Hydrogen gas leaking from a hydrogen-powered vehicle in a residential garage may form a flammable mixture with air. Passive, buoyancy-driven ventilation is one approach to limiting the concentration to a safe level. We explored the relationship between leak rate, ventilation design, and hydrogen concentration through laboratory testing, an algebraic analysis, and CFD modeling. We used helium to test slow, steady, low-velocity leaks in a full-scale test room under nearly isothermal, steady conditions, and we report the results in sufficient detail that other modelers can use them. The results show the importance and variability of stratification. Our algebraic and CFD models agree very well with the experimental results. We describe our CFD approach in sufficient detail for use by others. We tested under nearly isothermal conditions, but also discuss indoor-outdoor temperature difference as an important risk factor. Information about realistic leakage scenarios is needed to apply these results as safety recommendations.iv