When installing insulation between floor joists in a crawl space, the vapor retarder:

The correct answer is that the vapor retarder should be installed above the insulation in a crawl space. This configuration is important for moisture control within the space. By placing the vapor barrier on the warm side of the insulation—meaning above it in a traditional crawl space application—you effectively prevent moisture from entering the insulation from below, where ground moisture can accumulate. This helps in reducing the risk of condensation forming within the insulation material itself, which could lead to mold growth and deterioration of the insulation over time.

This approach is particularly critical in areas with high humidity or where the ground tends to be damp. The vapor retarder acts as a barrier to prevent moist air from reaching the cooler surfaces above, thereby protecting the structural integrity of the flooring and other materials in the building.

In addition, this method supports long-term energy efficiency by maintaining the effectiveness of the insulation, ensuring that it performs as intended in terms of thermal resistance. Proper installation of insulation and vapor retarders is a key aspect of building design to promote health and efficiency within the home.