Rob Firman, technical and specification manager at Polyfoam XPS, addresses contractors’ queries about inverted roofs and extruded polystyrene insulation.

Why is a different lambda value used for an inverted roof calculation?

In simple terms, the ‘declared’ lambda value refers to the thermal conductivity of the product when it leaves the factory.

When laid over the waterproofing in an inverted roof, the insulation’s efficiency can be affected by rainwater draining between the boards. The declared lambda value is adjusted and the insulation assigned a ‘design lambda’ value, which is used in U-value calculations for inverted roofs. 

What is a water control layer?

The cooling effect of moisture reaching the waterproofed roof deck affects a roof’s thermal performance, so a water control layer is laid over the insulation to reduce the amount of moisture that can enter the build-up.

Water control layers, such as Polyfoam Slimline Zero, are membranes resistant to rot and UV decay, and impermeable to liquid but permeable to water vapour. They allow a favourable drainage factor to be adopted in calculations. The lower the drainage factor, the thinner the insulation required to achieve a particular U-value.

What coverings should be used above the insulation?

As the insulation and water control layer are loose laid on an inverted roof, rather than mechanically or adhesively fixed, a ballast is required to stop them being lost to the wind and prevent flotation after heavy rain.

A variety of roof coverings can be used, including green roofs and timber decking. The following roof coverings, however, are accepted as achieving the necessary fire performance without the need for further testing, and therefore are most commonly specified and installed:

  • Loose laid gravel at least 50mm thick, or with a mass greater than 80 kg/m2 (subject to maximum and minimum aggregate sizes).
  • A sand/cement screed at least 30mm thick.
  • Cast stone or mineral slabs at least 40mm thick.

Why do I need to know the site location before requesting a U-value calculation?

The average rate of rainfall during the heating season, based on location-specific data from the Met Office, is included in a U-value calculation. It is used to help calculate the amount of rain water the insulation is likely to be exposed to and the effect it could have on performance.

For example, a building located in East Anglia is likely to be exposed to significantly less rainfall than one in Western Scotland. There can be wide variations in rainfall in specific regions too – the average rainfall for Manchester is lower than the North West as a whole, showing the benefit of using location data to improve the accuracy of the U-value calculation.

Why is drainage important?

Building Regulations and British Standards require roofs to drain properly. If water is allowed to pond, the roof can be exposed to issues such as greater loads than it was designed to accommodate, accumulation of unsightly silt and algae, and freeze-thaw cycles that place stress on roof coverings.

For inverted roofs, drainage should be provided at both the water control layer and waterproofing levels. Drainage outlets should be positioned at the low points of the roof, not just the roof edge. The low point on precast concrete decks, for example, is most likely at mid-span because of deflection due to the deck’s weight.

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This article first appeared in the October 2018 edition of Total Contractor magazine