Details:RainReductionFactor

Rain Reduction Factor

The r.r.f. takes into account that some of the rain water hitting an inclined surface splashes off on impact and is therefore not available for capillary suction.

WUFI calculates the rain load on the component from the data in the climate file and multiplies it by the r.r.f. The resulting rain quantity is offered to the component for capillary suction. (The component may imbibe the offered quantity or less, but not more).

The r.r.f. depends on the roughness, orientation and inclination of the surface, and on the nature of the precipitation (rain, snow...).

With horizontal surfaces, the r.r.f. is usually one, since the scattered drops fall back to the surface.

With vertical surfaces, it is nearly zero for snow and hail, for other conditions it must be estimated. A value of 0.7 might be a good starting point.

Since most component surfaces - with the exception of unrendered masonry and natural stone facades - only have a moderate water absorption, they cannot imbibe all the offered water anyway. In these cases, an exact knowledge of the rain quantity is not essential (whereas the duration of the rain is still important) and estimates for the r.r.f. and the position-specific driving rain coefficients may be sufficient.

WUFI automatically determines the r.r.f. from the inclination and the construction type; a user-defined value may be employed as well.

The rain reduction factor is entered in the dialog "Surface Transfer Coefficients".