A Case study | Energy Efficiency and Cost Effectiveness of a Mediterranean Green Roof Ecosystem

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10 minute read

This is a copy of a study conducted by Vanya Veras, owner of VIVACITY & peer-reviewed by Benjamin Gill, MSc, MIEMA, C.Env .
If you are short of time then scroll down to the CONCLUSIONS.

the difference in summer was dramatic. Before the Oikostegi Green Roof System, we couldn’t sleep at night. The fans were only throwing back the hot atmosphere that had been gathering during the day. When summer 2009 came, we couldn’t believe the difference...’

Back in 2008, I was asked to assist in the installation of a Mediterranean green roof ecosystem developed by Oikosteges, on a property in the centre of Athens.

The new owners had recently bought a 2-storey house. It consisted of 120m2 per floor with a small room on the roof of 25m2.

The house was heated by natural gas, they used an electrically powered boiler for hot water, cooled using ceiling fans and one air-conditioning unit for a 25m2 space. The home owners were after a more sustainable, energy efficient household.

Background to the Thermal behaviour of the building

The ownership of the building studied, changed in May 2007, increasing the number of occupants and necessitating the use of all living space. Being environmentally conscious, the new occupants chose to install ceiling fans rather than air-conditioning in the bedrooms which are located on the floor beneath the roof. The only air-conditioning unit in the house was in the small room on the roof-top, which is most exposed to weather conditions.

The occupants had intended to use the roof-top room as an office but this had not proved practical:

'I had wanted to have my office up in the small top floor room, but it was so unbearably hot that even with the air conditioning it was impossible to work up there. Even on sunny autumn days it was uncomfortable. The result was that we completely abandoned the use of the roof and its room.'

Between May 2007 and October 2008, energy consumption during winter and summer months was exceedingly high. Heating expenses alone that winter were €1453 (5.48 tons CO2).

In the summers of 2007 and 2008, as the house had no air-conditioning units, the ceiling fans were not helpful in cooling the building as they were simply circulating air, estimated by the occupants to be between 35 and 45 ̊C.

Around 50% of heat transfer occurs through the roof.

Around 50% of heat transfer occurs through the roof.

As a result of this discomfort, the occupants were seeking an insulation system for their roof to solve the problem and had seriously considered leaving the house, when they learnt of green roofs and their potential.

Following research of the available systems, the occupants decided to install a Mediterranean green roof ecosystem developed by Oikosteges
Their choice was motivated by the following factors:

  • our technique's extensive research and proven implementation,

  • very low watering needs,

  • Oikostegi’s green roof’s self-sustaining nature and

  • its intended use as an effective insulation method, with a secondary function as a roof-garden.

Implementation of the Green Roof

The available roof surface area was 80m2 (excluding the small room) and in October of 2008, we installed a Mediterranean green roof ecosystem on all of the available roof.

For the purpose of this study, winter is taken as the period from October to April.
As there is no air- conditioning, no reduction in consumption for cooling can be measured. Reductions shown here result only from a reduced need to heat.

Winter 2007-2008

House without our green roof garden developed by Oikosteges
Living conditions in the house were uncomfortably hot in the summer and the winter was cold.

Winter 2008-09
October 2008, installation of our Oikostegi, Mediterranean Green Roof System.

Over the first winter following installation, the house was warmer than it had been the previous year, coupled with a reduction in the heating bill.
When the following summer came around, the occupants of the property were able to observe the real effects of the Green Roof Oikostegi System.
In their words:

' the difference in summer was dramatic. Before our Oikostegi Green Roof, we couldn't sleep at night. The fans were only throwing back the hot air that had gathered during the day. When summer 2009 came, we couldn't believe the difference...'

‘...Since then, the top floor room has become the most popular room of the house and we decided to move one bedroom there, as it is the quietest, (our Oikostegi green roof garden system also has sound insulating advantages) the coolest in summer, and with a beautiful garden to look at. The air conditioning is
rarely used up there; it’s only used during heat waves.'

Winter 2009-10

Second Winter after installation

In the second winter with our green roof garden installed, the occupants decided to connect their hot water to the natural gas boiler. Despite this additional load and the cold winter, the heating bill was further reduced.

Table 1: Reduction in natural gas comsumption

Table 1: Reduction in natural gas comsumption

The winter of 2007 was cold, with snow-fall in the northern suburbs of Athens and an average temperature of 10 ̊C from October to April 2008. The following winter was mild, with average temperatures of 15 ̊C and little snowfall at high altitudes; whereas from Oct 09 to Apr 10, the average was once again low for Greece, at around 11 ̊C with very high rainfall. Comparing the years of 2007/8 and 2009/10 where the winter temperature was similar there was a 40% reduction in gas consumption – all of which can be directly attributed to the Oikostegi Green Roof system.

Fig 1. Reductions in expenditure and carbon emissions from natural gas use.

Fig 1. Reductions in expenditure and carbon emissions from natural gas use.

Embedded energy of the Oikosteges System

An Oikostegi Green Roof ecosystem can be watered to a maximum of 5lt/m2/yr. For the 80m2 roof in question, this would be 400 litres per year. This Oikostegi green roof was watered to this capacity for its first 17 months, which in terms of CO2 emissions, amounts to 0.51kg CO2 in total.

After these 17 months, it was ascertained that this amount of watering was actually harmful to the development of the green roof, reducing the growth capacity of the plants belonging to the system and attracting weeds. For this reason, watering was stopped and the plants could be seen to be developing again.

Watering: For the Oikostegi's green roof root system to develop, 6 weeks of watering are necessary following installation and a further 3 months in each of the first two summers. If these guidelines are followed, the total carbon footprint from the watering of 80m2 of Oikostegi need not amount to more than 0.005 (at 3lt/m2) to 0.015kg CO2 (at 5lt/m2).

Waterproofing:

The waterproofing layer represents a further 538 kg embedded carbon.Taking these two components as a basis, the embedded carbon of this Oikostegi green roof amounts to 538.51 kg CO2 (or 0.51 tons CO2).

Conclusions

  • The Mediterranean green roof ecosystem developed by Oikosteges has saved €726 from reduced energy consumption. These savings are only from heating requirements. This is a reduction which will be continued yearly.

  • A Mediterranean Oikostegi Green Roof system helps to reduce the internal temperature of a building in summer creating a much more pleasant internal environment.

  • The embedded carbon of a Mediterranean green roof ecosystem developed by Oikostegi when taking into account full watering for one year and the waterproofing layer is 538.51 kg CO2

  • During its first year, this Mediterranean green roof ecosystem developed by Oikostegi avoided 635kg CO2 from natural gas alone.

  • Over the period studied, assuming that the baseline is an average year in terms of temperature, this green roof developed by Oikostegi avoided a total of 2.58 tons CO2 from being emitted into the atmosphere.

  • For each subsequent year, the green roof ecosystem will continue to avoid at least 1.95 tons CO2 per year, which represents a reduced emission for Greece and less demand on the country's energy supply.

  • For the purpose of calculation, if we round that up to 2 tons CO2 avoided per year, this house alone will have avoided the emissions of 20 tons CO2 by 2020, the year that Greece must prove 12% reductions on 1990 levels as part of its pledge to the EU and as a signatory to the Kyoto Protocol.


As this is one of the few houses in Athens without air-conditioning, the energy reduction capacity of the Oikostegi Green Roof System is not entirely representative of the majority situation in Athens, where cost and CO2 savings would be considerably higher as a result of a reduced need for cooling. For this reason I have made a rough and conservative estimate of what the additional CO2 savings could be in a residence with air-conditioning.

  • If it is assumed that the residence has 2 air-conditioning units of 1kW power each and that they operate for 8 hours a day, 3 months per year, the electricity consumed would equal around 1.2 tons CO2 per year.

  • Added to the reductions achieved through reduced heating, this amounts to 3.15 tons CO2 avoided per year.

  • As this figure is an assumption, it will not be added to the conclusions but will be the subject of further study.

  • However, for the purpose of discussion, I will conclude that this combined saving would amount to 31.5 tons CO2 avoided by 2020. A full 11.5 tons more avoided by removing the need for air-conditioning for just 3 months of the year.

Another conclusion which can be drawn from this study is that, for an Oikostegi in its second year, which has been applied to the entirety of a roof surface, each m2. of Oikostegi is responsible for the avoidance of 24.4 kg CO2. per year, simply from the reduced need to heat.

As a result of increased occupancy, electricity consumption from May 2007 to September 2009 increased. When the water heater was linked to the natural gas boiler in the autumn of 2009, this achieved an additional reduction in energy costs to the household of €149.41 (845.6 kg less CO2 as Greek electricity generation uses coal).


Fig 2: Evolution of emissions from electricity consumption from 2007 to 2010

Fig 2: Evolution of emissions from electricity consumption from 2007 to 2010

References

  1. Natural Gas and Electricity Bills
  2. Greek calculation for the kg CO2/kWh (from DEFRA: the UK Department for the Environment)
  3. UK (UK Department for the Environment) calculations for the kg CO2 per unit of other items in the study.

This study was conducted by Vanya Veras, owner of VIVACITY & peer-reviewed by Benjamin Gill, MSc, MIEMA, C.Env .

Vanya Veras