It’s no surprise that many people who have the financial resources to build big houses are nowadays building them with environmentally friendly materials and systems that will reduce their reliance on the grid. Some are energy efficient, some are loaded with renewable-energy systems, and many incorporate both.In recent weeks we’ve mentioned a few such houses that, as it turns out, all weigh in at around 5,000 sq. ft.: a six-bedroom in Southern California (4,900 sq. ft.), with a LEED for Homes Platinum rating, and the first- and third-place winners (based on their Home Energy Rating index scores) of the 2010 edition of the Connecticut Zero Energy Challenge (4,539 sq. ft., and 4,944 sq. ft., respectively). Also, in late 2009, we posted a brief about a 5,329-sq.-ft. home in Avon, Connecticut, that was designed to operate at near net zero energy with a lot of help from a 15 kW wind turbine.On prime Long Island turfAnother entry in the big-with-green-ambitions category is a project called the HGA House, a 4,980-sq.-ft. lodge-like home in Southampton, New York. HGA stands for Hamptons Green Alliance, a nonprofit forged by the project team that designed and built the house, which replaced a home destroyed by fire in December 2008. The group’s mission, Hamptons Green Alliance says on its Web site, is to promote the design, construction, materials-selection, and performance standards that, in December, landed the home a LEED Platinum rating and a HERS index of 25.“Though the Hamptons are probably best known as the summer playground for the rich and famous,” the group says, “a home devastated by a tragic fire is being transformed into what will be a sustainable home that will set a building standard for others to follow.”The exterior walls of the four-bedroom, six-bath house are insulated with open-cell foam and fitted with windows (from Green Mountain Windows) with U-factors ranging from 0.26 to 0.29. The roof’s 2×12 rafters are insulated with closed-cell foam. The house is equipped with a ground-source heat pump for space heating and cooling, a solar hot water system, and a 10-kW photovoltaic (PV) array (including 6 kW supplied by thin-film PV material applied to the structure’s metal roof).The building’s LEED for Homes score, as mentioned in a post by The Energy Collective, was 104 points, well above the 90 it needed for a Platinum rating.