Cellular glass is in the first place known as the best thermal insulation in the roof but is less popular on (in) facades. And within the facade world, it was for a long time not supported as thermal insulation under a rendering. The reason for this is quite clear:
Most (if not 99%) rendering products for application on walls and thermal insulation contain cement as bonding agent. Cement based products hardens in a totally not flexible way and have a tendency to let shear off the cellular glass. During drying, the rendering shrinks and induces an enormous tensile stress on the cellular glass. In case the layer is thicker than a few millimeter, the cellular glass shears off because the tensile strength of the cellular glass is exceeded. In reality, a rendering must be at least 1 cm thick to get a long life and cellular glass became obsolete in this application.
(Portland) cement changed our life with its great strength and short settling time and let us create strong but still elegant constructions. But there was a time before Portland cement where almost pure hydraulic lime was used as bonding agent. Buildings made with this bonding agent were able to deform without creating a crack, they were (are) in a certain way flexible. Foundations were much smaller or non-existent while today a house without proper foundations will show important cracks when the masonry is performed with ordinary Portland cement.
Rendering, properly based on this old recipe, don’t shear off from cellular glass, even if the layer has a few cm thickness. A thick layer with coarse grained sand is put on the cellular glass and the finishing is done with fine grained layer. The fine grained layer will dry the wet thick base layer by the difference in surface tension.
Both systems are a perfect solution for those customers who want a rendering on thermal insulation which resists a foot ball, feels like a real wall and does not burn. Both solutions are guaranteed by an ETA and are insured.