green roof save energy

 green roof save energy

Flood Risk

Climate change will see a predicted increase in winter rainfall and extreme rainfall events. Due to the large areas of impermeable and hard surface development, plus the limited capacity of London’s storm drains and combined sewer system, the consequence of more intense rainfall will be an increase in surface and sewer flooding.

At the same time, the consequence of flooding will also increase as the amount and value of development in flood risk areas continues to grow. The government has recognised the importance of taking an integrated approach to storm water management through ‘Making Space for Water 2004’ [Ref:3.4]. Living roofs are a proven source control technique and as such can make

a positive contribution to the increased risk of flooding

in London caused by our changing climate.

Temperatures

London has already experienced episodes of significantly high temperatures that have affected the capital’s health, economy and environment. Climate change will cause average summer temperatures to rise to a point where our current extreme events will be average summer temperatures by the middle of the century, and heat waves will be even hotter. Summer temperatures for the South East are predicted to be up to 3.5°C warmer by the 2050s and up to 5°C warmer by the 2080s (United Kingdom Climate Impacts Programme 2002, UKCIP02).

All urban areas experience an Urban Heat Island Effect (UHIE): this is the increased temperature of a built-up area compared to its rural surroundings. The centre of London can experience temperatures up to 9°C higher than the surrounding greenbelt and climate change is likely to increase the frequency and duration of these effects. [Ref: 3.5]

The ‘Green Roof Effect’

Conventional roofing surfaces, hence referred to as ‘warm-roofs’, absorb sunlight and heat-up quickly. The absorption of radiation and the release of the radiation back to the atmosphere during the night is a major factor in UHIE. Where a building has poor insulation and poor ventilation this can lead to increased use of air conditioning and therefore increased energy use (thus green roofs make an indirect contribution to climate

The performance of green roofs can benefit both the buildings on which they are installed and the wider environment. Both are interrelated as outlined below. Energy use within a building may exacerbate the effect of the UHIE. Conversely a positive reduction in the UHIE is likely to lead to a reduction in the need for energy for summer cooling within a building. The details of these benefits are described in sections 2.2 and 2.3.

Energy balance and UHIE reduction are likely to become increasingly important considerations for building professionals within the capital. Elsewhere in the world, in North America and the Far East, green roofs are being considered as an important element in how cities can cope with the UHIE and reduce energy demand within buildings. Over the last few years the implications of climate change are becoming more pressing and, as is the case elsewhere in the world, green roofs are likely to be an integral element of building design in the future for both policy makers and businesses as they develop strategies to adapt to climate change.

2.2 Energy Balance and CO2 Reduction

Green roofs have a substantial thermal mass and a moderate insulation value. These combined properties significantly reduce diurnal temperatures at the boundary between green roof and building structure (the diurnal temperature being the daily maximum to minimum temperature range).

The diurnal temperature range for a conventional construction ‘warm-roof’ waterproof layer can be very large; for example, the surface of a typical bitumen waterproof layer may exceed 50°C during a sunny summer’s day, whilst falling to just above 0°C at night.

A roof with a low level of insulation below the waterproof layer will allow the space below to heat up quickly in hot, sunny weather. The increased internal temperatures in the floor below the roof contribute to making the internal building environment uncomfortable for the building’s occupants. Overheating can lead to increased use of air-conditioning, which in turn will lead to an increase in energy consumption. During cold weather, the opposite effect applies, resulting in a demand for extra heating of the floor directly below the roof and, hence increased energy consumption. The energy used for heating and cooling has a financial as well as an environmental impact.

The green roof has the same energy providers as a conventional roof, but it has the additional energy consumers of evapotranspiration and photosynthesis. Unlike a conventional roof, the green roof is a living system that reacts to the environment in a number

of important ways:

• water is stored within the substrate and is used in evapotranspiration by the vegetation layer; this process utilises a considerable proportion of the incoming solar