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When calculating the U-value of an inverted flat roof, the thermal conductivity of the insulation layer should be the product’s design lambda value and not its declared value. Because of the insulation’s position above the roof waterproofing layer, it is expected to be exposed to some rainfall and therefore a moisture correction factor should be applied accordingly.
A water flow reducing layer (WFRL) is a loose laid membrane used as part of inverted flat roof constructions. It is installed over the roof’s thermal insulation layer, and below the roof covering. A WFRL is a barrier to rainwater, reducing the volume of water that can reach the waterproofed roof deck. It also stops fines from entering the roof system.
There are two ways in which rainfall alters the way that U-values should be calculated for inverted flat roof build-ups. An inverted roof should drain at the level of the water flow reducing layer (WFRL) membrane but, because it is not a waterproof layer, the insulation will be exposed to some rainwater that drains at the waterproofing level instead.
The volume of rainwater that reaches the waterproofed deck of an inverted roof depends on two things. First is the building’s location, and the average rate of rainfall that location experiences during the heating season. The second is the result achieved from testing the thermal insulation and water flow reducing layer (WFRL) in combination to see how effective a barrier to rainfall the system is.
The water absorption of inverted roof insulation varies depending what insulation material is used. Extruded polystyrene (XPS) is the most common type of insulation used on inverted roofs, while expanded polystyrene (EPS) is also popular. Technical guidance related to inverted roofs usually mentions both, though XPS has the longer history of use in the application.
A water flow reducing layer (WFRL) is not a waterproof layer, and technical guidance for flat roofing is consistent in reinforcing this. It is therefore not appropriate to design an inverted flat roof on the assumption that no rainwater will be able to reach the waterproofing layer.
BBA Information Bulletin No.4 is a technical guidance document issued by the British Board of Agrément. Its content describes two areas of inverted roof design and specification – the drainage of inverted roofs, and how to accurately calculate a rainwater cooling correction factor for U-value calculations.
After careful consideration, and in light of the rapidly changing market conditions, we at Polyfoam XPS have taken the decision to temporarily reduce the level of our operations.
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