Large glass façades are popular architectural features in building design nowadays. However, these façades can result in interior downdraught during periods with low outdoor temperatures. A rule of thumb exists to assess the downdraught risk, based on window height and window temperature . In this paper the validity of this rule of thumb is evaluated by an experimental and a numerical study. In the experimental part ten healthy male subjects (age 20–26 year) are exposed to two different downdraught conditions in a controlled climate chamber. Experimental results are also used to validate the numerical models. In the numerical (Computational Fluid Dynamics) part a parameter study has been performed to assess the influence of window height and window surface temperature beyond the range tested in the climate chamber. In addition, different floor temperatures have been investigated to evaluate the effect of floor heating as a possible design option to prevent downdraught. Based on both experimental and numerical results the existing rule of thumb is shown to be conservative. Furthermore, the numerical results reveal that an increased floor temperature (i.e. floor heating) can increase the downdraught risk. Therefore, it is recommended to modify the rule of thumb by incorporating the floor temperature as a parameter.