Air sealing is an important part of insulating the attic. Before we start blowing insulation we go through and make sure that penetrations are air sealed. In this case the furnace flue dropped through an empty chase that was open to the attic. Attics are areas of extreme temperatues and open chases allow those extremes into the home. With the area properly air sealed and insulated, this "open window" to the attic temps has been closed.