Do Not Confuse Mass With R-Value
Thermal resistance describes how strongly heat flow through the wall is resisted. Thermal mass describes the ability of the material to absorb and release heat.
They are different properties.
A heavy log wall can moderate temperature changes because changing the temperature of a large mass of wood requires energy. That does not turn thermal mass into additional R-value, and we will not pretend that it does.
Thermal resistance is discussed separately hereFirst, How Much Wood Is in the Wall?
Before calculating stored heat, we need the mass of the wall.
That starts with the actual wall geometry. From the volume of wood and the density of the species at the relevant moisture condition, we can calculate the approximate mass.
A wall built from substantial natural logs contains a lot of wood. That is obvious when standing beside one, but in this section we want to put a number on it.
Species, density and other material dataThe Calculation Is Straightforward
Once the mass of wood and its specific heat capacity are known, the amount of thermal energy associated with a temperature change can be calculated using a simple equation.
- \(Q\)
- Thermal energy stored or released (kJ)
- \(m\)
- Mass of the wood (kg)
- \(c\)
- Specific heat capacity of the wood (kJ/kg·K)
- \(\Delta T\)
- Change in temperature (K)
There is nothing mysterious about the equation. The important part is choosing defensible input values and showing them alongside the calculated result.
In a real log wall, temperature does not change uniformly throughout the entire thickness. Heat moves gradually through the wood, so the calculation represents a simplified estimate when a single temperature change is used for the whole wall.
What the Number Does — and Does Not — Tell Us
The calculation tells us how much energy is associated with changing the temperature of the wall by a given amount.
It does not, by itself, tell us how quickly that energy moves into or out of the wall, how much heat the complete building loses, or what heating system the building requires.
Those questions involve additional properties, geometry, temperatures, air leakage and operating conditions.
So we will use thermal mass for what it actually describes rather than asking one number to explain the entire building.
Next: Load Tests