In terms of aesthetic considerations, design permutations are limitless. When it comes to practical issues such as maintainability, one may be required to manoeuvre through access or structural issues (Choosing between intensive or extensive green roof systems, irrigation requirements, etc.), but there is still much leeway for design. The scientific approach to landscape design, however, is entirely objective. In the outdoor urban climatic domain, Mother Nature calls the shots. And she does it with the simple concept of heat transmission.
The U-value describes how well a building element conducts heat. The higher the U-value, the better it is at conducting heat. Therefore, materials with low U-values are preferred for better thermal insulation.
Greenery selection based on U-Value
Now that we are familiar with the concept of U-value, we simply need to measure the U-value of the different types of greenery to make an objective comparison.
From a published study conducted in Singapore (Wong et al. 2003), we know that the U-values for the concrete roof and greenery are as follows:
1. Concrete roof = 2.40
2. Turf = 1.19
3. Shrubs = 0.45
4. Trees = 0.70
By just looking at these numbers, we can see that shrubs are about 5 times better at providing thermal insulation when compared to concrete. Energy simulations performed by Wong et al. (2003) show that savings in annual cooling energy for a hypothetical five storey building can be up to 15% by virtue of greenery selection (Figure 1). Multiply that by SGD 0.2745 per KWh (with GST) and you are looking at SGD 8235 worth of savings per year.
In the study, 100 % vegetation coverage is assumed for the rooftop. While 100 % tree coverage for a rooftop is unheard of, Figures 3 and 4 show examples of 100 % shrub and turf coverage respectively. One may infer from the cited study that the building in Figure 4 will enjoy a significant reduction in cooling cost if shrubs are added onto the roof.
Granted that most roof gardens will still end up looking like those in Figure 2, we hope that landscape design can start to be developed along objective guidelines for the betterment of our urban environment.
Wong, N.H., Cheong, D.K.W., Yan, H., Soh. J., Ong, C.L., Sia, A. (2003). The effects of rooftop garden on energy consumption of a commercial building in Singapore. Energy and Buildings, 35:353-364
1. Wong, N.H., Cheong, D.K.W., Yan, H., Soh. J., Ong, C.L., Sia, A. (2003). The effects of rooftop garden on energy consumption of a commercial building in Singapore. Energy and Buildings, 35:353-364