SOLAR TODAY - June 2011

case study

Mass floors are 32 inches (0.8 meter) thick inside the insulated envelope. the 5-inch slab with radiant tubes sits on 27 inches of compacted gravel with fines. this system provides many times the heat storage of the slab alone and is intended as a backup for the space heating.

Conservation was the

the entire exterior of the house is one piece of r- 28 spray foam with traditional southwestern stucco. it sticks well to the mass walls, creates a breathable, waterproof surface that lasts a long time, plus it creates an airtight barrier.

guiding principal, followed by use of the smallest needed amount of energy to meet the home’s needs.

to avoid freezing within the energy-recovery ventilator (Erv), the unit is typically paired with an electric preheater. but to avoid this energy use, builders ran a 6-inch-diameter pre-heat duct through the solar-heated mass floor and then to the Erv.

design is simple and low-tech: We mount a flat reflector above the solar collector, which is tilted perpendicular to the average winter sun angle so as to maximize midwinter gain.

Concentrating the sun’s energy can be a fire hazard. But, because these are flat reflectors, there is no concentration or the associated fire hazard. Our northern New Mexico winters have gotten cloudier in recent years, requiring an increase in collection area. In years past, I installed solar thermal panels at 70 degrees to point at the lower range of the winter sun, to protect from summer overheating. Now I use the more powerful 55 degrees ( 20 degrees plus latitude), which is vulnerable to summer overheating. The partial shadow cast by the overhanging reflector protects from summer overheating, while still supplying enough heat for the domestic hot water. (More details in the June 8, 2010, issue of Solar@ Work: tinyurl.com/sw-collector.)

The system is also designed to trickle some heat into the radiant floor’s deep zone during the summer. It may seem as though that would add heat to the air in the house, but as long as the floor temperature remains lower than the inside air temperature, no heat is transferred to the living space. With 270 tons of mass in the floor, some heat is needed in the summer to maintain the temperature at about 70° F ( 21° C). The high-mass floor takes longer to heat than a typical 4-inch slab, so preventing it from overcooling is important.

Industry data reveal that absorbing heat with a concrete floor to cool a room in summer is ineffective; therefore giving up the cooling effect of the floor is not an issue.