Your THERMOGRAPHY COMFORT LEVEL – 1st and 2nd Law of Thermodynamics! Why important?
There are two very important laws of thermodynamics all thermographers need to understand: the 1st Law and the 2nd Law. And they are not difficult to understand. The 1st Law of Thermodynamics states that energy can be transformed or changed from one form to another, but it cannot be created or destroyed. To illustrate: let’s say you have $500,000.00 in the bank stored in five safety deposit boxes. There is $100,000.00 in each box, some in paper money and some in gold. You can move it around and you can change from paper money to gold or vice versa but you can’t spend it or add more to the total. If you move $50,000.00 from one box to another box or you exchange $50,000.00 of paper money for gold you still have a total of $500,000.0 in all five boxes, right? Why is that necessary for those of us who use infrared cameras to understand? Since energy cannot be created or destroyed, where does it go? In what direction does it move? This question is answered by the 2nd Law of Thermodynamics and this law is critical for the thermographer to understand. The 2nd Law of Thermodynamics states that HOT goes to COLD. You’ve known this all of your life, especially when you touched a hot stove. To illustrate: you put a cup of hot coffee (your infrared camera reads the surface at 180°F) in a room that has an ambient temperature of 70°F and you leave it there for a while. What happens to it? The coffee cools off every time, right? You can count on it because of the 2nd Law of Thermodynamics. The energy in the coffee transfers to the air; hot goes to cold. If you put a glass of ice (your thermal imaging camera reads the surface at 32°F) in the same room, what happens? The ice melts. Why? The energy in the air moves into the ice due to the 2nd law of thermodynamics; hot to cold.
This 2nd law of thermodynamics becomes very helpful when we are using an infrared camera (thermal imaging camera) to investigate building envelopes for insulation issues or air leaks. Learn to think on a molecular level and ask the appropriate questions. The first question you need to ask when you look at a wall is “Where’s the heat source?” There are two sides to every wall. Which side is warmer? The one you are looking at or the other side? That piece of information tells you which direction the energy (heat) is moving, outside to inside or inside to outside. Heat will travel from the warmer surface to the cooler surface and take the path of least resistance. In a fully insulated wall with framing studs the path of least resistance, the area that is least insulated will be the framing studs unless there are areas of missing insulation. So, let’s say I’m inside the house and I take my Fluke Ti32 and point it toward an exterior wall. I’m using the red/blue palette and I’ve established that the other side of the wall (the outside surface) is warmer than the inside surface. The 2nd Law of Thermodynamics tells me that the heat will be moving from outside to inside. I’m looking at the bays and the framework. What color should the bay be and what color should the framing studs be? Let’s say the bays are R-19 and the 2×4 wood studs are R-3.5. The bays should be the cooler color because they are insulated and therefore resisting the heat movement from outside to inside. The stud constitutes the path of least resistance (or R factor) and therefore presents itself as warmer in color or shade on your IR camera. What if you see warmer areas in the bay? Then you have an “area of investigation”, an “anomaly”. Perhaps you have missing or compromised insulation. Wet insulation also reduces the R-factor. Understanding the 2nd Law of Thermodynamics certainly raises a thermographer’s comfort level.
This image was taken with a Fluke TiR1 IR Fusion camera. The outside surface is cold and the inside surface is warm. So, the heat is moving from inside to outside. In a fully insulated wall the path of least resistance should be only the studs. The bays should be consistently warmer than the framing. Therefore, the studs should show up blue (red/blue palette) or cool (absence of heat due to its movement toward the outside). And yet we see bays that are cooler in certain areas. This is typical of missing and compromised and wet insulation.