Green Facade Cooling: Evapotranspiration vs a Bare Wall

Facade Conditions

Solar irradiance 700 W/m²

Leaf area index (canopy density) 3.0

Soil moisture / stomatal opening 60%

Wind speed 2.0 m/s

Live Readouts

Bare wall

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Green facade

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Cooling ΔT

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Latent heat flux

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Ambient air fixed at 30°C. Wall color follows a thermal-camera scale: blue ≈ cool, red ≈ hot.

How it works

Each surface obeys a steady-state energy balance between absorbed sunlight and heat lost to convection and thermal radiation, solved every frame by Newton–Raphson:

h(T − T_air) + εσ(T⁴ − T_sky⁴) = Q_abs
h = 5.7 + 3.8·v        (wind convection, W/m²K)
σ = 5.67×10⁻⁸ W/m²K⁴   (Stefan–Boltzmann)

The living wall's canopy intercepts sunlight following the Beer–Lambert law, with extinction coefficient k ≈ 0.6:

Shading fraction  SF = 1 − e^(−k·LAI)
Q_leaf = α_leaf · SF · R_solar
LE = f_transp · Q_leaf        (latent heat carried away as water vapor)

Only the unshaded (1 − SF) fraction of sunlight reaches the wall directly; the canopy also blocks wind and sky view at the wall surface, so its effective convection and radiation coefficients drop with SF. The result: shading cuts the energy load, and evapotranspiration removes what the canopy does absorb as latent heat instead of sensible heat — exactly why living walls measurably lower building-envelope temperatures in the real world.

Solar irradiance — incoming shortwave flux on the facade.
Leaf area index — canopy density; higher LAI shades more of the wall.
Soil moisture — how much of the canopy's absorbed energy the plant can shed as evapotranspiration versus sensible heat.
Wind speed — raises the convective heat-transfer coefficient on both surfaces.

Cool Hot

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