Achieving a net-zero lifestyle may seem an unapproachably ambitious goal, particularly for the vast majority of us living in older homes, or renting them. But if we stand a chance against climate catastrophe, architects, engineers, and the buildings they create will necessarily play a huge part in moving the needle–and not just in shiny new buildings, but more crucially, by retrofitting existing structures as well.

Energy efficiency is a key factor in reducing emissions and curbing the effects of climate change. Residential and commercial buildings alone accounted for about 39% of total energy consumption last year in the U.S., according to the Environmental and Energy Study Institute, and a recent report from the UN found that 65% of projected building stock expected worldwide in 2060 has already been built.

Do the math and the answer is clear: We need to act to improve existingstructures–and quickly.

A growing number of institutions, schools, and industry groups are hoping to spur that kind of action.

The latest is Harvard University’s Center for Green Buildings and Cities (CGBC) and Graduate School of Design (GSD), which this week launched HouseZero, a new working prototype and “living laboratory” of an older structure that’s been renovated to become an ultra-efficient machine for living. It makes a necessary and approachable case for renovating existing homes into energy-positive structures–or buildings that actually produce more energy than they consume over a lifetime of use, and offset the emissions created by renovating them.

Harvard’s HouseZero isn’t a cold or stark vision of survivalism, but rather a warm, wood-shingled 1940s building that’s been expertly refurbished to meet performance demands of our time.