The term “adaptive” in the name of Ruth Mandl and Bobby Johnston’s Brooklyn-based firm may sound like a buzzword but is an apt description of their work. They focus exclusively on renovating and adapting existing buildings. Though one of their first projects was the renovation of a classic 19th-century Brooklyn brownstone, historic preservation was aligned with a substantial reduction in greenhouse-gas emissions and an elegant, even whimsical, way of reusing existing elements. “Asking what sustainability can be has been a journey for us,” said Mandl, 39, on a tour of one of their recent projects.