The effectiveness of the insulation can depend on the workmanship of the carpenter. Compressed blankets of batts change the density and reduce the insulation value. Blown in insulation with too much air in the mixture results if fluffy application that's less dense than required. In time it settles and isn't thick enough to provide the desired R-value. Installing the batt or blanket under floors over crawl spaces with the vapor retarder facing the wrong way.
Ventilation is overlooked by many people.
If moisture is in air and is soaked into insulation, the R-value decreases from the compaction. Improper placement and fitting of batts or blankets where there's cross bracing between joists and purlins, or knee bracing between studs. Leaving void spaces between the framing members when batts or blankets aren't fitted correctly Stapling batt or blanket flanges improperly, resulting in a poor vapor retarder. Staple with enough staples so there are no gaps and insulation is held securely in place. There are many types of insulation available today.