Building Science-Thermodynamics

Building science 101 is going to be a continuing discussion about building science and how it relates to building in a cold climate.  We must start at the beginning, which unfortunately has to do with physics.  A nasty word, but I’ll try my best keep things simple. The basics of building science mostly deal with the laws of thermodynamics.

The second law of thermodynamic (there are four of them that I’ll list at the end of this blog) is the most important.  It states:

“The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value of equilibrium.”

Got that, me neither.  So, let’s talk about is in layman’s terms.  Heat moves from a warm place to a cold place.  A lot of people think that heat rises, it can, hot air does rise, but heat can move down, or sideways.

There are three main ways heat moves, through radiation, conduction, and convection.  It sounds complicated, but if you stop and think about it, the basics of heat movement are really quite simple.  I’ll give some examples.

First there is radiation.  Most of us living in a cold climate have felt a chill while standing in front of a large picture window on a really cold day.  That’s chill you feel is heat leaving your body and moving to the cold surface of the picture window.  The heat is radiating from your body to the window.  The opposite can also happen.  Ever been around a big bon fire on a cool night.  Even if you’re standing far from the fire, you can still feel the heat of the fire on your exposed skin.  That’s heat radiating from the hot fire moving to your cooler skin.

Next we have conduction.  When a warm object is in direct contact with a colder object, heat moves from hot to the cold.  Ever been burned by a frying pan, heat from the pan burns your skin when you touched it. How about cold hands, if you stick them in your armpits, the heat from your armpit moves to your hands.  You  learn that from living in a cold climate.

The last form of heat movement is convection.  This happens when there is a difference in temperature between an object and the air around the object.  This is how our homes stay warm in the winter.  A fan moving air across a heat exchanger in a furnace is a good example.

Now that I’ve discussed heat movement, I’ll try to explain the movement of moisture.  Moisture moves from a wet place to a dry place.  The evaporation of water is an example.  A puddle in my driveway after a rain will eventually go away.  Some of it absorbs into the soil, the rest will evaporate into the air.  Both the soil and air dryer than the liquid water, wet to dry.

Our last important term is air pressure.  Air wants to move from an area of high pressure to an area of low.  A balloon can be used as an example.  Blow up a balloon, and you’re compressing the air inside the balloon.  The air being contained by the balloon is at a higher pressure.  When you open the end of the balloon, the air will escape, returning to the area of lower pressure.

What does any of this have to do with a house? The answer is a lot.  It explains everything from how much it costs to heat (or cool) your home to how your refrigerator (or air source heat pump) works.  Temperature, moisture and air pressure all have an effect on durability of your home, indoor air quality, and whether your home is comfortable or not.  Understanding the basics will help to understand my future building science blogs dealing with the stack effect (an important subject for builders in a northern climate).  I’ll also be discussing blower door testing, heat loss and drying potential through building assemblies in the future.  But now, I promised the four laws of thermodynamics.

Zeroth Law:
“If two systems are in equilibrium with a third, they are in equilibrium with each other.”

First Law:
“Energy can be neither created nor destroyed. However, energy can change forms, and energy can flow from one place to another.”

Second Law:
“The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value of equilibrium.”

Third Law:
The entropy of a perfect crystal of any pure substance approaches zero as the temperature approaches absolute zero.

Got all that, good.  Thanks to Wikipedia for the laws of thermodynamics. Another article for those who would like to learn more is The Four Laws of Thermodynamics by Allison A. Bailes III, Phd on the Green Building Advisor website.

As always, if you want to discuss anything I’ve talked about here in more depth, or have any questions, let me know in the comments.