THERMAL MASS: Why is wood better than other mass materials?

Thermal Mass refers to materials that have the capacity to store heat (thermal energy).

If the properties of a Thermal Mass material allow it to effectively absorb heat from a source and then release the heat slowly over a period of time, then the Thermal Mass can be effectively used in a structure to spread heat from the time the source is available to periods when the source is not available. In this form the Thermal Mass can mitigate high and low temperatures to help maintain a comfortable temperature within the structure.

WHY IS WOOD BETTER?

  • Dry wood used in construction is not a uniform material. It has millions of tiny cellular pockets filled with trapped air. These air pockets resist the flow of heat stored by the solid mass structure of the wood.
  • On a weight basis, softwoods used in construction can store 60% – 90% more heat than concrete and 300% to 400% more than steel. However, because of their density, on a volume basis both steel and concrete store more heat than wood, which in fact carries their sun-heated temperature beyond the comfort level of humans.
  • Dense materials conduct heat so rapidly to the cooler surface where the heat is emitted, that the mass becomes cold, performing as a huge energy-consuming sump for your heating system when the sun is not providing its radiant energy. Conductance of concrete is 10 – 15 times that of wood, while conductance of steel is 300 – 500 times that of wood.
  • Low-density insulation materials have no mass to store heat. Therefore, their only function is to provide a resistance to the flow of heat by their formed structure of trapped air spaces.
  • Wood has a significant inertia to heating and cooling created as a result of its high heat capacity on a solid material weight basis but low conductance on a volume basis due to its millions of trapped air-filled cells. As a result, wood never cools to the point of becoming a heat sump for your heating system or warms beyond human comfort, which would create a demand for your cooling system. It is an exceptionally compatible material for a human environment.
  • Wood mass provides a building envelope that is highly energy efficient in performance against today’s energy code standards, due to the combination of the Conductance/Insulating Factor, the Time-Delay for absorption – conductance and emission, and the Decrement Factor provided by the slow absorption and conductance that creates a process of re-radiation back toward the source rather than passage through the material.