The common life expectancy of knob and tube wiring is 25 years, due to the rubberized cloth insulation used on the wires beginning to break down. While the wiring itself remains functional, the deterioration of insulation increases the risk of injury and damage that can cause more serious issues. As this type of wiring is an outdated option, dating back to the early to mid-1900s, the homes that still have it today are living on borrowed time.