Improved predictions about moisture behavior in reverse roof insulation

© Fraunhofer IBP
© Fraunhofer IBP

Reverse roof insulation does not employ additional seals to protect against the effects of weather. The outer layers required for the weighting of the insulation, for instance made of paving stones, gravel or substrates, also serve to store more moisture from precipitation, which means the area above the insulation usually experiences only short dry periods.  

As a result of this construction method, insulating panels that are more diffusion-inhibiting can also exhibit a long-term accumulation of moisture, thanks to the thermal gradients in the existing moisture film under the insulation. This is particularly important if the layers do not dry out, and was taken into account at the method’s accreditation several years ago, when a value was added on for the material’s thermal conductivity. With the help of that value, the resulting thermal conductivity at the midpoint of the construction’s expected lifetime can be factored in.

Using hygrothermal simulations, it is now possible to reliably predict even long-term moisture behavior in structures when the conditions are known.

Collaboration with the Munich research institute for thermal insulation, FIW, arose as part of a project funded by the German Centre of Competence for Construction (DIBt). The project was entitled “Estimating moisture values for the thermal conductivity of reverse roof insulation on the basis of object examinations and hygrothermal calculations.” New computational models have been developed on the basis of outdoor testing and lab measurements for the assessment of reverse roofs.

These models take into account the specific installation situation of reverse roofs, including the moisture balance of covering layers and the formation of a moisture film under the insulating panels. This enables a more accurate quantification and differentiation of the added values in relation to the material properties, and considers not just the type of material, but also the insulating material’s exact installation situation (layer structure, separating layer, planting, etc.).

 

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