First, the insulation value is low. Can the glass fiber batts for sale provide as much insulation as possible. It's not that you can't manufacture higher performance levels of glass fiber; in fact, it is being manufactured and sold at higher R-values. But large glass fiber insulation material manufacturers are not yet ready to apply this high R-value glass fiber board insulation material to current buildings. Of course, poor installation is also bad, which means installing batts sloppily into stud cavities, over-compressing insulation materials or leaving gaps that allow convection, sloppy trimming will also obstruct and damage the insulation effect.
And the installers are only partially responsible. Because rock wool board material is soft and fluffy, it requires more care during handling, cutting and placement to install well. But it rarely gets this care. Therefore, it is usually installed quickly and cheaply. And the result left by the cheap installation process is poor air tightness and poor gas barrier.
In this process, someone came up with the good idea of combining a vapor retarder with glass fiber insulation batts. It combines two construction steps into one, theoretically saving labor and thus money. The problem is that once installed poorly as described above, and cutting the vapor retarder around electrical boxes, the final thing obtained does not seal well against air leakage or vapor diffusion.
Overall, I don't need my insulation material to do air sealing, because I used that good house wrap on the outside. And there are no problems, and it doesn't help with the air sealing your vapor retarder needs. In this case, choosing rock wool board plus standardized construction is very necessary.
English