Estimate Wind-Driven Rain (WDR) on building facades from Choi (1994). Adjust rainfall, wind speed, droplet diameter, building height, wall orientation and material to see water flux, facade pressure, 24 h uptake and capillary penetration depth in real time — a starting point for envelope waterproofing and material selection.
Parameters
Hourly rainfall R
mm/h
Horizontal rainfall. 10 mm/h is normal rain, above 30 mm/h is heavy.
Wind speed U
m/s
Wind speed at 10 m. 10 m/s is "rather strong", above 20 m/s is typhoon class.
Mean drop diameter d
mm
0.5 mm for drizzle, 2-4 mm for heavy rain. Sets the terminal velocity.
Building height H
m
Taller buildings raise the wind-speed profile and concentrate WDR on upper storeys.
Wall orientation theta
°
Angle between wind direction and wall normal. 0° = head-on, 90° = parallel (zero WDR).
Facade material
Changes the absorption coefficient A_w and relative wetting risk.
Absorption coefficient A_w
kg/(m²·√s)
EN ISO 15148 stage-I capillary uptake. Brick 1-3, concrete 0.3-1.
Results
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Terminal velocity (m/s)
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Incidence angle (°)
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WDR intensity (kg/m²/h)
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Facade load (Pa)
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24h uptake (kg/m²)
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Penetration depth (mm)
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Facade section — wind, droplets, penetration
Wind (blue arrow) blows from the left; droplets fall along trajectories tilted by terminal velocity and wind, then strike the facade on the right. The cyan band inside the wall shows the capillary penetration depth.
Wind sensitivity — WDR intensity vs. wind speed U
Material comparison — 24 h uptake and penetration depth
R_WDR: facade water flux [kg/(m²·h)]; R: horizontal rainfall [mm/h]; U: wind speed [m/s]; theta: angle between wind and wall normal; A_w: absorption coefficient [kg/(m²·√s)]; m: uptake per unit area [kg/m²]; t: exposure time [s].
Gunn-Kinzer (1949) raindrop terminal velocity v_t [m/s] and trajectory angle phi from vertical. d is the drop diameter [mm]. Higher wind raises the horizontal component and tilts phi further.
Combined facade load q [Pa]. First term is the wind dynamic pressure (Bernoulli); the second is the momentum flux from droplet impact.
Wind-Driven Rain (WDR) Facade Design
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I have never heard of "Wind-Driven Rain". How is it different from ordinary rain?
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Good question. Wind-Driven Rain (WDR) is "rain that has been pushed horizontally by the wind", and for the building envelope it is a far more aggressive water load than vertically falling rain. Add 10 m/s of wind to 20 mm/h of rainfall and Choi's formula R_WDR = 0.222 R U cos(theta) gives the facade about 44 kg/m²/h — that is roughly 44 mm of water per hour, more than twice what hits the ground. So a brick or concrete wall can look "merely wet" and still be soaking in a serious amount of water inside.
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Wait, more than what falls on the ground? That sounds like the wall is drenched on the inside!
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Exactly the reason WDR design matters. Porous masonry follows the stage-I capillary law m(t) = A_w sqrt(t), and a brick A_w can reach 1-3 kg/(m²·√s). If you let it absorb non-stop for 24 hours, this tool predicts hundreds of kg/m² of uptake. In reality wet and dry alternate, so the net is smaller, but in European regions where the Driving Rain Index (DRI) exceeds 500 mm/year a rain-screen wall (two-leaf with an air cavity) or hydrophobic treatment is essentially mandatory.
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Tall buildings have stronger wind, so WDR gets worse on the upper floors?
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Yes. In the atmospheric boundary layer wind speed follows a power-law U(z) = U_ref (z/z_ref)^alpha with alpha around 0.14 to 0.3, so at 100 m the speed is 1.5-2x the value at 10 m. Around the building the flow wraps around corners and accelerates near the roof edge and the windward upper corners. CFD (RANS, k-epsilon) often shows local Catch Ratios (CR) above 2 there. Blocken and Carmeliet (2005) is the classic reference, solving WDR with an Eulerian droplet phase coupled to the wind field.
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From the material side, what are the options to defend against it?
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Geometry first: drip edges, parapet copings and overhangs to "shed water away from the wall". On the material side three approaches dominate: (1) silane / siloxane hydrophobic impregnation that drops A_w by an order of magnitude; (2) a rain-screen wall with a ventilated cavity so the outer panel can get wet while the structural layer stays dry; (3) a cavity wall that drains and dries through breather paths. For numerical work, hygrothermal solvers such as WUFI, Delphin and HygIRC couple heat and moisture transfer using annual climate data, sorption isotherms and vapour-diffusion resistance mu. Key standards are EN ISO 15927-3 (DRI evaluation) and ASTM E 514 (wall water-permeance test).
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Is WDR still important for Net-Zero Buildings and Passive Houses?
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Even more so. With thick exterior insulation, any moisture that gets in cannot dry out easily. In EIFS or CLT (cross-laminated timber) construction, a tiny WDR ingress can lead over months to mould, wood decay and reduced insulation performance. The Passive House Institute recommends keeping the moisture content of wood members below 18 % using WUFI simulations. Metal panels and curtain walls have A_w close to zero but still leak through joints and frames, so they need separate water-tightness tests such as JIS A 1414 or ASTM E 331 to certify the joint design.
Frequently Asked Questions
Wind-Driven Rain (WDR) is rain that carries a horizontal momentum component from the wind and impacts vertical building surfaces. Looking only at vertical rainfall suggests that walls receive almost no water, but in practice even moderate wind (5-10 m/s) tilts droplet trajectories beyond 45 degrees and the water flux on a facade equals or exceeds the rain hitting the ground. The standard quick-look index is Choi's (1994) empirical formula R_WDR = 0.222 R U cos(theta), where R is horizontal rainfall (mm/h), U is wind speed at 10 m (m/s) and theta is the angle between wind direction and the wall normal.
Typical WDR-driven failures include (1) efflorescence and freeze-thaw spalling from repeated wetting-drying cycles, (2) mould and decay where capillary uptake carries water deep into the wall, (3) leakage at sealant joints, (4) loss of thermal performance when insulation gets wet, and (5) reinforcement corrosion in concrete due to chloride and CO2 ingress. In Europe the Driving Rain Index (DRI) is used to map annual WDR by region; areas above 500 mm/year typically require a rain-screen wall or hydrophobic surface treatment.
0.222 is a volume conversion factor derived from the raindrop terminal velocity and a Marshall-Palmer drop-size distribution. It converts the product of rainfall intensity (mm/h) and wind speed (m/s) into a water flux on the facade in kg/(m^2 h). Choi (1994) used CFD and wind-tunnel data to show that the free-stream WDR (away from any building) can be approximated by this expression. Around real buildings the flow accelerates over corners and roof-edges, concentrating WDR on upper corners; a Catch Ratio (CR) from CFD is then applied to correct the free-stream value. This tool returns the free-stream estimate.
This tool uses the Karman / Karol stage-I capillary law m(t) = A_w sqrt(t) for one-dimensional uptake under 24 h of continuous wetting from a dry initial state. Real walls experience alternating wet and dry periods, so long-term net penetration is much smaller. For design-grade studies use a hygrothermal solver such as WUFI, Delphin or HygIRC together with annual climate data, sorption isotherms and vapour-resistance values. Treat this tool as a screening calculator for early material and shape selection.
Real-World Applications
High-rise office envelope design: For 100 m-plus buildings in maritime climates such as Tokyo, Osaka, Fukuoka, New York or Hong Kong, designers run CFD to map the wind-speed profile and Catch Ratio, then strengthen the secondary water-control layer where WDR concentrates on upper corners and below parapets. In real projects ANSYS Fluent or OpenFOAM with an Eulerian droplet phase visualises the WDR distribution; high-risk nodes are flagged for thicker sealant grades and tighter joint tolerances.
Historic masonry restoration: Medieval brick churches in Europe and brick townhouses on the U.S. East Coast use highly absorptive masonry with A_w of 2-3 kg/(m²·√s). WDR water that penetrates in autumn then freezes in winter and causes frost-decay spalling. Silane impregnation that lowers A_w to 0.1 or less is the dominant repair, and toggling A_w between 0.1 and 2.0 in this tool literally shows an order-of-magnitude shift in penetration depth.
Passive House and timber CLT buildings: Walls with more than 200 mm of exterior insulation may take months to dry out any moisture that gets in. The Passive House Institute recommends keeping the annual mean moisture content of structural timber below 18 % and the Mould Index at 3 or less, evaluated with WUFI. Comparing "metal panel with ventilated cavity" against "brick with direct-applied insulation" in this tool already shows why a rain-screen approach is worth the extra cost.
Coastal and typhoon-prone housing: Okinawa, South-East Asia and the Caribbean coast experience 25+ m/s winds together with 50 mm/h rain. Plugging U = 25 m/s and R = 60 mm/h here pushes WDR to about 330 kg/m²/h, and window frames are then at extreme risk of forced leakage. Designers verify watertightness at 500 Pa or higher pressure differential with JIS A 1517 / ASTM E 331 and adopt pressure-equalised rain-screen drainage details.
Common Misconceptions and Pitfalls
The first trap is to treat the Choi WDR estimate as what the wall actually receives. R_WDR = 0.222 R U cos(theta) is a free-stream value; on real buildings the flow distorts and WDR concentrates on upper corners while shedding from central low-rise zones. The CFD studies of Blocken and Carmeliet (2005, 2007) show Catch Ratios from 0.3 in sheltered zones to 2.5 on exposed corners — almost an order of magnitude. Use this tool for the average free-stream condition; for critical projects run a CFD or instrumented panel measurement to obtain CR.
Second, do not assume that "small A_w means safe". Metal panels and glass do not absorb water at the surface, but rain still enters joints and frames through four mechanisms — capillary, gravity, momentum and pressure difference. Pressure-Equalised Rain Screen (PER) design ventilates the cavity behind the outer skin so that the wind pressure trying to push water through any gap is eliminated; without it, even A_w = 0 cladding leaks. Always combine material choice with detail design.
Finally, do not equate "uptake" with "leakage into the room". The 24 h uptake here is the total water absorbed by the outer surface, not what reaches the interior. In porous walls the absorbed water re-evaporates in the next dry period; the long-term outcome depends on the moisture balance between uptake and drying. Realistic leakage estimates need annual hygrothermal simulations (WUFI Plus, Delphin); use this tool as an upper-bound screening number and read it together with the Sd value (equivalent vapour-diffusion thickness) and the ventilation rate.
How to Use
Enter hourly rainfall intensity (0–50 mm/h) using the rainfall slider; typical storms range 5–20 mm/h.
Set wind speed (0–30 m/s) via the wind slider; 15 m/s represents gale-force conditions on exposure.
Adjust raindrop diameter (0.5–5 mm); maritime climates typically generate 2–3 mm drops, while drizzle is 0.5–1 mm.
Input building height (5–100 m) to account for wind speed amplification with elevation per Choi (1994) correlation.
A 25 m commercial facade on Norway's west coast: hourly rainfall 18 mm/h, wind speed 22 m/s, 2.5 mm droplets. Terminal velocity calculates ~8.6 m/s; incidence angle reaches 68° from horizontal. WDR intensity yields 156 kg/m²/h at facade surface. Estimated facade load: 94 Pa dynamic pressure. Over 24 hours, cumulative uptake is 3.74 kg/m² on vertical cladding. Penetration depth into 1200 kg/m³ brick veneer predicts 18 mm wetting front, requiring drainage cavity design ≥25 mm.
Practical Notes
Choi (1994) model assumes vertical facades; sheltered lee sides show 30–50% reduction in WDR intensity compared to windward elevations.
Droplet diameter increases with wind speed due to coalescence above 15 m/s; model defaults conservatively assume stable 2–3 mm in moderate gales.
For porous materials (sandstone, terra-cotta), 24-hour uptake >2.5 kg/m² triggers capillary transport risk; specify hydrophobic treatments or external protective coatings.
Building height wind speed adjustment uses logarithmic profile (z₀=0.5 m terrain roughness); high-rise facades (>60 m) in urban zones experience 40–60% higher WDR loads than reference 10 m anemometer data.