it may be more the conductivity of the concrete itself, the thermal mass (heat capacity) of the earth beneath is certainly not entirely trivial in this case. I mean, if there was no long term seasonal thermal buffering provided by the earth there would be higher temperatures beneath the slab in summer than there are.
All thermal mass does is to damp (reduce) the overall temperature swings. There is no magic creation of energy, it just stores energy and releases energy similar to a battery (actually closer to a capacitor in electrical terms). I've used this before on chilled water systems by using a large tank of water in the cooling loop. The purpose of the tank is to slow down the temperature rise of the system so that the time between a power outage and chiller restart (on generator power) doesn't cause the facility to get excessively hot.
The same thing happens with a concrete mass in a house, but I agree -- insulation is likely to be more beneficial. Thermal mass can help you (release solar warmth at night), but it can also hurt you (slow down the time it takes to warm the house up), which isn't an issue with insulation.
Bill