Green Roofs

By Maria Morgan: Product Manager (Polyfoam XPS and XPE) – Knauf Insulation Northern Europe

Green roofs can be an attractive prospect as they not only provide amenity space for building users, but critically, reduce heating costs, CO2 emissions and cooling loads in a building. Whilst the inclusion of a growing medium and vegetation can form a crucial component of SUDS drainage systems, as they slow rainwater runoff.

Green roofs can retain up to 75 per cent of rainwater; gradually releasing it back into the atmosphere via evaporation and transpiration, while retaining pollutants in their soil. Controlled evacuation to the main drainage system prevents localised flooding.

There are many insulation choices for flat roof constructions however the demands on the insulation are significantly different for inverted roof applications as the roof insulation is placed over the waterproofing membrane. The insulation chosen therefore, needs to have high compressive strength and a closed cell structure for low water absorption in order to maintain its performance criteria for the lifetime of the building.

When designing an inverted green roof it is recommended that the roof is designed in line with European Technical Approval Guideline (ETAG) 031-1 which sets out the performance criteria for inverted insulation kits. ETAG 031 references two types of insulation that it considers suitable for inverted roof applications – XPS and EPS.
ETAG 031 highlights that XPS is suitable for all inverted roof applications

• Untrafficked;
• Pedestrian;
• Green Roofs;
• Roof Gardens;
• Parking Decks.

However, due to the limited history of EPS insulation in Green roof and Garden roof applications, ETAG 031 suggests that it should only be used for the following inverted roof applications

• Untrafficked
• Pedestrian

It is clear when referencing the guidance in ETAG 031 that the most suitable insulation product for inverted green roof and garden applications is XPS.

Knauf Insulation’s Polyfoam Roofboard range of XPS products that we recommend for inverted roof applications are almost 100 per cent closed cell resulting in low water absorption over time i.e. 0.6% moisture by volume. The Polyfoam boards also have a high compressive strength of 300 kPa suitable for the heavy loads associated with green or garden roof applications.

In addition, the thermal performance is unaffected by site conditions such as wide fluctuations in outside temperatures and repeated freeze thaw cycles, making Polyfoam Roofboard ECO Extra products the ideal solution for green roof applications.

Furthering the eco credentials of green roofs, specifiers should look to incorporate insulation products that have third party environmental certification. The Polyfoam ECO range of products are the only extruded polystyrene products to be independently assessed as a certified BRE Green Guide rating of A allowing specifiers to have the utmost confidence that the products provide a proven sustainable, environmentally friendly solution.

Further reading
Green Roof Organisation
BRE Green Guide Live
ETAG 031 inverted insulation kits

One thought on “Green Roofs

  1. With 1.5 million m2 added to UK rooftops in 2012, Green Roofs are a hidden growth gem for UK construction, in an otherwise depressing economic outlook.

    80% are Extensive roofs and 20% being Intensive. For both types it is important that Specifiers follow Maria’s advice. This is a specialist application where the most appropriate product should be used in order to prevent potential costly correction works in years to come.

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