Know the Second Law of Thermodynamics to Design Better Building Envelopes

The laws of thermodynamics outline the fundamental rules for understanding heat. If you want to design better building envelopes, then it’s important to understand the building science. 

More specifically, the Second Law of Thermodynamics, which describes the flow of energy from a hotter body or space to a colder one.

To design building enclosures that work, we need to know in which direction heat, air, and moisture will flow most of the time. In analyzing failures, we can rely on that flow direction to help us understand what happened.

Dr. Joe Lstiburek, Principal at Building Science Corporation, explains the Second Law in more detail and why you need to take proper consideration of the specific climate in which you build. Otherwise, you risk violating the fundamental laws of physics and designing a building enclosure prone to failure.

A summary of the Second Law and building enclosure design

To Dr. Joe, the Second Law of Thermodynamics is the most important for architects, general contractors, and engineers designing an energy-efficient, durable building enclosure.

But it’s difficult to understand when most definitions are like the following: ‘In an isolated system, a process can occur only if it increases the total entropy of the system’.

Nobody knows what that means. 

Fortunately, Dr. Joe has translated that to create a simple definition to understand the concept. Basically, the Second Law states that heat moves from warm to cold. This applies to moisture moving from warm to cold, moisture moving from more to less, and air moving from a higher pressure to a lower pressure.

When it comes to building enclosures, we’re concerned with moisture. And there are two components to that:

  1. Moisture moves from warm to cold, and
  2. Moisture moves from more to less

This typically acts in the same direction but sometimes in opposite directions. When this happens, more to less beats warm to cold. 

Rock beats scissors, scissors beats paper, paper beats rock, more to less beats warm to cold. 

For the most part, building professionals will be able to know which parts of a building will be warmer and which parts will be colder. They can look at a set of drawings and say this part will be warm, and this part will be cold. 

What it tells you is that most of the time, the moisture ends up in the cold spot.

Let’s say you’re in Canada, where there are two seasons – this winter and last winter. The inside is warm, the outside is cold, moisture is moving from the inside to the outside.

Now let’s say you’re in Florida or Texas, where air conditioners work full-time. That makes the inside cold while the outside is (unbearably) warm and wet, meaning the moisture will move from the outside to the inside. 

This is why, in cold climates, it’s not a good idea to have a vapor barrier on the outside if it’s cold. However, you can have a vapor barrier on the outside of your framing in a cold climate if you put a lot of insulation on the outside to make it warm enough not to cause issues. 

In warm and wet climates, it’s a terrible idea to put something impermeable on the inside, like vinyl wall coverings, epoxy paint, or oil-based coatings or paints. This can cause mold in air-conditioned buildings.

So, when you’re designing a building enclosure, remember that moisture moves from warm to cold, moisture moves from more to less, rock beats scissors, etc.

Continue leveling up your building science knowledge with Dr. Joe

About Dr. Joe Lstiburek 

Dr. Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. Dr. Lstiburek holds a Bachelor of Applied Science in Mechanical Engineering, a Master of Engineering in Civil Engineering, and a Doctor of Philosophy (Ph.D.) in Building Science. Get the full scope of Dr. Lstiburek’s work, accolades, and contributions to the industry over at Building Science Corporation.