Piping operating below ambient temperature must be vapor sealed.

• A. Vapor sealed
• B. Insulated and moisture-protected
• C. Painted
• D. Not required

Answer: (A)

When a pipe operates below ambient temperature, moisture in the surrounding air can diffuse toward the cold surface and condense there. A vapor seal creates a continuous barrier to water vapor on the warm side, preventing that vapor from migrating into the insulation. By stopping vapor transmission, the insulation stays dry, its thermal performance stays intact, and moisture-related problems—like corrosion, mold, and degradation of insulation materials—are minimized. Simply insulating and protecting against moisture isn’t enough if vapor can still pass through; painting doesn’t stop vapor diffusion either. Not having a vapor seal would leave the system vulnerable to condensation and its damaging effects, so vapor sealing is the correct requirement.