In this project, waste heat represents expensive heated air that is leaving a home, instead of staying and keeping the house warm (Fig 7). Waste heat typically escapes through poorly insulated doors, windows, walls, ceilings, ductwork and electrical fixtures (i.e., pot lights). This is costly to the home owner, generates considerably more greenhouse-gas emissions than necessary, and is invisible to the human eye. So most people never consider it until they get a huge energy bill in the winter, and wonder why it cost so much.