A.Bruce Harley, an energy efficiency expert at Conservation Services Group, responds: Both statements are correct. When you compress fiberglass insulation, you increase its R-value per inch, up to a point. However, when you compress a batt of a particular thickness, the total R-value does decrease. For example, standard low-density batts, at their nominal rated thickness — R-11 at 3 1/2 inches, or R-19 at 6 inches — have an R-value of about 3.1 per inch. If you compress a 6-inch R-19 batt into a 3 1/2-inch cavity, you get about R-14, or 4.0 per inch. This is a much higher R-value per inch, but the total value of R-14 is still substantially less than the R-19 you started with. Even in a 5 1/2-inch wall, the R-19 (which is rated at 6 inches of thickness) gives you only R-18.

For the best performance, any batt should be installed carefully, to fill the entire space without compressing around wires, pipes, bridging, or other obstructions. For those seeking higher R-values per inch than standard fiberglass batts, one alternative to compressing large batts into small cavities is to use so-called high-density batts (R-13 or R-15 at 3 1/2 inches, or R-21 at 5 1/2 inches). Although they cost more than standard batts, they are stiffer, easier to fluff up, and easier to cut around obstructions in wall framing. These factors result in a bigger benefit in real-world performance than the factory ratings indicate.