It is called an insulator.
You are cold – you put a jacket on – you get warmer. Everyone knows this.
You get hypothermia, your core temperature drops to 97 degrees. You put on a space blanket, your core temperature increases to 98.6 degrees.
There is a principle that heat always flows from warmer objects to cooler objects, which is sometimes abused by skeptics in a mindless attempt to debunk a theory which is inconvenient for them to believe. A cooler object most certainly can increase the temperature of a warmer heat source.
Scientists need to understand the difference between temperature and heat.