Most of us know that a full refrigerator uses less energy than an empty one, because anything’s that denser than air will better store the cold. Opening the door of an empty fridge causes cold air (weighing less than an ounce) to rush out and fall to the floor. Warm air enters. When the door closes, the unit cycles on, and the process repeats. That’s sort of how thermal mass construction works. Beefy Styrofoam and concrete walls on an insulated foundation cut both heating and cooling loads. It’s vastly more efficient to keep a relatively massive structure at a steady temperature than it is to manipulate the air with energy-intensive HVAC systems. Read on for a closer look at this green building strategy!
A 3,000 square-foot home with 10ft ceilings has about 12.7 lbs of air inside that wants to dart about, changing temperature. But the walls of a same-size thermal mass house might weigh 350,000 lbs – and this weight stubbornly resist thermal gains or losses.
There’s also something called the thermal mass effect. Thick walls absorb heat during the daytime, which is released to the interior at night when it’s colder. That’s why desert dwellings have traditionally been made of adobe, or mud bricks. “We heat and cool our homes like an empty refrigerator, which doesn’t make any sense from a physics standpoint, because air has basically no mass,” says Joe Britt, an engineer and father of three who built his own thermal mass home in upstate New York.
Modern thermal mass homes feature walls and floors made from layers of concrete and insulating foam; everything’s embedded with radiant pipes. These inexpensive, familiar materials make for a home with a tight energy envelope that resists fire, termites and rot. The exterior can be finished any number of ways. While thermal mass construction, or TMC, costs about 20 percent more than a stick-built house, that’s quickly offset by the energy savings, particularly in extreme climates. Some thermal mass houses are round, but that’s a matter of taste, not structural necessity. Most contractors use manufactured precast panels delivered to the job site. Dow Chemical Company’s T-Mass is one popular brand; with the proper equipment, walls go up in a day. A caveat: last-minute design changes aren’t feasible.
Britt poured his walls in place using insulated concrete forms, known as ICFs. Building with ICFs is less expensive and more flexible than precast, but it takes 27 days to cure the concrete.
Touted as the evolution of residential architecture, TMC’s appeal is ironically something cavemen understood. “Once you get a massive amount of rock to temperature, it stays,” says Britt. “With the radiant tubing, using a simple loop in the ground plus a circulating pump, you can keep the interior temperature steady with minimal energy. We don’t even have a furnace, just a normal pellet stove, and in the summer, a small window unit air conditioner keeps the whole house cool.”
Britt says that when TMC is combined with smart design, solar hot water and photovoltaic components, a structure can easily be self-sustaining, i.e., off-the-grid. “That’s freedom,” says Britt, whose wife works for a utility company.
Lead photo from Monolithic