One of the most important things to remember about batt insulation is that from the moment you touch it the R-value starts to decrease. The material shouldn’t be compressed, yet some of that is inevitable. Say you start out with R-13. By the time it’s bagged, cut open, grabbed and installed you’re lucky if you have R-8 or R-9.
But the good news is that by using best practices during installation you can control a lot of that loss.
It’s important to place the batt carefully rather than just shoving it in the cavity. Narrow chinks all need batt as well, and again make sure it’s placed with care.
Another thing that we’ve found helpful to is place netting in the middle of the wall until sheetrock is installed. This prevents the batt from sagging downwards.
We see badly installed bat all the time, especially with some of the volume builders.