R-value is a material's resistance to transferring heat from one side of the material to the other. Roughly, the number given for an R-value of a material expresses how many hours a one foot square surface must be heated to change the temperature on the other side of the surface by one degree.
A piece of foam insulation board has an R-value of 5. This means that it takes five hours to increase the temperature on the other side of the material by one degree. This is very good insulation, and can dramatically improve a house composed of other materials with low R-values of, say, 1/4 to 1/2 (equivalent to 1/4 to 1/2 hours as described above).
Multiple Layers of Material
The R-value of multiple materials can be added together to give the R-value of a wall or roof. When adding the R-values of materials never forget to add the R-value of air if there is an air gap. Air is an excellent insulator and has an R-value of approximately one per one inch of air gap thickness.
Most materials have a standard R-value per inch of thickness, however some materials' R-values are measured for the material as a whole. Be careful when adding R-values that you are using the correct value for a material--the R-value of the wall or roof will be much higher or lower, depending on the numbers used to calculate the total R-value. Check the manufacturer's specifications for particular material R-values.
R-Value and Efficiency
As mentioned above, the higher the R-value, the lower the amount of heat that is transferred through a surface. Try to use very high R-values, e.g. four or five for single materials and 19 to 25 for complete walls or roofs, when choosing materials for a building project. With higher R-values, less energy is needed to heat or cool a structure, and the structure requires less money to operate.
Some building elements, such as windows use a U-value to describe the heat emission of a material. The U-value is the inverse of the R-value. Therefore, the lower the U-value, the less heat is transferred from one side of the element to the other. Double glazed windows have a U-value of between 0.40 and 0.30, which is equivalent to an R-value of 2 1/2 to 3 1/3.
Increased attention to sustainability and rising energy costs is pushing material manufacturers toward higher R-values. Materials such as silica aerogel and improved polyurethane insulations are providing R-values of between 8 and 10. Future materials will push toward the ideal R-value of 30 per inch of thickness. These individual materials will be assembled to give a wall a total R-value of 50 or more.