High Density Mineral Wool is Dimensionally Stable
Green Roof Durability
Furbish has performed several tests on the durability of mineral wool as part of lightweight green roof systems, per Appendices D and E of the report. Though these are not exhaustive tests designed to determine the ultimate strength of mineral wool or document exhaustive product characteristics of mineral wool, these tests present results consistent with research by Steponaitis and Vejelis. Mineral wool is a valuable component for green roof durability.
The basic material of mineral wool is highly durable rock, typically basalt, dolomite, or slag. The rock is blown into fibers that are compressed to a given density. Mineral fibers are compressed tightly in bats or boards that are easily handled while maintaining an open fiber structure. Mineral wool is typically used within some containment (in green roof applications, usually perimeter containment and below at least one other layer of material), and as such separation of fibers is not a significant threat to material integrity. Mineral wool examined in exposed and covered applications, after two to three decades of exposure to the elements, show a high degree of fiber structure integrity, similar to new material.
Mineral wool has a high degree of dimensional stability when manufactured at a high density with an appropriate binder and appropriate installation handling.
A shot demonstrating green roof stability throughout the entirety of the roof
Narrowly focusing on physical performance of mineral wool under stresses common in green roofs, and understanding that empirical results will yield the most useful information, Furbish focused on physical testing and case studies, documenting variables in density and binder, versus detailed documentation of fiber structure or production techniques. Interestingly, Steponaitis and Vejelis note that “compressive stress decrease[s] with increasing thickness of mineral wool boards” and “It can be assumed that deformation in mineral wool products is distributed unevenly. In weaker layers deformation is very high, and in stronger layers the deformation is very small.” Though claims of increased thickness have not been included in Furbish’s tests to date, there is a possibility that thicker installations of mineral wool will be even more resilient than thinner installations.
Furbish researched and tested densities of 2 pcf (pounds per cubic foot), 4 pcf, 8 pcf, 12 pcf, and 14 pcf. The most common density used in European green roofs is 8 pcf, as lower densities exhibit noticeable compression under light foot traffic. Laboratory testing and documentation of two- and three-decade-old roofs in Germany reveal that 8 pcf mineral wool bound with phenolic resin is highly resistant to compression in the absence of foot traffic, and subject to 15% to 35% compression when subjected to high levels of foot traffic. Laboratory and field tests of 8 pcf mineral wool produced with no chemical binder reveal that 8 pcf mineral wool compresses by approximately 25%, but with some noticeable rebound.
Appendix H of the report, Table H.1 summarizes observed density in several case studies. Furbish is actively monitoring thickness/density in several installations not listed, and all those installations are performing similarly. In all projects summarized, the initial density of mineral wool was nominally 7.9-8.0 pcf. In older projects not subject to much foot traffic, expansion of the material was observed in places, yielding effective densities as low as 5 pcf. In projects subjected to low or moderate foot traffic, effective densities remained approximately 8 pcf, or as high as 10.5 pcf. In areas subjected to very high foot traffic, effective densities reached as high as 12.2 pcf. Long-term project monitoring illustrates that 8 pcf is an acceptable density for most green roof applications, but that 12 pcf is a more appropriate density to reliably guarantee volumetric retention and thus stormwater retention. Dimensional changes observed in case studies are typically fractions of an inch, and all test data indicates that full stability is reached when using material in the range of 12 pcf.
In advance of industry-standard test protocols to accurately measure and predict the dimensional stability of mineral wool in exterior applications, Furbish designed and performed tests to determine mineral wool’s resistance to compression over the expected life of a green roof. The predominant threats to mineral wool’s dimensional stability are media weight and pedestrian impact.
Long-term weight of media was not tested due to the following observations: Mineral wool is primarily used to augment the water retention capacity of thin-profile, extensive green roofs, therefore it would be used in conjunction with media of 0 inches to approximately 4 inches thick, which would typically weigh no more than 30 lbs/sf. After manufacture, before shipping to jobsites, high density (8 pcf or higher) mineral wool is stacked and palletized such that the bottom layers of material may be subjected to several weeks or months of several hundred lbs/sf, with no measurable compression. Further, two- and three-decade-old German green roofs that have areas with as much as 2 inches of media exhibit the same compression as areas with 0 inches of media. Further, pedestrian impacts present far greater force applied to mineral wool at approximately over 800 psf. Therefore, additional testing focused on resistance to compression under the highest applied forces: pedestrian impact.
Tests were performed to simulate 37.5 and 45 years, which roughly correspond with the expected service life of a commercial roofing membrane under a green roof. Tests indicate that the anticipated compression rate is approximately 10% over 40 years, when using a 12 pcf or 14 pcf mineral wool bound with phenolic resin. A compression rate of 10% is likely very conservative due to the following observations:
- Two- and three-decade-old 8 pcf mineral wool, installations stabilized at effective densities of as low as 5 pcf, indicating that in the absence of continuous pressure, mineral wool may expand slightly.
- Two- and three-decade-old 8 pcf mineral wool, installations stabilized at effective densities not typically higher than 12 pcf, indicating that initial densities of 12 pcf may not be subject to any long-term compression.
- Tests were performed with compressive forces applied in rapid succession, without allowing the material time to rebound. Mineral wool has well documented elasticity, and thus would likely rebound between compressive forces over extended periods of time, versus the concentrated forces applied during testing.
The following laboratory compression tests were performed:
- Simulation of 45 years of actual foot traffic by workers, per Appendix D, and
- Simulation of 37.5 years of foot traffic, using calibrated compression device, per Appendix E of the report.
Testing mineral wool is fun! After conducting an equivalent of 30-years of compression resistance, our team incorporated dancing into the lab testing.
Similar Product Applications
Green roofs are not the only exterior applications that utilize mineral wool, as some manufacturers are offering high-density sub-grade mineral wool products. Roxul’s DRAINBOARD® is available in 8 pcf and 11 pcf densities and has been used in Denmark for 35 years, successfully intact, and virtually unaffected by compression.
Inert phenolic resin binders safely improve the properties of mineral wool.
Most mineral wool is manufactured with a binder that holds the fibers together and greatly improves dimensional stability as noted by Steponaitis and Vejelis, and also by Gardziella, Pilato, and Knop, and as supported by tests in Appendices D and E and observations in Appendix H.
The most common binder, phenolic resin, technically known as phenol formaldehyde (PF), is a product that is completely safe for use in green roof applications. See Appendix F for water quality tests.
Runoff from PF-bound mineral wool exceeds the EPA’s standards for safe drinking water.
Phenol, the primary component of PF, is used in cough drops, throat losenges, mouthwashes, and pharmaceuticals. Though formaldehyde is a component used to manufacture PF, the finished product of PF has negligible free formaldehyde, as formaldehyde binds tightly with phenol to form a new compound. The tightly bound formaldehyde in PF is no more available than toxic chlorine is available in table salt (sodium chloride, NaCl).
PF is a stable, inert, non-toxic compound present in many common household items and construction materials, including floral foams, plywood, laminated veneer lumber (LVL), glulam, fiberglas and mineral wool insulation, spray insulation, plastic toys and figurines, laboratory countertops, billiard balls, dishware such as Bakelite, firefighter protective gear, and gaskets for furnaces and ovens. PF is a highly durable, waterproof, inert industrial plastic with excellent chemical- and flame-resistant properties that make it an invaluable component of so many useful products. PF has a long track record of resistance to the elements. PF is used extensively in engineered lumber, particularly for exterior applications due to the materials excellent resistance to water, wetting and drying cycles, temperature extremes, and biological degradation.
Binders are typically sprayed onto mineral wool fibers and perform like spot welds at fiber intersections, rather than coating fibers uniformly. In order to be sprayable, PF is diluted with water and urea into a sprayable form. The diluted (extended) product is then known as “urea-extended phenol formaldehyde” (UEPF). UEPF is by far the most common binder used in mineral wool, particularly in Europe when green roofs first began to use mineral wool.
UEPF is a stable diluted form of PF. UEPF retains the same properties of the parent PF when appropriate urea concentrations are incorporated; however, if urea concentrations are too high a loss of strength can occur, resulting in loss of compressive strength and dimensional stability.
In addition to improving the sprayability of PF, the introduction of urea to PF greatly helps to reduce formaldehyde emissions at the manufacturing plant, per Pilato “depending upon the P/F ratio, the reaction mixture upon condensation contains 5-15% unreacted formaldehyde,” therefore scavengers, such as urea, are added to the P/F mixture, to minimize the free formaldehyde level.
The Rockwool Group, the makers of Roxul insulation, note: “The [binder] compounds are used in a fixed or ‘cured’ form, so they are not emitted from the product. Studies show there is no appreciable increase in the levels of formaldehyde in buildings where Rockwool insulation is used and thus does not represent a risk to the health or well being of occupiers nor has it any negative impact on the indoor climate. Tests confirm that Rockwool products meet the most rigorous standards in Europe classifying the release of formaldehyde.”
UEPF meets strict European Union standards for environmental quality in green roofs. LEED v2.2’s EQ Credit 4.4 allows PF, commonly used in interior applications such as hard-surface countertops, and melamine formaldehyde (MF), commonly used in plastic laminates, as both MF and PF are highly stable with negligible free formaldehyde. Conversely, as formaldehyde is less tightly bound in the less stable UF, LEED’s EQ Credit 4.4 precludes the use of UF.
Mineral wool’s compression resistance and long-term durability is determined principally by density, and secondarily by binder. As UEPF is less rigid than undiluted PF, mineral wool bound with UEPF with a urea concentration of approximately 1-3% per mass has exhibited continued elasticity and resistance to compression over three decades of use, particularly at a density of at least 8 pcf.
A variety of binders other than UEFP are used in the mineral wool industry, including sodium silicates, polyesters, melamine urea formaldehyde, polyamides, and furane-based resins. Key differences between PF and urea formaldehyde (UF) are worth noting. UF is commonly used as a coating for slow-release fertilizers. UF is easily decomposed within the temperature range of 70-90 degrees Fahrenheit by microbes common in most soils, such as Ochrobactrum, rendering UF an unstable binder to use in conjunction with mineral wool in exterior roof applications that demand long-term dimensional stability. UF is also commonly used in several engineered wood products that are not typically exposed to moisture, such as particle board. UF is relatively unstable and is subject to offgassing of free formaldehyde. Due to offgassing concerns, LEED and other environmental rating systems and regulations are actively working to remove UF from interior environments. UF is not recommended as a binder for use in mineral wool in green roofs.
Several companies are actively using or investigating the use of non-petroleum-based and formaldehyde-free binders. At the time of publication, the authors are unaware of availability of binders other than PF that have demonstrated stability in the exterior environment; however, future technologies will likely produce viable alternatives to PF.
A broadleaf sedum plug laid out to be planted on a living roof
Appendix A of the report includes monitoring data for one installed project using UEPF-bound mineral wool, documenting the material’s excellent retention and rewettability.
Appendix G of the report compares 40x magnified images of UEPF-bound mineral wool fibers of new material with fibers exposed to 3 years of weathering; no discernible differences are present.
Appendix H of the report documents two- and three-decade-old mineral wool in green roof applications, which has physical characteristics nearly identical to new material.
Appendix F of the report documents water samples collected from a 3-year old green roof sample using mineral wool bound with UEPF. The water collected exceeded the EPA’s safe drinking level for formaldehyde by 200 times and for phenol by 12 times.