Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow. So it is thought that the higher the R-value, the greater the insulating effectiveness. The R-value of thermal insulation depends on the type of material, its thickness, and density. In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added.R-Value is a very accurate and reliable expression of how insulation materials perform in a laboratory apparatus. But people don't live in laboratories. They live in homes with real walls and ceilings, and in the real world of buildings R-Value is only one factor in the actual performance of insulated building assemblies. Just because a material has a higher R-value does not mean it will hold heat in a house better. If the insulation is not tightly sealed, air can traverse through it in and out. In a mild climate that is just fine, but in the great white north where the winters are harsh and the summers can get blistering hot, air flow is a very bad thing. This is where polyurethane spray foam is a cut above the rest. It completely seals in the thermal envelope, reducing heating costs by up to 50%.
Many factors are taken to account when calculating the insulating effectiveness of a system. These include:
• Total R-Value of all system components.
• Air Infiltration due to leakage through gaps in the system. • Air infiltration due to permeability of system elements.
• Convective flows within insulated systems.
• Thermal bridging across the building envelope.
• Thermal mass of building components.
Where a fibreglass insulation may have a stellar R-value, it also has a risk of allowing convection air currents to run through it. U.S. scientists have proven that convective flows in very light density attic insulation can reduce its performance by more than 40 percent under winter conditions. Canadian researchers have reported a similar effect in walls.
R-value is an important aid to consumers when comparing proposals and specifications for different brands or suppliers of the same type of insulation. It is less useful when comparing different insulation materials. R-for R, all insulation is NOT created equal!
Click to read "Beyond R-Value: The truth about insulation performance in the real world of buildings" by the
Cellulose Insulation Manufacturers Association.