Wind-driven Rain?

Wind-driven rain is an important boundary condition for HAMT modelling and the main moisture source that affects the hygrothermal performance and durability of building envelopes. This is particularly true for porous materials, that potentially hold high level of moisture.

As rain is driven into a façade by wind speed and direction, it can penetrate through to sensitive building materials, which potentially damages the building envelope and reduces its performance and service life. In ASHRAE 160, we account for the with the 1% driving rain assumption.

HAMT is limited as to its current capacity to provide for wind-driven rain, as it does not consider surface water absorption or precipitation during or after a rain event. In addition, since precipitation is not considered, the natural phenomenon of solar driven moisture is also limited.

To acknowledge this limitation, Speckel has created a unique function that allows the relative humidity of the external facing material to be elevated via a function to define an offset (%) for the external relative humidity climate conditions based on the climate file for the study.

While this does not correspond to rain events and active surface water absorption, it does allow the higher relative humidity of the external facing material to be higher all year round, reducing its thermal conductivity (when temperature-dependent) while increasing the liquid transport coefficients (suction and redistribution).

Our testing results suggest that all project should assume at least a 10% increase in relative humidity based on accounting for wind-driven rain and to account for out dated climate data with lower then typical precipitation and relative humidity.