In the last century, the only cost-effective insulation for the home was mass insulation like fiberglass, cellulose and foam that reduced heat transfer by convection and conduction. The “R-value” was the measure of how good a job those insulators did. Yet R-value measures only the smallest part of residential heat transfer. E-value is the measure of emissivity, radiant heat transfer, the principle source of energy loss. New technologies make it practical to achieve extremely low emissivity in window glass and in a reflective film ideal for the attic. As these low-E technologies advance, the codes will catch up and E-value will replace R-value as the primary measure of energy efficiency.