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Conclusions:

  • Dry Mineral Wool is light weight and requires ballast on a roof.
  • When saturated it is as heavy as a specifically engineered drainage board that has increased water retention or as heavy as saturated LWA (Lightweight aggregate).
  • The relative air content is very low when saturated comparing to LWA– this saturation can cause problems over the long run, especially with succulents like Sedum (they don’t  mature, like Peat Moss in pre-vegetated Sedum mats).
  • The ballast (mentioned under 1) can compress Rockwool overtime and this can reduce water retention, but more importantly it can further reduce air content.
  • It is proven that the high water content (when saturated) of Mineral Wool is hardly available for the plants.
  • Mineral Wool has very little or no ion exchange capacity (dead material).
  • Mineral Wool can contain a large amount of chemicals that make the material hydrophilic (how long will this last?).
  • The insignificant air content of Mineral Wool – open space – can be compressed or filled with fine particles.
  • No equivalent or compressive strength compared to LWA or specifically engineered Green Roof Drainage boards.
  • Drainage capacity of Mineral Wool is poor (below the requirements of FLL)
  • High costs.
  • Difficult to recycle once in place over decades.
  • High energy consumption in production (over 1600°C/2900°F)
  • Potential health issues of fibers (during install and in runoff water).
  • Attracts birds and rodents for nesting material.
  • No sufficient tests in North America (two years or less).
  • Material is not used anymore in the German Green Roof industry by professionals over the last 2 decades – most existing Green Roofs have been replaced by other systems. 


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