Mineral Wool as a Stormwater Management Component in Green Roofs
Naturally hydrophilic, mineral wool predictably and efficiently retains water.
Mineral wool typically possesses a maximum volumetric water capacity (VWC) of 80-94% when tested per ASTM E-2397. When monitored in field conditions, actual water retention reliably peaks to over 70% VWC, and has been field-documented to retain up to 94% VWC, an extremely high retention efficiency for green roofs.
Research conducted at the University of Maryland in 2012-2014 documented the VWC of all major brands of lightweight aggregate (LWA) green roof media common in the mid-Atlantic region. Research by Starry (University of Maryland, 2014) included testing of VWC of multiple commercially available green roof medias in the mid-Atlantic region, including the top selling brands in the mid-Atlantic, and including some medias that advertise VWC as high as 65% when tested per ASTM E-2399. No commercially available green roof media tested demonstrated VWC above 25%. This is likely due to the fact that green roof aggregate medias are designed to drain very rapidly in the field, but ASTM E-2399 tests the media’s ability to retain water after being fully saturated, a condition that is highly unlikely to occur on most green roofs.
Conversely, mineral wool that has been tested to retain 94% VWC per ASTM E-2397 (the corollary to E-2399 for fibrous materials) has been documented to retain up to 94% VWC in the field. Mineral wool’s extremely high efficiency is likely due to the fact that mineral wool rapidly absorbs rainwater, and generally only drains once the material approaches saturation, versus lightweight aggregate, which absorbs water more slowly and drains very rapidly.
Volumetric water content (orange line) and rain events (vertical blue bars) for July – September 2014. Left axis is VWC expressed as percent of total volume. Right axis is total daily inches of rainfall. Horizontal axis is time. See the full report for more details
| Material | VWC per ASTM E-2397-9 | VWC as Field-Verified | Field-Verified VWC / ASTM E-2397-9 VWC |
|---|---|---|---|
| LWA | 35% | 20% | 57% |
| LWA | 45% | 25% | 55% |
| LWA | 65% | 25% | 38% |
| MW | 94% | 85% | 90% |
| MW | 94% | 94% | 100% |
Total daily rainfall (inches) and green roof system volumetric water content (%) of a 20,000 square foot green roof in Washington, DC (VWC n = 10 and rainfall n = 1). See the full report for more details
Appendix A of the report describes monitoring of a 20,000 SF installation of EcoCline in Washington, DC, utilizing a 2-inch layer of mineral wool. Monitoring, performed in conjunction with the University of Maryland, reveals that mineral wool very effectively both captures and releases stormwater. Rainfall fills the mineral wool to approximately 70-80% VWC, then VWC quickly drops to approximately 30-50% over a few days, then continues to draw down to approximately 20% over the course of a week. Nearly all reduction of VWC can be attributed to evaporation and evapotranspiration, as visual monitoring of drains reveals negligible flow of gravitational water after rain events. Drawdown of VWC occurs during all seasons of the year.
The project monitored in Appendix A of the report included ten sensors, some of which measured VWC as high as 94%. Data presented averages all ten sensors together, including sensors at the top and bottom of profile, and sensors at high-points and low-points of the roof.
The vertical axis represents volumetric water capacity (VWC) expressed as percentage of water of the total volume. The horizontal axis is wet/dry cycling trials described in the full report. Note that the two Bound samples (Samples 1 and 2) average approximately 85% VWC, which is consistent with actual field data illustrated in the full report
Rewettability
Furbish simulated 20 years of saturation/desiccation cycles, simulating worst-case rewettability scenarios of full desiccation of the mineral wool annually, per Appendix B of the report. These tests, performed on mineral wool bound with phenolic resin, indicate no significant loss in material rewettability resultant of wet/dry cycling. Various informal field measurements of mineral wool taken from 2012 to 2015, and various European applications sampled in 2015 support this data, as noted in Appendix H of the report.
Field monitoring as described in Appendix A demonstrates precisely measured, reliable rewettability of mineral wool over the course of one year; further Appendix A indicates that volumetric water content does not routinely drop below 15 or 20% or approach desiccation. An antecedent moisture content (AMC) of 15-20% provides significant water to plants between rain events, and is high enough to ensure a very high degree of absorption during the following rain event, preserving maximum hydrophilia, as AMC is an accurate predictor of a green roof’s capacity to retain the next rainfall. Figure 3 illustrates mineral wool’s highly efficient retention characteristics.
A contrast between a typical living roof weight and an EcoCline living roof weight saturated with equivalent volumes of water
“Our desire is to develop systems that perform as efficiently as nature would on its own.”
~ Michael Furbish
Sedum floriferum flowering on a living roof
The microscopic fibers within a section of 14 pcf rock wool
