The effects of rooftop garden on energy consumption of a commercial building in Singapore

Abstract

DOE-2 energy simulation program was used to determine the effects of rooftop garden on the annual energy consumption, cooling load and roof thermal transfer value (RTTV) of a five-story hypothetical commercial building in Singapore. The thermal resistances (R-values) of turfing, shrubs and trees were estimated using data from site measurements, and the effects on the building energy consumption of a rooftop garden with these three types of plants were simulated. Two soil types with different soil thickness on the building roof were also simulated. The results showed that the installation of rooftop garden on the five-story commercial building can result in a saving of 0.6–14.5% in the annual energy consumption, and shrubs was found to be most effective in reducing building energy consumption. The results also revealed that the increase of soil thickness would further reduce the building energy consumption and the moisture content of soil can affect the outcome quite substantially.

Introduction

The New Concept Plan 2001’s vision calls for an even greener Singapore city. This blueprint is designed to turn Singapore into a thriving world class and very green city in the next 50 years [1]. One key aspect of this greening process is the use of more aesthetically pleasing planting with trees, shrubs and grass in our urban environment.

With a current population of 4 million and a projected population of 5.5 million in 40–50 years’ time, all within a tiny island state of 682.7 km², the trend would inevitably move towards higher density housing exploiting almost every inch of land available. However, to maintain a pleasant living environment, the balance between vegetation and concrete built-up areas cannot be overlooked. With the fundamental layout of cities unlikely to change for some years to come, planners face the challenge of finding other means of increasing and enhancing the amount of greenery in urban areas.

One promising option for dense urban settings is the greening of buildings [2]. Roof gardens, though not a new concept, increase the percentage of greenery in urban built-up area and bring back the vanishing urban green space. Sprucing up the originally under-utilized portion of the buildings, they can ‘create a new network of vegetation linking roofs’ and increase the ratio of greenery to population. To a certain extent, roof gardens do contribute to the National Parks Board (NParks) target to develop 0.75 ha of parkland per 1000 population.

Roof gardens, more commonly known as green roofs in European countries, are gaining foothold in North America while widely popular and established in European countries especially Germany, France, Austria, Norway, and Switzerland. A large amount of research has also been undertaken in an attempt to improve the performance of green roofs. By comparison, the incorporation of green roofs technology into current design and construction practice through research and application to local context is still in its infancy phase, with considerable gap from our European counterparts. Nevertheless, the extensive research conducted in Europe does provide us with significant insights of green roof technology commonly adopted there.

Despite the growing interest in green roofing and that Singapore does have all the materials required for green roofs, many developers are often held back from including rooftop gardens in the design brief mainly by concerns like high initial costs and structural capacity [3].

In considering for a green roof, cost savings, it provides, should be scrutinized. Also, the challenge is to look beyond the first cost comparison between conventional waterproofing and green roofs [4]. Patterson mentioned that even though first costs of green roof range from three to six times the cost of a typical roofing system, in the long term, green roofs might be less expensive and outperform conventional roofing [5]. Barbara and Boyles also observed that a short-lived, low first-cost product is often not the cost-effective alternative [6]. A higher first cost may be justified many times over for a durable product with minimal maintenance [6], and the cost of creating a green roof must be weighed against its benefits [2].

Green roofs provide a large range of benefits from amenity to ecological, technical advantages to financial aspects [2]. Roof gardens act as filters to particles, alleviating the problems of poor air quality, the quality of storm water can be improved and volumes reduced. The most significant benefits of green roofs, such as storm water retention and a cooler microclimate in urban areas—are hard to quantify or to put a dollar value on them. However, these benefits, combined with the improved roof longevity and thermal insulation of a green roof, can easily outweigh the increased first costs for most installations [4].

By constructing green roofs, the plants can absorb large quantities of solar energy through biological functions. The remaining solar radiation that would affect the internal temperature of the building is much less than that of a bare roof. Of the total solar radiation absorbed by the planted roofs, 27% is reflected, 60% is absorbed by the plants and the soil, and 13% is transmitted into soils [7]. That is to say, with a green roof, the insulation value is in both the plants and the layer of substrates. Even without considering the increased thickness of soil due to additional layers of soil and drainage, the plants layer can shield off as much as 87% of solar radiation while a bare roof receives 100% direct exposure.

Patterson noted that green roofs modify building behavior as the structure is not subjected to temperature extremes. The insulation value of the soil on the structure lowers the cooling energy costs [5]. Heat transmitted through a barren roof would be more than a rooftop with plantings due to the additional layers of drainage, substrates and vegetation that act as insulation [8].

Meier reviewed research on temperature reduction and energy savings from landscaping on building surfaces. According to Meier [9], strategically placing plants on building surfaces can significantly reduce building surface temperature by up to 20 °C, and can save air-conditioning energy use by up to 80%, although 25–50% were more common.

Akbari et al. [10] monitored peak power and cooling energy savings from shade trees in two houses in Sacramento for 129 days. According to the collected data, trees at these two houses produced seasonal cooling energy savings of 30%, and peak demanding savings for these two houses were about 27% in one house and 42% in the other. Akbari et al. [10] also used building energy analysis program DOE-2.1E to model these two monitored houses. The analysis revealed that the shade trees dramatically saved 29% of the total daily cooling energy use of the two monitored houses over the entire monitoring period.

Parker [11], [12] explored the savings of landscaping around a nursery school in hot-humid Miami. A multi-layer canopy of fully-grown shrubs and small trees (2–8 m high) was planted around the building, and the landscaping shaded windows and walls. Parker found that 2 days’ measured air-conditioning savings exceeded 50% for comparable hot days, but long-term savings were about 25%.

Takakura et al. [13] investigated the cooling effect of greenery cover on roof models in summer. Four concrete roof models each with a surface area of 0.81 m² were constructed and different coverings were arranged for each model: bare concrete, soil layer, soil layer with turf, and soil layer with ivy. Measured results showed that the maximum difference between room air temperatures beneath the bare concrete roof and the ivy-covered roof was around 15 °C. Takakura et al. [13] also developed a one-dimensional non-steady-state computer model to simulate these four roof models and evaluate the cooling load. Simulated results revealed that for the soil covered, turf-covered and ivy-covered roofs, the heat flow was mostly from inside to outside, while for the bare concrete roof the heat flow was mostly from outside to inside.

A California based study by Simpson and McPherson [14] showed that tree shade has the potential to reduce annual energy for cooling by 10–50% (200–600 kWh) and peak electricity use up to 23% (0.7 kW). The research by Hull et al. also suggested that strategic planting of lawns and other landscape plants could reduce total air conditioning energy requirements by 25% [15]. These studies have clearly demonstrated the important role tree shade play in reducing the energy consumption of buildings.

As research studies have shown, under a green roof, indoor temperatures (without cooling) were found to be at least 3–4 °C lower than hot outdoor temperatures of between 25 and 30 °C [16]. The depth of the topsoil and type of plants chosen, however, would vary the insulation values owing to the different U-values. Both the plants and substrates increase the R-value resulting in energy cost savings [17].

The objectives of this study therefore are:

• to identify the benefits of rooftop garden in reducing the heat gain into the building;

• to examine the quantitative impact of rooftop garden on the building energy consumption;

• to examine the impact of rooftop garden on roof thermal transfer value (RTTV); and

• to study the cost savings in cooling energy of building due to the utilization of rooftop garden.