What does R-Value mean?
R-Value is a measure of resistance to heat flow through a given thickness of material. In theory, the higher the R-Value, the greater that resistance.
Unfortunately, R-Value has become, in consumer’s mind, a universal method for comparing insulations - the higher the R-Value, the better the insulation. But all R-Values are not created equal, because they measure only one of the factors that determine how insulation will perform.
Insulation is, first and foremost, meant to stop the movement of heat. The problem with using R-Value as the sole yardstick of insulation’s effectiveness is that heat moves in and out of your home or commercial building in the following ways:
- by conduction (which R-Value measures), and
- by convection (not measured by R-Value)
- by radiation (not measured by R-Value)
- by air infiltration (not measured by R-Value)
The R-Values of insulation materials are measured in a lab and do not take into account other factors like wind, humidity, and temperature changes. These factors create pressure differences between the interior and the exterior of the building due to things like hot air rising, wind, and mechanical systems forcing air through every little opening.
While a building may look solid, there are in fact thousands of tiny gaps, cracks and penetrations between building materials. Even when a fiberglass batt is installed at R-19, heat can still be lost because of air infiltration, with heat being transported around (or bypassing) the fiberglass batts through convection. Foam insulation can also lead to convection if it is not installed to the entire thickness of the wall cavity. In very low density materials like loose blown fiberglass, heat will actually radiate right through the insulation, and this, along with convection, significantly reduces fiberglass installed performance and your comfort.
A superior insulation system will have good R-Value (prevent heat loss via conduction), will be pneumatically or spray applied, fully filling the building cavity (prevent heat loss via convection), and will be densely packed (prevent heat loss via air infiltration and radiation). Fiberglass meets the first criteria, but not the other three. Cellulose meets all four of these critical performance criteria!
The chart below shows the recommended levels of insulation for different parts of the United States:
(Sources: US Department of Energy)
Note: These values can be achieved in new home construction but may not be able to be reached in existing homes. These recommendations are design to provide the best opportunities for energy savings.
