Improved Thermal Performance
The use of a green roof compared to conventional roofing surfaces can have a significant impact on the energy balance within a given building and on the immediate environment. This is particularly relevant if a building has poor insulation and poor ventilation, which can lead to increased use of air conditioning and therefore increased energy use.
Studies have shown that the membrane temperature beneath a green roof can be significantly lower than where the membrane is exposed.
Study of temperatures under membranes of a conventional and a green roof
Winter / Summer
- Mean Temperature: 0°C / 18.4°C
- Temperature under membrane of conventional roof: 0.2°C / 32°C
- Temperature under membrane of green roof: 4.7°C / 17.1°C
www.greenroofs.co.uk (Nottingham Trent University)
The National Research Council of Canada also noted temperature fluctuations during spring and summer on a conventional roof were of the order of 45ºC whilst under a green roof the fluctuations were in the order of 6ºC.
The positive effect on the temperature of the membrane under a green roof not only protects the membrane from the effects of UV, frost and sunlight, but also moderates heat flow through a building by shading, insulation, evapotranspiration and thermal mass.
Although green roofs do provide potential energy savings by improving building insulation characteristics, it is difficult to give them a fixed U-Value rating. This is because green roofs are dynamic systems and their properties vary through the year.
Studies in Germany have provided various estimates. Figures attributed to ZinCo estimate that 2 litres of fuel oil are saved per m2 of green roof per year.
A more recent study of domestic buildings in North East German with flat roofs suggests that there is a 3-10% winter saving on fuel bills. The results of the study over five years suggest that there is a maximum saving of 6.8kWh/m2 (1.5kg/m2 CO2 e tonnes) and a minimum saving of 2.0kWh/m2 (0.44kg/m2 CO2 e tonnes) during the winter.
This correlates with the Ottawa study referred to above, which compared a conventional roof system with a green roof system
‘The average daily energy demand for space conditioning caused by the reference roof system was 20,500 BTU to 25,600 BTU (6 kWh – 8 kWh). However, the green roof system’s growing medium and plants modified the heat flow and reduced the average daily energy demand to less than 5,100 BTU (2 kWh) – a reduction of more than 75 percent.’
The City of Toronto has estimated that the direct energy savings citywide, through reduced energy for cooling through wholescale city greening, would be in the order $22 Million and equivalent to 4.15kWh/m2 per year (a CO2 emission saving of 1.7kg/m2). There would also be a reduction in peak energy demand in the order of 114.6MW leading to fossil fuel reductions in the region of 56,300 metric tonnes per year.
Studies by Environment Canada have shown that the upper floor of a building with a green roof is likely to save 20% of its energy demand by reducing its need for cooling. This study also calculated the reduction on floors below the upper floor. The summer savings on the top floor of the building would be in the order of 20%. If a building is five storeys or more, the total summer savings would be in the region of 6% and with a two-storey building, the total summer savings would be between 10 – 12%.
An energy study undertaken by the City of Chicago estimated that, with whole scale greening of the city’s rooftops, energy to the value of $100 Million could be saved each year due to reduced demand for air conditioning. This would to a reduction in peak demand in the order of 720MW.
A 1999 study undertaken by the City of Chicago also estimated that the greening of all the city’s roofs would save around $100 Million in energy each year, especially due to a reduction in the need for air conditioning – the equivalent energy consumption of several coal-fired generating stations, or one small nuclear power plant.
The considerable energy benefits of a green roof should be considered as part of any assessment of a building’s long term energy use.