The official unveiling this week of the KB Home GreenHouse: An Idea Home Created with Martha Stewart was preceded by a months-long drumroll touting its virtues: energy-efficient yet affordable, and, of course, representative of Stewart’s notions of comfort and style.Judging from the photos of the 2,700-sq.-ft. house — which opened for tours on Wednesday to participants at the International Builders’ Show in Orlando, Florida — it does seem to deliver solidly on the traditional, Stewart-inspired styling and amenities found in many communities built by KB Home, which has had a marketing alliance with Stewart for some years. But the advertised energy efficiency of the house — net zero energy — is new to KB Home, which, along with Pulte Homes, topped a recent survey by Calvert Investments, a specialist in socially responsible investing, intended to rank the green practices of the nation’s 10 biggest publicly traded home builders.Adding insulation, increasing airtightnessThe thirteenth project in Builder Magazine’s Concept Home series, KB Home GreenHouse started attracting media attention last fall, while construction was still underway. KB combined the production-home strategies it uses to help control costs with techniques to increase the thermal resistance and airtightness of the concrete-block exterior walls and the roof, which includes a mix of concrete tile and, in some sections, photovoltaic tiles, plus a solar thermal system.The interior surfaces of the CMU walls were insulated with foam panels. The exterior surfaces were covered with housewrap and topped with fiber cement siding. The roof was insulated with open-cell foam. According to Concept Home videos about the project, the home’s HVAC system includes a heat-recovery ventilator as well as a highly efficient dehumidification system. GreenHouse is located in Windermere, not far from Orlando, which sees very mild winters, comfortably warm weather in the spring and most of the fall, and hot semi-tropical summers.As the first net-zero-energy home in the KB lineup — and as the main showcase for this product line — the company designed and built the house to qualify for a number of green-home credentials, including the Environmental Protection Agency’s Energy Star and WaterSense labels, a LEED for Homes Platinum rating, and an Environments for Living green certification.Of the home’s $380,000 listing price, a premium of about $70,000 was spent to achieve net-zero-energy performance and the green-home labels mentioned above. A KB official noted that the price premium will shrink significantly as the company’s NZE category grows.