Photos: MIT's solar house under construction

A passive solar home needs thermal mass, material that can absorb and emit solar energy gathered during the day to heat home at night. The MIT team has built a "warm light wall" made up of 1-foot-thick tiles that will altogether hold about 60 gallons of water. The water and a layer of aerogel insulator will help even out fluctuations in interior temperature.

Photo by Martin LaMonica/CNET News.com