Heat is transmitted via conduction, convection, radiation or a combination of them. Heat always goes from the hot to the cold. Without insulation, heat will be radiated into the atmosphere. For example an uninsulated 400 degF hot steel surface of 1 sqft will radiate around 400 W (1400 BTU) into the atmosphere.
Insulating this surface would drastically reduce heat radiation and thermal energy cost. Thermal conductivity “k” is expressed in how many BTUs of heat will travel through one square foot of material of one-inch thickness in one hour. (BTU/in/hr/sqft/degF) Every material has its own value. Insulation (“R”) is the reciprocal of thermal conductivity.
Example: One type of “thermal insulation wool” has a thermal conductivity of 0.29 @ 100degF. (1.31 @ 600 degF) This means that a 1 inch thick piece of insulation wool of one square foot surface would permit 0.29 BTU’s of heat to move through it in one hour if there is a temperature difference of 1 degree F on either side. If the Delta T would be 10 degree F on either side, then 2.9 BTU’s would move through it in one hour.
Air is a fantastic insulator because of its thermal conductivity of 0.16. Trapping the air in small chambers, like in foam, provides good insulation.
Delta T is working with two basic type of insulation: