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All you need to know about: Inverted roof correction factors

by Jennie Ward
Rob Firman.

Designers and installers of flat roofing solutions are often told what they ‘should’ know about products and standards. In this ongoing series, Rob Firman of Polyfoam XPS looks at different aspects of flat roofing design and construction and helps to demystify them. This month he looks at correction factors applied to U-value calculations for inverted flat roofs.

How can moisture affect inverted roof performance?
Calculation of the thermal transmittance (U-value) of an inverted flat roof accounts for the effect of moisture through two corrections. The first is any change to the insulation’s thermal conductivity due to moisture absorption; the second is the cooling effect of rainwater running over the waterproofing layer.

Obtaining design thermal conductivity by moisture correction
The thermal conductivity of an insulation material in a U-value calculation should reflect the conditions in which the product will be used. In an inverted roof, the thermal insulation is positioned above the waterproofing layer, exposing it to moisture.

“Moisture is an effective conductor of heat, so moisture absorption can worsen the thermal conductivity of an insulation material”

Moisture is an effective conductor of heat, so moisture absorption can worsen the thermal conductivity of an insulation material. Different insulation materials demonstrate different moisture absorption characteristics, which is why it’s important to select a suitable insulation material where the effect is minimised.

ETAG 031 describes the procedure for correcting the insulation’s thermal conductivity for use in U-value calculations. Declared thermal conductivity is multiplied by a moisture conversion factor to give a design thermal conductivity.
In last month’s issue of Total Contractor we covered BS 6229:2018, and section 4.6.2.2 of that standard says: “Calculations of the thermal transmittance of specific roof constructions should be carried out … using design thermal conductivity (including moisture conversion factor Fm).”

The factor is derived from an insulation’s moisture conversion coefficient, and a calculation value for moisture content based on the average tested value for long-term water absorption by diffusion and freeze thaw.

What information is needed to calculate a rainwater cooling correction?
BS EN ISO 6946:2017 describes the combined method for calculating U-values. Annex F gives a correction procedure for inverted roofs “due to rainwater flowing between the insulation and the waterproofing membrane.” The rainwater absorbs heat energy from the structure and increases the rate of heat loss from the roof.

This correction is completely separate to the moisture conversion factor described above. That is applied to the thermal insulation separately, whereas rainwater cooling is calculated as a correction to the U-value for the roof build-up as a whole.
Calculating the rainwater cooling correction requires three values:

• p (measured in mm/day) is the average rate of rainfall during the heating season. It is based on location-specific data and varies significantly across the UK. Both BS EN ISO 6946 and BBA Information No.4 explain where this data can be derived from.

• f is a drainage factor. It expresses the percentage of the rainfall (p) that reaches the waterproofing membrane.

• x (measured in W.day/m2.K.mm) is the factor for increased heat loss. It is a standard value of 0.040 W.day/m2.K.mm, as given in BE EN ISO 6946.

Rainwater cooling is a fundamental part of the combined method for calculating U-values. The overall U-value correction and its subsequent impact on insulation thickness is strongly dependent on the three values listed above.

Inverted roof systems typically feature a water flow reducing layer (WFRL), which is tested to establish the value for f that can be included in U-value calculations. See the May 2021 issue of Total Contractor to read our summary of WFRLs.

For both moisture conversion factors and rainwater cooling corrections, inverted roof system manufacturers obtain third party assessment and certification to provide reliable information about their impact on system performance.

Contact Polyfoam XPS about inverted roof U-value calculations.

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