The dispatch says a residential structure fire. First due engine with a crew of 4 reports smoke and fire visible from the road upon arrival. An interior initial attack is begun with fire extending through the attic. Command is about to pull the interior crew out because of the volume, velocity and color of the smoke when a backdraft explosion occurs. The backdraft blows the wall on the D side about 2 feet out from the structure and windows are blown 20 feet from the structure. Command tries to contact the interior crew repeatedly. There is no response. There is no RIT in place. This report details the event, the lessons learned and actions to reduce the likelihood of being put in this situation again. The end result is no one was injured. The interior crew had lost their radio and were evacuated without injury.