Mineral Wool is a Reliable Horticultural Medium
Mineral wool-based green roofs support a wide range of green roof plants as part of a lightweight green roof system with high stormwater retention. Mineral wool has been used as a horticultural growing medium almost as long as it has been used as insulation. Companies such as Grodan specialize in production of mineral wool media for nursery, greenhouse, and crop production. The most common type of binder used in mineral wool, phenolic resin, also has a long history of horticultural use; phenolic resin is an inert plastic used to create floral foam, a ubiquitous green foam found in floral arrangements.
A living roof with both Sedum album and Bryum argenteum growing successfully
Mineral wool is commonly used in living wall applications, such as the Sage Living Wall and Sempergreen’s Flexipanel, both of which are used in interior and exterior applications.
Handreck and Black document the components of growing media as:
- mineral particles – the inorganic fraction,
- organic matter, the remains of living organisms,
- water, the ‘soil solution’ in which nutrients for plants are dissolved,
- air, which fills the space between solid particles not filled with water, and
- living organisms, smaller animals and microbes.
Once some organic matter is added to mineral wool, and the material is exposed to naturally abundant microbes, mineral wool meets all relevant criteria for growth media.
Mineral wool is highly root permeable and absorbs up to 94% of its volume in water.
When used as part of a green roof assembly, mineral wool is typically used as a water retention layer, water retention and drainage layer, or sometimes as the surface growth media. Appendix H of the report documents green roofs with no media or soil above the mineral wool, a configuration which has reliably supported vegetation for decades.
Water held in pore spaces between mineral fibers is readily available to plants. As plants absorb and evapotranspirate water, volumetric water content (VWC) drops to approximately 20%. Though short-term VWC is very high after a rain, thirsty drought-tolerant plants “stock up” on water and the air-to-water ratio returns to approximately 50%/50% quickly after a rain, as documented in Appendix A, and as evidenced by healthy vegetation observed in Appendix H of the report.
Pictures of a thriving living roof with a rock wool water retention layer displaying plant root penetration
Mineral wool can be very valuable on a green roof not only for stormwater retention, but for plant irrigation between rain events, increasing the viability of rooftop vegetation and possibly increasing the potential for a diverse plant palette even in a very thin profile. For example, as mineral wool’s VWC is typically between 20% and 50% when used in the eastern US, plants are supplied with abundant water to fuel growth, versus most aggregate green roof medias which only retain a maximum of 25% VWC, even when ASTM tests report higher VWC (Starry), and which rapidly drain below 10% VWC.
Broadleaf sedum growing in an EcoCline living roof