A.Ned Nisson responds: The total R-value of fiberglass insulation does drop when it’s compressed. At the same time, however, the R-value per inch increases. For example, some R-19 batts are 6 1/4 inches thick, so they are compressed to 5 1/2 inches when squeezed into a 2x6 stud cavity. In this case, the effective R-value drops to about 18. If compressed into a 2x4 cavity, the R-value drops to about 13. The total R-value is lower because you have fewer inches, but the R-value per inch of that insulation is higher. Similarly, at full thickness, one of the newer "medium-density" batts has a higher R-value than the conventional batt — about R-21 for the 2x6 batt and R-15 for 2x4 batt. These medium-density batts are sized at 5 1/2 and 3 1/2 inches wide, so they don’t have to be compressed further.

Ned Nisson is president of Energy Design Associates, a New York City-based consulting firm, and editor of Energy Design Update, a monthly newsletter on energy-efficient building design and construction.

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