A Green Roof can be thought of as a external “heat” insulation material. The adiabatic efficiency of typical insulation is measured by a) the materials heat resistance value and b) its thickness. But these measures do not apply to a Green Roof.

The insulating efficiency of “dry” soil is ~1/3 to that of commonly used insulation materials. However, soil is different in that as its moisture content increases its insulating efficiency also increases. Because of this inconsistency, soil is not considered an effective insulation.

In the diagram below, refer to the “100mm DRY Soil” illustration, the heat conducted through dry soil is surprisingly high. Thus showing that soil with a poor vegetation cover and low water content does not provide effective heat resistance.

In the “100mm MOIST Soil” diagram, the surface of the moist soil is well-covered with vegetation. A great deal of “Latent Heat” is reflected off of the surface of the vegetation, and thereby provides superior heat resistance. The thickness of the arrows representing “Latent Heat” in each diagram is an accurate representation of the ratio of heat transferred away from the roof. With moist soil, heat that reaches the soil is reduced to a point where measurable cooling energy savings are realized.

The amount of “Buildng Frame Heat Conduction” in the “25mm DRY Soil” and “Exposed Roof Surface” diagrams is virtually identical. This is due to the low water content in the soil which does not provide adequate evaporation to consume heat.

Ultra-thin “non-irrigated” soil bases (≤25mm) provide no adiabatic effect, poor plant health, poor plant surface coverage and could promote heat conduction into the building. This is called the “Inversion Phenomenon”. This is caused by the lack of plant coverage due to “dry” thin soil’s inability to support healthy plants. Dark exposed soil has a higher heat conduction value than light coloured concrete, thereby promoting heat into the building.

However, when an ultra-thin system is irrigated, its insular value is increased significantly (see the graph). A healthy plant base shades the soil and water evaporation cools the surface. The effectiveness of moisture’s cooling effect gradually increases as soil depth increases from 25mm to 200mm. Depths above 200mm provide no further significant increase in the adiabatic effect.

The thickness of a buildings roof deck and insulation are also important. On a thick roof deck or with thick insulation (>30cm), adding a Green Roof will provides almost no added heat resistance.

In the correct climate, the adiabatic effect of a Green Roof is significant, and can eliminate the need for air conditioning. However, accurate estimates of energy savings must be made based on the water content of the soil at any given time.

Source: Professor Hiroyuki Yamada, Engineering Department, Wakayama University

As outlined above, energy savings provided by a Green Roof is significantly effected by the moisture content of the soil at a given time.  When the G-SKY Extensive Green Roof System's soil is moist and it has a healthy plant base, you can expect at least a 2.0°C decrease in thermal load.  This works out a reduction of 0.052kWh/ft² (0.56kWh/m²) in a single day.  Thus we can use the below example as a basis to calculate energy savings.

Once you've calculated this number using your local energy providers electricity rate, you need to figure our your air conditioning usage schedule and calculate your yearly energy savings, as shown below. 

In a single year, we break down the average usage of the Air Conditioner into three states, Quasi-usage, Full-usage and Quasi-usage (in most areas, this translates to Spring, Summer, Fall).  You can figure out your average air conditioning usage days and usage percentage by calling your local electricity provider. 

Period	Days	Savings	Usage	Calculation
1	85	0.25¢/ft2	60%	85 x 0.25 x 0.6	=	12.75¢/ft2
2	85	0.25¢/ft2	100%	85 x 0.25	=	21.25¢/ft2
3	85	0.25¢/ft2	60%	85 x 0.25 x 0.6	=	12.75¢/ft2
Yearly Savings per Square Foot					=	46.75¢/ft2

During summer, hot walls cause temps to rise inside buildings increasing demand on cooling systems and consuming more energy. A Green Wall surface temperature is reduced by up to 10°C when covered with plants and moist soil. In 1979, Green Wall research by Akira Hoyano (Professor, Tokyo Institute of Technology), a pioneer in passive and low-energy architecture, revealed that the heat energy that passed through a Green Wall was significantly lower than a concrete wall (see below graph).

The Tokyo City Government recently undertook a study to measure the effects of Green Walls on the Heat Island effect, and in essence, to confirm Professor Hoyano's earlier findings. They not only confirmed the findings, but they were able to derive the significance of Green Walls in cooling buildings and combating the Heat Island Effect.

With the Green Wall tests shown here, it was discovered that Green Wall panels reduce the wall temperature by 10°C (see the below graph). It was also concluded that Green Wall panel reduce energy transfer into a building by ~0.24kWh/m². This is approximately 60% less than that of a Green Roof. The above calculations can be used in the same manner, however, Green Wall energy savings calculations depend greatly on the direction the wall is facing, the sun's angle in your local region, and many other factors that make calculating Green Wall energy savings complex. Please contact us for more information if you would like to hire our consultant services to help you calculate Green Wall energy savings.