An object that dose not allow energy to pass through it

Insulator 

Insulation isa property of objects that inhibits heat transfer and its counter property is conduction. 
As much hot air is pressured into the room space by a condiitioner, the heat accumulates. The objects we have at home like cloth, wood and other objects inhibits or gradually slows the heat to escape and going onto the cold areas outside. This prevents the room temperature from shrinking from its desired state.