Building ventilation code requirements for crawl spaces were reviewed from 1937 to today and though remain largely unchanged, provide designers and builders flexibility in moisture control methods. This study evaluates the current building ventilation code requirements for at-grade and below grade crawl space using computational fluid dynamic (CFD) software with experiment inputs. The research first tested the soil moisture evaporation rate from two monitored crawl spaces in Colorado, US, which produces an average moisture load of 13.75 grains/(ft²·h) (9.6g/(m²·h)) and a maximum load of 42.7 grains/(ft²·h) (29.8g/(m²·h)). The soil moisture evaporation rates identified align well in magnitude with those recorded in the literature, supporting the estimation method used. The experiment reveals that plastic ground cover can effectively reduce the moisture load from the soil by an average of 93%. The study then developed a CFD model of the monitored crawl space to assess the necessity and effectiveness of various ventilation code requirements. The space effective leakage area to the exterior was determined through field pressurization testing and CFD analysis to be approximately 0.26in.²/ft² of floor area. The CFD predictions, validated with the measured data, verify that the building code requirements for at-grade crawl spaces appear sufficient, but have limitations for below grade crawl spaces. Sealed crawl spaces perform better in humid climates, supporting previous research, and mechanical ventilation is justified for below grade crawl spaces only. The paper provides suggestions for the revisions to the current building code to recognize below grade underfloor spaces.