The Role of Thermal Mass in Building Materials

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In construction design, thermal mass is an inherent property of the construction material that allows it to retain heat, which in turn gives it a lower efficiency against climate changes. In layman’s terms, thermal mass means the amount of force that is required to move a system of mass in one direction. For example, let’s say you take a piece of rope and wrap it around a log twice its size. Once it is wrapped, what is the amount of force required to move the log? Well, once the log is wrapped, there would be no more power to pull the log to the west or east (the direction it is always going in anyway) and the same force would be needed to push the log back north or south (also in the direction it always goes). In this scenario, thermal mass provides the impetus that keeps the project moving forward.

thermal mass

However, what if we take this one step further and substitute our log for a mass…say, the thermal mass of the average man. What is the amount of force required to keep him from getting burned? Of course, his skin and his internal organs are also subject to heat, so it is possible that he would feel burning even without having his arms and legs immersed in the heat of the summer. And what if, because of insulation, his body becomes cooler when the sun shines on him instead of when it reflects off of the walls?

The end result is that the human thermal mass has been greatly reduced. He is cold because of air compression and cooling (he is warm because of water vapor condensation), not because his body is encased in heavy layers of fat. And yet, because buildings can be designed more adequately to encourage comfortable temperatures, indoor temperatures rarely ever reach those extremes. Why? Simple: buildings are constructed with ventilation and temperature control in mind, and this ventilation and temperature control ensure that indoor temperatures are always optimum.

Thermal mass, on the other hand, is neither mass nor temperature; rather it is pressure. Think of the huge air conditioning systems in large office buildings. Large sections of those air conditioning units are made up of foam, or some kind of thermosetting plastic. Thermoplastic is effective because it does not conduct heat well. It has low thermal conductivity and dissipates heat very efficiently; however, it is expensive and difficult to use.

Now let’s look at our table 2. This table shows the large variation in thermal comfort between various buildings and their occupants. On one end, you have a large building in an area with very little natural ventilation. On the other end, you have a building with excellent ventilation but very poor insulation. The large building with poor insulation has excellent thermal mass, while the small building with good insulation has poor thermal mass.

Here is the way the thermal mass on one slab of cold resistant construction varies from that of another slab in the same area. First, the cold resistant construction has more metal than any other building or part of that construction. Second, that structure has more brick than any other construction. Third, that structure has more stone than any other building or part of that construction. That is why the cold resistant slab of thermal mass has a much sturdier core than the cold slab of thermal comfort. Thermal comfort, because it does not incorporate air cooling, simply dissipates heat better than the thermal mass.

Since humans have a need for heat and comfortable temperatures, we are almost always at an advantage when it comes to constructing structures that provide both heat and comfort. However, all too often construction professionals ignore both heat and comfort when designing a new building or remodeling an existing structure. Perhaps this is why so many commercial structures during the last several decades have had problems with increased levels of heat or increased levels of comfort, despite increases in air temperatures.

One method for incorporating both heat and comfort in structures is to create a cavity brick construction. Cavity brick construction is often used in small office buildings as well as in some modular buildings, such as warehouses. It is also used in many pre-engineered buildings. This cavity has a heating component built into it, but the brick acts as a buffer that helps to prevent thermal mass from reaching the interior of the building. When thermal mass passes through the brick, the temperature is slightly cooler than the interior of the building materials, which helps to keep the inside of the building at a comfortable level, even when the outside temperatures are high.

Jenn Fontana
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