What does insulation do and why is it important?

Insulation
Material that has the ability to reduce heat or cold transmission

Insulation Terms:

BTU
Amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit.

K-Value
Laboratory determined value of thermal conductance of a material.

R-Value
Ability of a material to retard the transmission of hear. The higher the R-Value, the better the insulating performance.

U-Value
Amount of heat, in BTU, is transmitted through one square foot of material (the door) in one hour at a temperature difference of 1 degree F from one side to the other. The lower the U-value, the better the insulation.