We demonstrate a methodology to account for rain effects on wall heat gain using the DOE-2 building energy estimation software as a reference. The methodology involves modifying the boundary conditions used for estimating heat flow through exterior wall components. Information on boundary conditions during rain is obtained from a classification of rain patterns from local weather data. The DOE-2 computer code, with the appropriate modifications, is calibrated with experimental results obtained for both dry and wet outdoor conditions. Finally, the calibrated code is used to estimate the reduction in heat gain due to rain by a porous building wall for different rain patterns. The annual heat gain reduction is also estimated for a typical tropical climate.