The Math

How much heat transfers through an exterior wall into the environment in a given period of time? The theory of Thermodynamics, specifically the formulas for heat conduction using thermal resistance, offers a solution to this question. Thermal resistance (“K-value”) is a material property that can be obtained for common building materials from tables published in literature – it does not need to be calculated for every new application.

With that information and a little bit of math, one can calculate the energy loss of a structure over time, or the energy required to heat or cool the structure. While one can in theory make the walls of a home infinitely thick to make the home’s energy losses infinitely small, there is a practical and economical limit – i.e. it does not make economical sense to go beyond a certain thickness of insulation, because the cost will not be offset by any savings1.

Now, while Thermodynamics is a highly useful discipline in this context, it has its practical limitations. Calculations are always done based on approximations of the real world, by simplifying and omitting unknown or (in the eye of the beholder) seemingly insignificant facts. For instance, the R-Value of a wall is calculated from the R-value and thickness of the wall enclosures and the R-values of the insulation material or airspace in between. A more advanced calculation would even take the wooden wall studs into account. The problem with that is, though, that the calculation is only accurate as long as the wall is built well and the insulation is properly installed – an indeal that is rarely met by real-life circumstances (i.e. real-life contractors).

Therefore Dr. Heet is using a different approach. He is using common sense and guidelines derived empirically from observations to establish guidlines that would be difficult and costly to calculate with the means of traditional Thermodynamics.

1There is actually a mathematical optimum for the thickness of insulation as well, because adding insulation to the outside of a structure increases the surface area.