Insulation Primer
Insulation is a material designed to slow down the flow of heat. In the building envelope, the primary function of insulation in most North American regions is to keep heat in but insulation also plays an important role in keeping unwanted heat out during the summer and in hot climates. Most insulation materials are lightweight fibrous or cellular materials that enclose air or gas pockets. Heat flows through these materials by conduction, convection and radiation:
- Conduction occurs through both the gas and the solid material separating pockets of gas. Insulation is most effective at stopping conduction.
- Convection occurs as air circulates through the insulation material and is usually a minor component.
- Radiation occurs across air pockets in insulation or through air gaps in the building's shell.
Measuring insulation effectiveness
In considering the effectiveness of insulation materials, remember the following:
- R-value is the most common measure of the resistance to heat flow (thermal resistance) used in the U.S. The higher the R-value, the better the insulation ability (R-values for typical roofing systems are shown in Figure 1).
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Figure 1: Insulation value ranges of common roofing systems Source: Platts |
- U-value is the inverse of R and is known as thermal conductance, measured in Btu/ft 2-°Fahrenheit-hr.
- R-values of different components can be added (all the different layers of a wall, for example); U-values cannot be directly added.
- The primary insulator in most insulation is air and the goal in designing and installing insulating materials is to keep the air as still as possible.
- The highest R-value that can be achieved with conventional air-filled insulation materials is R-4.5 to R-4.7 per inch. However, advanced materials are under development with R-values that are considerably higher and these may find their way into the market in the coming years.
- Remember the law of diminishing R-eturns (Figure 2): Each additional unit of R-value contributes less energy savings than the previous one. As the graph shows, the U-value curve, the amount of heat that moves through the material for each degree Fahrenheit difference in temperature, quickly flattens as R-value continues to climb. In practical terms, this means that adding R-10 insulation to a wall that already has R-20 insulation will save very little additional cooling energy. Economic analyses that account for the cost of energy saved can help in determining the most cost-effective level of insulation.
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Figure 2: The law of diminishing R-eturns While high R-values sound attractive, the real effect of insulation—the U-value—diminishes with each additional inch of insulation. For example, an insulated 2 x 4 wall has an R-value of 13 and a U-value of .077. Upgrading to a 2 x 6 wall adds 5 points of R-value (18-13) but only drops the U-value by .022 (.077-.055). Adding another 5 points of R-value would lower U-value even less—by only .011. Source: Platts |
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Last Modified: May 9, 2009
