One measure of the effectiveness of insulation is its resistance to heat flow, the R-value. The higher the R-value, the greater the resistance to winter heat loss or summer heat gain. The following table shows typical R-value for various types and thickness of insulation.
R-numbers are additive. You can add an insulation rated R-11 to one that is rated R-19 to achieve a resistance value of R-30. The thermal resistance of an area covered with loose-fill or flexible insulation can change over the years. The insulation R value depends not only on the material but also on the amount of trapped air contained within the material. If the loose fill is disturbed or the flexible insulation crushed (because of items being stored on top of it), it will no longer be as thick as when it was installed. Consequently, its effective R-value will be reduced. To determine the current R-value of the insulation in your home, you should measure its thickness.
The insulation R-value recommended for your house can be determined from the map below. You might be surprised to learn how much insulation is recommended. The R-values, however, are based on current and projected fuel costs. If your house is already insulated, once you determine the amount of existing insulation, you can add the difference. Remember, the R-number are additive. In some homes, it might not be economically justifiable to increase the insulation to the recommended value.
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