With up to 80 % of all construction claims relating to the building envelope in the US and an estimated $7.5 × 10⁹ annually to repair roofs and walls in Canada, a better understanding of the causes of these failures and a better technical knowledge base to avoid them in the future would seem to be required at the design and construction phases of the building envelope. These two issues have been addressed in this paper. Of the four barriers: vapor, air, thermal, and runoff and rain penetration control barrier (RRPCB) necessary in the building envelope, the RRPCB is the most critical barrier. While the first three have algorithms correlating their respective drivers to their respective heat, air, and Moisture (HAM) movements, the RRPCB has no overall algorithm, against which the designer can evaluate his/her design. Through field observations and a case study, an empirical equation and empirical graph are presented against which the several buildings in this paper have been checked.