Only if it keeps the surface above dew point AND blocks vapour. On below-ambient service the insulation must be closed-cell or vapour-sealed (elastomeric with its built-in barrier, PIR with sealed joints, cellular glass which is inherently vapour-tight) — fibrous wool without a perfect barrier wicks vapour to the cold wall, condenses internally, and fails wet. Thickness is set by the dew-point calculation (ambient, RH, surface coefficient), not economics — the free calculator includes a condensation-control mode.
Compute your own case: free heat-loss calculator · materials library: 12 datasheets · related: What is CUI (corrosion under insulation)? · Does mineral wool recover after getting wet? · What is economic thickness of insulation? · R-value vs k-value (λ) — what's the difference?
