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Thermal MassThermal mass is the ability of a material to absorb heat. On the other hand, Thermal inertia is the rate at which heat moves through the material. Materials with high thermal mass also exhibit slow rates of thermal inertia. Because of the storage of heat, the material has a longer resistance to temperature change, from one side to the other. Of common materials, water will hold the most heat, and rock or heavy-weight concrete the second most. Denser building materials like concrete and heavy timber have thermal mass, or in other words a “mass effect.” The mass effect includes both the amount of heat stored in the material, plus the length of time required for the heat to pass through. Lighter materials like wood framing and light gauge steel have so little thermal mass, it’s considered to have none.
Insulation is distinctly different from thermal mass. It has virtually no ability to absorb heat, but rather acts as a barrier only. Conversely, thermal mass is stored heat which when released, modulates and smoothes inside room temperature as the result of it’s ability to release heat to the cooler side as temperatures swing. If temperatures are too extreme to be smoothed, the lag time for inside temperatures to change result in the Heating and Air Conditioning demand being satisfied late in the day or night when the demand is the least. Stored heat in mass is released to the cooler side—often in a temperate climate to the inside in the winter and the outside in the summer. (Except for extreme conditions) Because AAC has no additional insulation, summer breezes can carry heat away by convection. Homeowners living in aerated autoclave concrete homes report power bills one half their neighbors. AAC, a lightweight concrete product is less dense than concrete, but is also self insulating. Concrete is considered a high mass material and AAC, a medium mass material. However, concrete has to be insulated, which seriously degrades the ability of the concrete to gain and release heat, which has created an age old discussion about whether to insulate inside or outside of concrete or both. After applying insulation to concrete it’s no longer able to act as a high mass material—at least on the side that’s insulated. The diagram below illustrates the remarkable value of the “mass” effect. A 12” AAC wall is painted black on the sunny side of the building to gain as much heat as possible. As illustrated in the diagram, the outside wall reaches a maximum temperature of 180 degrees while the inside wall gains maybe 2 degrees maximum. The temperature varies wildly outside yet the indoor temperature remains moderate. What’s harder to see is that the heat stored in the walls moves into the cooler space as the sun sets. Notice the temperature in the house rises starting at about 1600 hours or 4:00 PM, as the outside temperature continues to drop. |
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