Ever heard someone refer to fiberglass insulation as "filterglass" because it doesn't stop the air from moving through it? This amounts to confusing the symptom with the cause.
So says building science blogger Allison Bailes. We've all encountered fiberglass that's been discolored from dirt particles picked up as air moves through the insulation. But the fiberglass isn't the cause of the problem, it's merely showing the telltale signs of a big hole somewhere in the wall that is leaking air. Plug the hole and the fiberglass will perform as intended.
Building components work together, and a problem in one can sometimes disguise a problem somewhere else. "A good building will have control layers that control the flows of heat, air, and moisture," Bailes writes. "Just because a failure in the air control layer (air barrier) shows up in the heat control layer (insulation) doesn't mean it's the fault of the insulation."