Joan Ferrer, Technical Sales Manager, Ravago Building Solutions, discusses the crucial considerations when calculating U-values for inverted flat roofs.
The robustness of U-value calculations are of paramount importance due to increased emphasis on energy performance of buildings and the growing adoption of BIM for building management, ensuring that structure and envelope performs as well in reality as in computer models.
The latest version of the BS6229:2018 ‘Code of Practice for flat roofs with continuously supported flexible waterproofing coverings’ came into effect in November 2018. Some of the revisions have been welcomed by the industry, yet there are also concerns.
European Products Standards thermal test results provide a reliable and consistent approach in assessing thermal conductivity of insulation products, overcoming the inherent degree of variability of solutions. The resulting declared lambda (as specified in BS EN 13164 for XPS insulation) incorporates ageing of materials, further reinforcing the robustness of subsequent calculations.
Sometimes referred to as the 90/90 value, declared lambda represents 90 per cent of production achieving the quoted conductivity value at a confidence level of 90 per cent. However, when an inverted flat roof is specified additional correcting factors are required to account for the unique end use conditions.
Detailed and accurate information should be available from the manufacturer of proprietary materials, though in lieu of official guidance it may be appropriate to use generic figures for the materials. Confusion about the thermal conductivity value representing the thermal insulation material must be avoided at all costs because the thermal insulation layer contributes the majority of the total thermal resistance in the final U-value.
Building location impacts energy performance, as two identical buildings constructed in two different climates would perform differently. When considering an inverted flat roof the principle concern is rainfall levels and the ensuing rainwater cooling effect.
There are many advantages to placing insulation above the waterproofing layer, however water flowing beneath the insulation boards to drainage outlets is an additional heat loss mechanism, meaning that the volume of rainfall is integral to thermal performance. A correction (Delta U) can be calculated as set out by BS EN ISO 6946:2007, considering average rainfall and the percentage of water that will subsequently reach the waterproofing layer, known as the drainage factor (fx).
A proportion of rainwater can be assumed to flow underneath edge-profiled interlocking boards for calculation purposes, European Technical Approval Guideline (ETAG) 031-1 offers a figure of 30 per cent. In instances where a water flow reducing layer (not to be confused with waterproofing) is used, the flow of water can be reduced to typically around 1 per cent. This layer between the insulation and ballast significantly reduces the resulting Delta U.
Paragraph 126.96.36.199 of BS6229:2018 examines how thermal performance is impacted by rainwater penetrating the insulation layer and reaching the waterproofing layer as a result of poor workmanship on the water flow reducing layer (WFRL). Ravago Building Solutions believes there is no evidence to support the assertion of routine poor workmanship, and find the 10 per cent increase in insulation thickness to be entirely arbitrary and unfounded until evidence has been gathered and tests carried out to substantiate the claims.
The potential for interstitial condensation is reduced due to the waterproofing layer being on the warm side of an inverted flat roof, serving as an effective vapour control layer. Furthermore, the fundamental design of an inverted flat roof considerably reduces the risk of condensation, but it does not eliminate it, as there could be sudden drops in temperature due to the rainwater cooling effect.
Ravago Building Solutions offer a free U-value calculation service for prospective customers.