The vapor barrier of insulation should be installed?

• A. On the cold side of the wall
• B. On the top 2/3 of the wall
• C. On both cold and warm sides of the wall
• D. On the warm side of the wall

Answer: (D)

Moisture in building assemblies moves from warmer, more humid spaces toward cooler areas. Placing the vapor barrier on the warm, interior side stops interior moist air from diffusing into the wall cavity where it could condense on cold surfaces. If the barrier were on the cold exterior side, interior humidity would migrate into the wall and condense, increasing the risk of mold, wood rot, and insulation degradation. Blocking the vapor path on the warm side helps keep the wall drier and reduces moisture-related damage, whereas options that place the barrier elsewhere or on both sides don’t address the direction of moisture diffusion.