[zdragnar]

A poor fit, minor damage, or any defect allows convection within the fiberglass. R-value assumes zero air movement, which is why it is far less useful comparing fiberglass to foam or aerated cement. In real world settings, aerated cement will outperform fiberglass.

[Retric]

The same thing can happen when concrete cracks, which is quite common. All of this a question of basic building maintenance rather than the building materials chosen.

Cracking is why single family homes generally avoid concrete walls in the permafrost.