Low E glass is a wonderful new technology. Window glass underwent revolutionary changes in the 1970's. Insulated glass (two or more pieces of glass with a dead air space between) made its debut in the early 70s. Low E glass (Low emissivity glass) was introduced in 1979. Second generation Low E glass (dubbed Low E squared or Low E2) was introduced in 1983 with several companies claiming to have invented it. Since then the major glass companies have made improvements yearly. Low E glass works by reflecting heat back to its source. It does this by utilizing an ultra thin metallic coating on or in the glass.