Peak Electric Savings Raise the Importance of Increased Thermal Mass and Passive Solar

Two of four advanced prototype houses where built with T-Mass walls. These walls have 4” of concrete on the inside, 4” of STYROFOAM in the middle, and 2” of concrete on the outside. One of the other houses was built with Structural Insulated Panels (SIP) and the fourth home was built with 2x6 wood framing. Two of the homes have 2 stage evaporative cooling systems, one has an air conditioner with an evaporative cooled condenser and one has an air conditioner with a high SEER, 20.5. The T-Mass house that has the evaporative cooled condenser, also has a NightBreeze system.

After super cooling three of the four homes in a test to 72°F, air conditioners were turned off. On the day of the test the outside high rose to 105°F, yet the high thermal mass home’s interior surface of exterior walls rose only 5°F between 12:00 Noon and 10:30 when the thermostat was programmed to turn the air conditioner on again.

These four homes are each equipped with 3.1 kW AC photovoltaic systems (PV). This means that the owners of the high mass homes, if pre-cooled, can sell most of the electricity produced from Noon to 10:30 at night back to the utility at peak rates where time of use rates are offered. As peak demand, driven by residential air-conditioning, becomes more and more critical, utilities may advocate the benefits of thermal mass and off peak cooling along with alternative means of cooling like night cooling and modern 2 stage evaporative cooling or evaporative cooled condensers.

A high thermal mass house (exterior walls and floor) coupled with nighttime cooling to discharge (cool) the mass or off peak cooling, can result in significant energy and cost savings in homes that can benefit both the home owner and utility. If the high thermal mass home is also designed to admit and absorb the winter sun, theses cooling benefits are added to passive solar heating benefits and further energy savings. With the documentation of cooling and peak energy savings there are more reasons to support the widespread good design and use of thermal mass than in recent times:

1. Cooled mass acts as a heat sink minimizing temperature rising throughout the peak demand period.
2. Decreases mean radiant temperature significantly enhancing summer comfort when cooled.
3. Energy cost savings for the home owner and utility.
4. The mass will also serve to absorb solar heat through windows and skylights in winter to provide stored warmth at night. When heated, thermal mass will provide a higher mean radiant temperature that will significantly enhance comfort during under heated periods of the year.

While these benefits are significant, the cost increases are very substantial. It is conceivable that increased thermal mass above the levels of normal wood frame construction with ½ inch drywall can be achieved less expensively and more effectively than with exterior T-Mass walls.

For example all walls, exterior and interior could have 5/8 inch drywall installed. For pre-cooling at night, surface area of thermal mass is more important than thickness, so increased thickness of drywall combined with tile floors may perform well too. Use of thicker drywall, interior plaster walls, even phase change materials in walls and ceilings need to be further tested and may be far more likely to be employed by production builders than exterior concrete walls. A BAP home is under construction in Fresno, CA that will test the concept of increased surface area of thermal mass by employing concrete Structural Insulated Panels (SIPS) walls.

References:
Borrego Springs Zero Energy Demonstration Homes - www.clarumzeroenergy.com