What Is High Thermal Mass Concrete?

What is high thermal mass concrete? Thermal mass concrete collects and expels heat to maintain a constant temperature. Frederick Bernard, the owner of Acorn Builders, a custom home designer, builder, and...

Frederick Bernard, the owner of Acorn Builders, a custom home designer, builder, and remodeler, says, "Thermal mass is a hard surface that collects heat." He explains that "heat is energy, and cold is the lack of energy. Light and heat are both energy." Thermal mass "collects and expels heat."

During the day, sunlight enters a passive solar house through the windows. The sunlight heats the air and the thermal mass materials which are used in the walls and the floors. The thermal masses absorb and hold onto the heat. Usually, by the time the sun goes down, the thermal masses have collected a substantial amount of heat. As the air inside the rooms cools down, the thermal masses slowly release their heat into the rooms, which helps keep the temperature of the house more comfortable.

Bernard explains that "high mass" material means material that is "very dense. It's very hard and very solid. The aggregate used in high thermal mass material, which would be sand and gravel, would be very dense stone rather than a light stone. Clay products such as rammed earth and adobe are medium mass, but their thermal properties are different in terms of 'thermal lag,' or the time it takes heat to transfer. They absorb and expel heat more slowly. Clays also absorb and expel moisture from the air, so on a humid day, it would be more comfortable in a clay mass than in a concrete mass because concrete does not absorb and expel moisture in the way that clay does."

When asked whether there are any drawbacks to a house made of high thermal mass concrete or clay as compared to a home made from lumber, Bernard answered, "The walls will be thicker. You'll have more square footage per area of room, so it will cost more to build. As sustainable as stick framing is, the advantage of high mass is that you have more protection from hurricanes and tornadoes. Wood is also a soft product, and it's organic, so it will deteriorate, whereas earth only moves from one place to another. It may deteriorate or wash out over a long period of time, but it's still dirt, so it won't burn, bugs won't live in it and bugs won't eat it. So the fire safety and the insect deterrence are two factors, along with the energy it saves in heating and cooling, that will save you money in your energy costs and safety."

There are also other factors that create differences in construction costs for houses made of high thermal mass materials and houses made from lumber. The U.S. Department of Energy, on its Energy Efficiency and Renewable Energy (EERE) website, says, "To make the best use of the thermal mass, additional insulation materials are typically added," such as insulating concrete forms. "Therefore, insulating a building using a thermal mass system usually costs more than insulating a typical wood-frame building. But some other construction costs may be lower. For example, buildings constructed with pre-cast concrete walls will have lower costs from less construction time and limited use of drywall."

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