ISO 15927-3:2009

ISO 15927-3:2009 - Driving rain index for vertical surfaces (Overview)

ISO 15927-3:2009 is part of the ISO 15927 series on hygrothermal performance of buildings. It specifies two validated procedures to estimate the quantity of wind-driven (driving) rain impacting vertical building surfaces and to present that information as indices useful for moisture and rain-penetration assessment. The standard covers both an hourly-data method and a coarser method based on average wind and the meteorological present weather code for rain. It also defines corrections for topography, local sheltering and wall/building type.

Key topics and technical requirements

• Two calculation methods:
  • Clause 3 (hourly method): Uses coincident hourly wind speed, wind direction and rainfall data to compute
    • Airfield annual index (influences moisture content of absorbent surfaces)
    • Airfield spell index (influences likelihood of rain penetration during worst spells)
    • Requires long-term hourly records (≥10 years; preferably 20–30 years).
  • Clause 4 (present-weather method): Uses half-day periods, average wind, and present weather codes to identify moistening and drying half days and to calculate a reference spell with a 10-year return period (Gumbel distribution fit).
• Index conversion: Multiply airfield indices by roughness coefficient, topography coefficient, obstruction factor and wall factor to obtain wall annual and wall spell indices (Clause 5).
• Definitions and terms: spell, airfield index, wall index, terrain roughness categories, wall orientation, present weather code.
• Statistical treatment: 67% percentile used for 3-year spell index; Gumbel fitting for 10‑year reference spell.
• Applicability limits: Not suitable for sheer cliffs/deep gorges, regions where >25% annual rainfall comes from severe convective storms, or where precipitation is predominantly snow/hail.

Practical applications and users

Who uses ISO 15927-3:

• Building envelope designers and façade engineers
• Hygrothermal modelers and consultants
• Meteorological data providers and climatologists
• Building code authorities, insurers, and facility managers

Practical uses:

• Assessing moisture risk and likelihood of rain penetration for walls, windows and joints
• Input data for hygrothermal simulation and building performance modeling
• Guiding material selection, detailing and maintenance planning for façades
• Producing driving-rain maps and climate datasets for design

Related standards and references

• ISO 15927 series (Parts 1, 2, 4, 5, 6) - related climatic data and hygrothermal calculation methods
• WMO Guide - recommended measurement methods and present weather code definitions
• Informative annexes in ISO 15927-3:2009 include spell characterization, method comparison, and mapping procedures

Keywords: ISO 15927-3, driving rain index, wind-driven rain, hygrothermal performance, wall annual index, spell index, building envelope.