Fiberglass Compression Issues

Fiberglass Batt Insulation

A fun fact about fiberglass is that it does not insulate through the glass itself, but through the tiny air pockets trapped between the fibers. Squeezing a 6-inch batt into a 4-inch cavity pushes much of this air out. Compressing a standard R-19 batt by 50% drops its performance to roughly R-11 to R-13. This creates a double problem: lower R-value in the squashed zone and air gaps along the uncompressed edges. Research shows that just a 4% void in fiberglass batt insulation can result in a 50% reduction in overall effectiveness. Research from Oak Ridge National Laboratory has shown that compressing fiberglass batts significantly reduces their thermal resistance. Poor installation, including compression and gaps, can dramatically reduce the real-world performance of batt insulation, even if the insulation itself has a high rated R-value. So what does that mean? It means that it is very difficult, like really hard, to insulate properly with batts. That's why we generally don't recommend them.

Main source https://nationalinsulationauthority.com/fiberglass-batt-insulation