In Insulation 101 we talked about how everything is flowing. Now let’s talk about the flow. The dynamics. To get this point of Insulation 102, we need to cover Heat transfer 10… We are talking about the Zeroth Law. Let’s get right into it and then just stick to it. This is not Heat Transfer 101, just Insulation 102. As we know, Heat Transfer, Thermodynamics etc. can get super complex. The most important thing to understand here is the Thermal Equilibrium, the Zeroeth Law and the relationship between Insulation 101 and Insulation 103. That’s it.

“Two physical systems are in thermal equilibrium if no heat flows between them when they are connected by a path permeable to heat”

WHY THIS APPLIES TO US: First, the two systems – inside the home and outside the home. Second, heat flows – from a region of higher concentration to lower. As in if it is 68F inside the home and 20F outside (winter) the home, it will flow outside. If it is 68F inside the home and 90F outside (summer), it will flow in. It will keep flowing until it reaches equilibrium (equalizes) or something stops it from flowing. Third, a path permeable to heat. As long as nothing stops it from flowing or changing, it will keep flowing. Permeable to heat is an important term to keep in mind.

winterflows summerflows

It keeps changing. Never Stops. It keeps flowing…from a higher temperature to a lower temperature. It goes from the inside to the outside in the winter and outside to the inside in the summer. This flow of heat from a higher temperature to lower is called heat transfer. The change that Heraclitus told us about!


Keep scenario A in mind, when its Winter. It’s not the cold air that’s necessarily coming through that we need to think about, we just need to think about the changing and flowing heat. It flows from the higher temperation (68F inside) to the lower temperature (20F outside).

INSULATION 102: Cliff’s notes: Everything is always flowing and changing. The air from your home is flowing in and out all the time and thus its changing. It always flows from the higher temperature to the lower temperature as its always trying to equalize. So inside to outside generally in Winter and outside to the inside generally in the summer through a path permeable to heat.

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