Bellerbys College, Brighton

CHALLENGE

A 3,500m² protected membrane roof has also been specified for the soon-to-be-completed extension to Bellerbys College in Brighton. Its new multi-million pound International Study Centre will house 62 classrooms, library, exam hall and lecture theatre, 375 en-suite student bedrooms, cafeteria, internet cafe and common room.

The roof will have some ballasted areas and some green roof construction. Green roofs can be flat, pitched or curved and are generally finished with sedum mats/plug plants (extensive systems) or trees and shrubs (intensive systems). They deliver a number of well-established benefits:

• Thermal insulation
• Acoustic insulation
• Rainwater retention/management of surface water run-off
• Reduction of urban temperatures (albedo effect)
• Improvements to air quality
• Provision of extra open space

SOLUTION

By marrying together Polyfoam extruded polystyrene rigid board insulation with the Slimline membrane, Knauf Insulation has delivered a straight-forward solution to a problem bugging the industry – that of water finding its way into protected membrane and green roofs and having a ‘chilling’ effect within the roof construction.

Before the 2002 changes to Part L of the Building Regulations, manufacturers  generally counteracted this ‘chilling’ effect by recommending a thicker insulation, typically 120mm when using XPS.

However, updates toPart L revised the conventions for calculating U-values and advised designers to take more account of the chilling effect, potentially pushing insulation thicknesses up to around 180mm. This increased depth would have created its own problems – wind uplift calculations and upstand sizes had to be revised and access to roof spaces became more difficult etc.

Knauf Insulation’s solution was simple – swap the filtration layer used in this roof construction for its Slimline membrane. The high-performance, non-woven polyethylene geotextile membrane is water-resistant but vapour permeable (water vapour molecules are small enough to diffuse through the fabric, liquid water molecules are not). This prevents water collecting within the roof build-up, allowing for reduced insulation thicknesses to to provide high thermal and structural performance roofs.