SUSTAINABLE BUILDING

WHAT LIES BENEATH:

Insulation for Green Roofs

G

reen roofs are becoming ever more commonplace within urban areas in the UK as they offer major benefits over traditional

roof constructions. Accessible roofs provide more space in denser, more compact urban environments and London and Sheffield are two areas where planners are being particularly proactive in encouraging green roof designs. This was evident in the recent redevelopment of the roof of Barclays Bank Commercial Tower in Canary Wharf, where a green roof was retro- fitted to provide a site for a Biodiversity PhD Study.

However the UK is still some way behind the rest of Europe in the uptake of green roofs. The system originated in Scandinavia and Germany is now leading the way in their uptake. Studies there have proven 3-10% savings on winter fuel bills where green roofs save between 2 and 6.8 kWh of energy per m2

. The German

government offer significant tax incentives to encourage the use of green roofs, which are already estimated to account for 10% of all roofs. A study by Nottingham Trent University backed up the winter energy savings in Germany. With an outside temperature of 0°C, temperatures under conventional roof membranes were shown to average 0.2°C, compared to 4.7°C under green roof membranes. However, the thermal enhancement seen with green roofs is even more important in summer. The same Nottingham Trent University study demonstrated that an outside temperature of 18.4°C translated to 32°C under a conventional membrane and just 17.1°C under a green roof. The high energy consumption of cooling systems means that a 5° temperature reduction leads to a 10% cut in air conditioning energy requirements. The City of Toronto has carried out detailed research into air conditioning energy savings achievable through whole scale roof greening. Across Toronto it estimated that around $22 million could be saved, reducing CO2 emissions by 1.7kg/m2

.

Thermal performance is just one of the benefits of green roofs, another being

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SUSTAINABLE FM | APRIL 2010

A number of environmental and performance concerns are making green roof constructions a popular option for both commercial and domestic buildings. Here, Richard Lee, Sales Director at Jablite, explains how recent developments within Expanded Polystyrene (EPS) are making the material the preferred choice for insulating green roofs.

the reduction in water run. This is a particularly important issue in London, where the waste water network is struggling to cope and SUDS in an ever more

important issue. A green roof usually reduces run off by 75% or more, as rain water is absorbed by the plants. The speed of water run off is also reduced providing a natural control of flow into the drainage system. The bio-filtration provided by green roofs also filters out water-borne pollutants including heavy metals found naturally in rainwater, a further benefit to the waste water system. Careful choice of design and planting on a green roof can enhance biodiversity and provide additional habitat for wildlife species, whilst reducing CO2

and

increasing oxygen levels. However, by trapping airborne dust particles and pollutants, green roofs can have an even greater impact on air quality as 1m2

of

green roof can remove 0.2 kg of air- borne particulates from the air every year. A further benefit of green roofs is acoustic insulation. Tests have shown that indoor sound can be reduced by as much as 40 decibels, providing particular benefit to buildings in noise impacted areas close to airports or industry. Although a green roof will reduce energy consumption for cooling in summer and heating in winter, Building Regulations in the UK mean that thermal insulation is still required within the roof build up. Expanded polystyrene (EPS) is becoming a more popular choice because it achieves an A+ rating in the BRE’s Green Guide to Specification, the highest environmental standard available. EPS was rated highly due to its low

environmental impact. It has an Ozone Depletion Potential (ODP) of zero and a Global Warming Potential (GWP) of less than 5, which means it qualifies for BREEAM credits.

Green roofs increase the lifespan of the weatherproofing membrane by two to three times, reducing temperature variation at the membrane and protecting from UV damage. Guarantees of 40 to 60 years can therefore be obtained for this type of roof build up. This benefit is also helping to increase the popularity of inverted roofs, where the insulation sits above the weathering membrane. At Jablite we saw an opportunity to develop a product to suit the particular needs of inverted roofs, which requires a modified EPS with reduced water absorption properties that can also resist the effects of freeze/thaw. The new product, Jablite Flat Roof Inverted, has reduced water absorption properties compared to our standard EPS range (less than 1%) and meets all the criteria for an insulation material in an inverted or green roof application. It also comes with the benefit of low GWP and an A+ rating which is unique in inverted roof insulation. Due to this latest development, EPS insulation is now available for any type of flat roof construction, which means that any green roof can now benefit from the greenest insulation material available. As environmental legislation increases, green roofs will increasingly become part of residential and commercial buildings. Selecting the right insulation material for these structures is vital, not just in enhancing the thermal efficiency, but also in maintaining the highest environmental credentials of the roof itself.

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