Where should wall penetration flashing ideally be installed?

The ideal placement of wall penetration flashing addresses critical points in a building envelope where moisture can enter. Installing flashing at doors, windows, and where different wall materials meet is essential because these areas are common vulnerabilities for water intrusion.

Flashing acts as a protective barrier, directing water away from penetrations and preventing it from infiltrating the structure. At windows and doors, proper flashing ensures that any water that reaches the opening does not enter into the wall cavity but instead is channeled away. Likewise, where different wall materials meet—such as where siding transitions to brick or stucco—flashing serves to manage moisture migration between dissimilar materials, which may have different expansion and contraction rates.

This comprehensive approach to flashing installation mitigates the risk of water damage to the building's interior and structural integrity.

In contrast, limiting flashing to only specific areas, such as just windows or horizontal seams, does not adequately address all potential entry points for moisture, leading to increased risk of damage and failures in the building envelope. Therefore, the correct option emphasizes the importance of installing flashing at multiple critical locations for effective moisture management.