What enhances the R-value of the foundation wall overall?

The enhancement of the R-value of a foundation wall is best achieved by installing vapor impermeable insulation on the exterior. This method effectively reduces thermal bridging, which is the transfer of heat through materials that are more conductive than insulation. By placing insulation on the outside of the foundation wall, it helps to keep the cold soil from affecting the wall's temperature, thus improving the overall thermal resistance of the system. Moreover, exterior insulation also protects the wall from moisture infiltration, creating a more consistent and effective thermal barrier.

Other methods noted in the options, while beneficial in different contexts, do not directly augment the R-value of the foundation wall as successfully as hard insulation does. For instance, waterproof coatings primarily serve to prevent water infiltration rather than increase thermal resistance. Reflective barriers can assist in managing radiant heat but are less effective in insulation compared to rigid board or dense batt insulation. A thicker layer of fiberglass batt can help, but it may still be compromised if placed internally due to potential condensation issues and thermal bridges. Thus, exterior, vapor impermeable insulation stands out as the most effective choice for enhancing the R-value of foundation walls.