Building Science and the Laws of Thermodynamics, Part Zero

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water ice cube bizarre property

Building science is an odd subject. Few colleges and universities teach it. The majority of those who work on buildings call themselves engineers, architects, and contractors, not building scientists. And many of those who do invoke the term can explain at least one formulation of the second law of thermodynamics (we'll get to that below) but may not know what the other laws of thermodynamics are, why their numbering is so peculiar, or even how many there are. Do you?

Well, today let's address this deficiency. If you're going to speak of one of the laws of thermodynamics, it's your duty at least to know the others well enough to tell a curious person what they are and which ones have some bearing in the world of building science. Before we launch into this subject, though, let's get a little perspective on thermodynamics from the physicist Arnold Sommerfeld:

Thermodynamics is a funny subject. The first time you go through it, you don’t understand it at all. The second time you go through it, you think you understand it, except for one or two points. The third time you go through it, you know you don’t understand it, but by that time you are so used to the subject, it doesn’t bother you anymore.

With that, let's begin this little four-part series on the four laws of thermodynamics. Part one is on the first of the laws, which is not the first law of thermodynamics. (I told you it was peculiar!) It is...

The zeroth law of thermodynamics

Let's start with the obvious question: Why isn't it called the first law of thermodynamics? As it happened, this law was discovered after the first and second laws but considered to be more fundamental. So we have a zeroth law.

Now, the statement of the zeroth law:

If two systems each are in thermal equilibrium with a third, they are in thermal equilibrium with each other.

And that, of course, raises the question of the definition of thermal equilibrium. As an example, let's start with a glass of lukewarm water. If you drop an ice cube into it, is the ice in thermal equilibrium with the water? No. It absorbs heat from the water. The ice melts and the water gets colder.

But if we think about that ice cube sitting in the freezer before we pulled it out. The ice is surrounded by cold air in the freezer. Once the ice has been in the freezer long enough to freeze completely and then reach the same temperature as the air in the freezer, no heat transfer happens between the ice cube and the freezer air. Now we have a case of thermal equilibrium.

Thus, two systems that are in thermal equilibrium are at the same temperature, are in thermal contact with each other, and have no net heat flow from one to the other. One consequence of the zeroth law is that temperature measurements are kind of a big deal.

An aside on adiabatic walls

If you do any energy modeling, the idea of thermal equilibrium may make you think of adiabatic walls. If you're doing a heating and cooling load calculation on an apartment building, for example, the wall that separates two units in the building is often called an adiabatic wall in the software. We assume the two units are at the same temperature, and that's why there's no net heat flow across that wall.

In building science, the word "adiabatic" means there's no heat flow, but there could be.  In the strict thermodynamics meaning of the word, though, an adiabatic wall is one that does not allow heat flow. That means you wouldn't see any heat transfer across such an adiabatic wall even if you had a temperature difference.

Building science and thermodynamics

The concepts of heat transfer, thermal equilibrium, and temperature are the important takeaways from the zeroth law of thermodynamics. This law undergirds all of thermodynamics.

Building science is concerned with heat flow through building enclosures and heat supplied to or removed from conditioned space. The laws of thermodynamics lay down the fundamental rules for understanding heat, and that means if you want to understand building science better, you need to know some thermodynamics. When you get far enough into it to know you don't understand it but are used to it, then you can pat yourself on the back because you've reached Sommerfeld's third level of thermodynamics education.


Other Articles in This Series

Building Science and the Laws of Thermodynamics, Part 1

Building Science and the Laws of Thermodynamics, Part 2


Related Articles

Why Doesn't Heat Flow Backwards?

How the Heck Does a Heat Pump Get Heat from Cold?!

What IS Heat Anyway? - Building Science Fundamentals

The Easy Way to Convert between Celsius and Fahrenheit


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I object to the phrase "building science". Science applies to everything. There is nothing unique about buildings when it comes to science. You can (and should) apply science to buildings, and applied science is often called "engineering". According to, Engineering is defined as "the art or science of making practical application of the knowledge of pure sciences, as physics or chemistry, as in the construction of engines, bridges, buildings, mines, ships, and chemical plants." So even though many people like to call themselves "building scientists", they are really "building engineers". The difference between engineers and scientists is that engineers like to do something useful with science whereas scientists like to expand scientific knowledge just for the sake of the knowledge itself. Watch Big Bang Theory, it explains all of this quite nicely. By the way, I am an engineer.


Roy, you're right that buildings aren't unique in terms of science. But I think the term "building science" does have a place in our lexicon. The field of building science includes a lot of the narrower disciplines of science and engineering: physics, materials science, mechanical engineering, psychrometrics... The big deal with building science is the building-as-a-system perspective, which many of the individual disciplines don't have.

By the way, I hate The Big Bang Theory.

By the way, Big Bang Theory is my favorite show, even if it is about a bunch of scientists making fun of the lone engineer in the group. So far, Howard is the only one that has done anything useful on that show ;-)

My comments were somewhat tongue-in-cheek, but I have engineering degrees and you have science degrees, so I guess that we are on opposite sides of the fence on this one. I prefer the term "building engineering" or perhaps "engineered buildings". I just can't figure out why Joe L. calls it building science when his background is engineering. Perhaps "Building Engineering Corp." was already taken when he started his business?

My Canadian company was called Building Engineering Corporation (BEC) and my US company was called Building Science Corporation (BSC). I ended doing more work in the United States and ultimately moved to the United States and became a citizen. It was much easier to register BSC in the US than use "engineering" in the corporate name due to individual state engineering licensure issues. In terms of building science as a body of knowledge or as field of discipline note that it involves much more than might have heard of architecture and material science and run across a few master builders in your travels...

Gosh and here I was hoping you were going to explain that funny looking ice-cube!

Joe, I have to disagree. Architecture has nothing to do with engineering or science. Architects are just artists that like to get paid, and engineers and builders (usually just builders) have to figure out how to build it. "Material Science" is a misnomer. It should be called Material Engineering. As a mechanical engineer, I had courses in physics, chemistry, math, materials, thermodynamics, structures, etc. When I use that knowledge to do something useful, it is called ENGINEERING. By the way, my father was a builder and I spent my summers and vacations working with him until I got too smart to do anything useful.

Roy, to really make your day, over in Europe they call it building physics...

My goodness Roy, it's difficult for this architect to see you waaaaaaaay up on that mighty engineering horse you ride, but I must say, I have thoroughly enjoyed this article, this conversation, and a thoughtful new perspective on the term "building science". I have a lot of respect for all three of you gentlemen (Allison, Joe, and Roy!). I would not be the architect I've become without the teachings and musings of gents like you. If you ever need someone to put some icing on your engineering, please give me a call, I love making engineering look lovely. I also like very much, applying science to art and architecture to make our world just a little better.

Thanks guys!

Well, as Sheldon Cooper would say, physics is the basis for all other sciences. But as Howard would say, engineering is the useful application of science.

Timothy, I have to admit that I enjoy making fun of architects. But I also admit that it is partly jealousy due to my complete lack of artistic ability. I love to build things, but I have to have some kind of plans from others, otherwise I just end up with square boxes, of which even I don't like the looks. So I guess that I am a degreed engineer with knowledge of science, a somewhat experienced builder, but definitely not an architect or artist. I admit that we need all of these skills and talents to make a decent house.


Are you actually referencing Big Bang Theory to validate your point? I'm with Allison. I hate that show. True nerds do not need a laugh track to know something is funny. Heck, we don't even need to react externally. So right now, RoyC, maybe we are laughing at you. But you wouldn't know, would you?

Allison, suppose one of your colleagues discovers an even MORE fundamental law than this one. What will we call it? The double-zero? Or the more American, double-oh? Or, in this twenty-first century, are all of the laws just trying to be equal?


Jeff, if we have to put another law before the zeroth it would of course be the negative-first law of thermodynamics. (Or would we call it the negative-oneth?)

Jeff, I guess to each his own. I am not ashamed to admit that I like The Big Bang Theory. I don't understand your comment about the laugh track, especially since I didn't realize that it had one. But since there does not seem to be any other fans of this show on this blog, I will quit mentioning it. However, I will keep keep defending engineers over scientists until Allison quits letting me comment.

Your numbering system sounds rational to me.

RoyC, I'm queuing the laugh track for you now...

Since you brought the subject up in this, another of your many informative articles, I thought it might be an excellent opportunity for me to announce an ongoing project I have been laboring over in my garage for the past ten years.

In an effort to revolutionize the construction industry, my project to build an adibatic wall continues apace and I'm pleased to report numbers to date approach characteristics of a Dewar Flask and although it seems I have "hit a wall" so to speak, I'm confident further development will result in a truly unique structure, one easily replicated with contemporary building practice, whether on-site or in a factory environment.

Readers of Energy Vanguard will of course be given preferential treatment when my IPO is announced.

Watch this space.

No matter where you go there you are, I think!

So here is an interesting question that I have seen raised from time to time. Assume that you have air flowing through an adiabatic duct (you can consider it very well insulated or that it is a return duct in a conditioned space with no significant temperature difference across the duct walls). However, the duct is undersized as usual, thus there is a significant pressure drop due to fluid friction in the direction of flow. How does the temperature of the air in the duct vary in the direction of the flow?


According to Joule and the other scientists who proposed that mechanical work is equivalent to heat, the temperature should rise as the mechanical energy converts to internal energy.

Allison is wrong! Engineering wins over Physics! There are two possible answers left. Any takers?


Dang! OK, I see it now. I answered too quickly.

I guess that no one else wants to take on this question. If you assume that air in a duct is an ideal gas (which is a very good assumption), then the temperature does not change. Conservation of energy says that the fluid enthalpy does not change since there is no energy (heat or work) exchanged with the surroundings, and enthalpy is only a function of temperature, not pressure, for an ideal gas, so the temperature does not change. If you really want to get into the details, if you have a large enough pressure drop in this adiabatic duct, you might get a detectable temperature change, but it will be a decrease, not an increase.

So why is the zeroth law "more fundamental" than the first law? Was, or is, this numbering order controversial? It's not immediately obvious to me why it should precede the first law, which I always think of as a kind of definition of energy's existential state ... and the zeroth law more related to the state (equilibrium) necessary to define prior to describing heat flow (the second law).


Great question, Clay. The answer lies in what I wrote about temperature above. The first and second laws lean heavily on temperature. If you read the second article in this series (part 1), you'll see that I talked about how the technical form of the first law is a relation between work, heat, and internal energy. You can't quantify heat without a way to measure temperature. The zeroth law basically defines temperature and thermometry.

Simply: we have already said there is equilibrium between the ice and the cold air in the freezer. Let's say there is also equilibrium between the ice and the ice tray. The zeroth law tells us that ice tray and the cold air of the freezer are also at equilibrium. They use the same scale to measure the temperature. This one law allows even mere scientists AND engineers to make important calculations.

-Neither a scientist, nor an engineer (and why do the numbers in the CAPTCHA keep going up as I keep commenting on this article?)

I see ... I did read Part 1 and maybe I was drawn into the reference to Colding and a mechanical equivalent to heat, and thinking "ok, shouldn't start with a base case of discussing *energy* before we start in on the specifics of one type (thermal)" ... but now see from your explanation that as a law specifically applied to thermodynamics, we need to start with a system that allows us to measure that specific kind of energy, and on that basis the order is starting to make more sense ... Ok now, forgive my continued exploration, just that the word "energy" (not "heat") in the first law piqued my attention: Does the first law really apply to all energy - kinetic and potential? ... or just all kinetic? (thermal, mechanical, magnetic, etc.)? ... or some subest of kinetic only? ... I do see the corelations between mechanical and thermal energy, and apperciate your examples of engines and ceiling fans and such. But wondering how far the first law can be extended to evaluate (eg. sniff out scams) in new systems using magnetic or chemical or sound energy. In my position, marketing folks and entrepreneurs throw out all kinds of crazy claims, the verasity of which is mostly easy to identify, but not always ... this is the reason for my asking; not sure if it needs to be answered here. Love your articles (and the commments of your community), Allison ... always stimulating.

Thank you, Jeff. Yes, measurement and calculation is what I am finally seeing it is all about (not philosophical whims and rhetorical fancy).

(I hear you about the CAPTCHA. I'm worried it's going to break out into finding derivatives and I'm going to have to search my shelves for an old textbook.)

First of all because of our sun, there is no equilibrium. Yes, it is nuclear fusion but the sun will run out eventually and is constantly emanating heat. There can be no equilibrium because of this.
You cannot recapture heat. And of course energy is neither created or destroyed and leads to more entropy ( misnomer if you ask me).
The more we transform building materials, the more entropy (heat) is released.
Whether it's energy efficient building, Solar electric, Solar thermal, there is no equilibrium unless you can prove that when the life cycle of that product can equal energy saved AND energy used to recycle that product in addition to the sun emanating heat.
Magnets, Solar panels, wood, steel and tile roofs, air conditioners, insulation, windows, etc have a life cycle.
There is no equilibrium and most importantly, how much more entropy is created when recycling?

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