Laura Schumann completed her MS thesis on the temperature, stormwater and cost benefits of green cloaks.

Thesis Title: ECOLOGICALLY INSPIRED DESIGN OF GREEN ROOF RETROFIT

What is a Green Cloak? It is a novel substitute for an extensive green roof. It uses fast-growing, suspended vine species to block solar power from heating the roof and to intercept rainfall, thus lowering the building's inside temperature and delaying the rate at which stormwater flows from the roof.

Schumann's Abstract:

Green roofs have become popular in the United States for their runoff and energy reduction abilities.  However, current designs have high installation costs, heavy load-bearing requirements, and restrictions to low-sloped roofs.  We designed a novel retrofit technology, the green cloak, which uses fast-growing vine species and a trellis to suspend vegetation above a roof.  We conducted field experiments, prototype testing, and mathematical modeling to determine the effect of the green cloak on stormwater runoff and indoor summertime building temperature reduction.  We also assessed energy and monetary costs and benefits.  The green cloak reduced July indoor building temperature by 11.3 °C (20.3 °F) which saved 73% of cooling energy costs.  The green cloak delayed the peak storm runoff from a 100-year, 24-hour storm (0.15 mm/min; 8.5 inches) by 1.8 hours.  We estimated that the green cloak will cost 38% less than an extensive green roof.  The green cloak demonstrated great potential for mitigating runoff impacts of iimpervious surfaces, reducing summer temperatures of buildings and creating urban greenery

Figure 1. This is an experimental Green Cloak with Virginia Creeper (Parthenocissus quinquefolia) growing in one of the University of Maryland's Plant Environmental Chambers where we investigated its effect on building temperature.

Some Highlights...

Green Cloak's Effect on Reducing Solar Power Load Transmitted to the Roof

The thick (i.e., high leaf area index) vine canopy of the Virginia Creeper Green Cloak greatly reduced the amount of solar radiation transmitted to the roof surface as shown in Figure 2 below. The Virginia Creeper had a leaf area index (LAI) of 3.17, which meant that it had 3.17 units of leaf surface for energy one unit of roof surface. We determined the hyperspectral (i.e., many small spectral bands) transmission using an Analytical Spectral Devices handheld spectroradiometer. The Green Cloak canopy only transmitted 4% of the ultraviolet (325 to 400 nm) and 5% of the visible (400 to 700 nm) radiation. The increase in transmission seen at the 700 nm wavelength is where visible light ends and near-infrared radiation (NIR) begins. It is typical of plant leaves to transmit more NIR than visible. The cloak vines transmitted 18% of the NIR radiation (700 to 1075 nm). For the entire spectrum shown, which is the vast majority of the solar spectrum, the canopy only transmitted 9.3% of solar power. In contrast, asphalt shingles absorb most of the solar energy and convert it to high temperature heat. When black asphalt shingles were fully exposed to July solar power their temperatures reaced 150 F, but the asphalt shingles protected by a Green Cloak only reached 105 °C (20.3 °F)F. When shingles on a roof heat up, they re-emit the heat into the house. According to Stefan-Boltzman equation (electro-magnetic radiation is proportional to 4th power of temperature), this temperature difference means the Green Cloak reduced the solar power load by 32%.

Figure 2. Radiance above (black line) and below the canopy (red line) of a green cloak with Virginia creeper and the percentage of light transmitted through the canopy (green line)

Figure 3. Diurnal reduction in indoor building temperature by Green Cloak (Virginia Creeper) (3 °C = 5.4 °F) .