Carl Warren has been designing & building homes, sun rooms, and additions since 1979. Initially, Carl designed sun rooms. These custom sun rooms were designed in many different styles, to complement the function and esthetics of their host structures.

In 1980 Carl designed his first complete house. Built in Belchertown MA, this house used an innovative solar heating technique called the solar envelope developed by Lee Porter Butler of Tahoe CA. Later called the convective loop house, heat collected in a south facing greenhouse circulated around the inner shell of the house, returning to the sun room through a crawl space beneath the house. The theory was that heat stored in the outer shell's mass would create a temperate zone, lowering the delta T (temperature difference), in effect blanketing the inner (living space) shell.

After extensive monitoring on this and similar structures, it was realized that the true benefit was the thermal coupling with the earth under the house. This thermal coupling helped maintain higher than expected air temperatures as well as contributing to a balanced, healthy humidity level. Another benefit was improved indoor air quality (IAQ) due to the natural filtering process of the plants in the south facing sun room.

Carl abandoned this strategy after this one house in favor of other solar heating techniques that were simpler and more cost effective. In 1982, in collaboration with Bruce Wilson and Norman Saunders, Carl designed and built an all solar house in Boxboro MA. This house utilized a high and low heat storage. The high heat store made up of one gallon glass jugs received