A while back I asked about adding my 1.5" thick pieces of mineral wool insulation batts ontop of my 12" thick fiberglass I am installing in my attic. As I have began doing this, I am seeing that the fiberglass is so fluffy that throwing on the 1.5" thick batts weighs the fiberglass down an inch or more. There is no way this insulation will not settle if it squishes that easily. Am I doing more harm than good with the extra layer? This 12" insulation is 10" at best.
Even though the fiberglass is getting slightly compacted by the mineral wool on top; the minor R-value that may be lost to compaction is easily outweighed by the increased R-value of the extra mineral wool. The only adjustment that could be made (and it is a laborious one) is to increase the height of the joists so they are above the fiberglass and support the mineral wool. Just to be sure the mineral wool is being installed at right angle to the fiberglass?
You are correct, the R value of any material depends on many things, but with fiber type insulation, the internal dead airspaces are critical. If you have a 12" insulation that has been compressed to 10", you have lost 2/12 of your R value, or about 17%. The coefficient of thermal conductivity hasn't changed for the material (glass, stone, etc), but the CTC of the trapped air has because there is less of the air in the insulation. If you started with an R30 and you lose 17%, you are down to an R25. This will decrease over time as well as the insulation compresses with age. As long as the mineral wool insulation is higher than R5, you are winning. However, for a 1.5" batt of mineral wool insulation like Roxul or something similar, you probably are less than that (but it does depend on the specific thermal insulative characteristics of your actual material. You would be much further ahead to add a dense Styrofoam like SM. It is light and you could put the individual sheets on little risers (lumber, strips of foam, etc) above the ceiling joists to ensure a seamless foam barrier and prevent future settling and thus compression of the fiberglass batts below.
As Personal says, the value of batt insulation is primarily in the air. So compressing definitely affects the R-value.
Fiberglass batt insulation has an R-value of 3.14 per inch.
12" will give you about 38 for an R-value.
10" will give you about 32.
Mineral wool gives you about the same R-value.
So, yes, you are likely doing more harm than good or...at best, no harm at all but just adding time and money...by going from 12" of insulation to 11.5".