Compressed mineral wool conundrum

This one's a forum post, not really a news story, so there isn't much to summarize for planning purposes. A homeowner asked a building science question: does squeezing mineral wool insulation into a tighter space raise its R-value per inch but lower the total R-value of the wall or ceiling it's in?

The short answer, which the poster figured out on their own, is that compressing insulation does pack the fibers closer together, which slightly increases R-value per inch. But because you're also reducing the total thickness of material in that space, the overall R-value for that section still tends to drop. Squeezing 4 inches of mineral wool into a 3-inch cavity, for example, doesn't give you the full R-value of 4 inches of material, because you've lost an inch of depth even though the material itself is is a bit denser.

This matters for anyone insulating a wall, attic, or rim joist with batt-style insulation: cramming batts into a cavity that's narrower than the batt's rated thickness generally doesn't help and can hurt performance. If you're evaluating insulation upgrades in your own home, the practical point is that fitting the right thickness of material to the actual cavity depth matters more than assuming a denser squeeze will make up the difference.