Insulation is a material designed to slow down the flow of heat. In most North American regions, the primary function of insulation in the building envelope is to keep heat in, but insulation also plays an important role in keeping unwanted heat out in warmer months. Most insulation materials are lightweight fibrous or cellular materials that enclose air or gas pockets. Heat flows through these materials through conduction, convection, and radiation:

• Conduction occurs through both the gas and the solid material that separates 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.

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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 United States. The higher the R-value, the better the insulation ability. (R-values for typical roofing systems are shown in Figure 1.)
• U-value is the inverse of R—the lower the U-value, the better—and is known as thermal conductance. U-value is expressed as the number of Btu that flow in one hour through one square foot of material with a temperature differential of 1° Fahrenheit.
• 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.
• R-values of different components can be added (all the different layers of a wall, for example); U-values cannot be directly added.
• 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 (Table 1).
• Although high R-values sound attractive, the law of diminishing R-eturns applies: Each additional unit of R-value contributes less energy savings than the previous one. As Figure 2 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. When upgrading a typical insulated 2-by-4-inch wall (R-13, U-0.077), for example, to a 2-by-6-inch wall (R-18, U-0.055), this divergence translates into 5 additional points of R-value but only drops the U-value by 0.022. Adding another 5 points of R-value would lower the U-value even less—by only 0.011. In practical terms, this means that the additional insulation will save very little 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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Conduct—or hire someone to conduct—an infrared scan to measure thermal leaks in the building envelope to verify existing insulation levels in a building. These tests, also called thermographic scans, can be performed by energy auditors or other qualified technicians and offer a nonintrusive way to check insulation.

An infrared scanner can reveal if walls, or parts of them, lack sufficient insulation by measuring the surface temperature of the wall. Areas that lack sufficient insulation show up with different temperatures than areas with proper insulation because they conduct heat differently. Infrared scans will also reveal points in the building envelope where air is leaking through.

If insulation levels are found to be inadequate, an insulation contractor should be consulted to determine the best course of action for fixing the problem. Refer to the Purchasing Advisor topic "Building Shell: Insulation" for more on choosing insulation.

In some cases, minimum insulation levels will be dictated by local building codes. For areas without such stipulations, consider complying with the building envelope requirements in the latest version of ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Standard 90.1, "Energy Standard for Buildings Except Low-Rise Residential Buildings." This standard outlines minimum insulation levels based on climate zone. Installing higher levels of insulation may be cost-effective depending on the amount of the building's internal heat gains, its size, the construction of its envelope (including such things as the amount of glass it has), and the climate. However, to determine the cost-effectiveness of additional insulation would require conducting a building energy simulation.