Best Practices for Usage of Mineral Wool in Green Roofs
Mineral wool can be a valuable component within a green roof assembly when used according to best practices. As is the case with most construction materials, context, application, and handling are important variables.
When selecting any green roof system, select a system whose components have been engineered to work together. In the case of mineral wool, select a green roof system that has been developed and tested to perform as intended with mineral wool as a component; simply adding mineral wool to a green roof profile without testing may lead to unexpected results. Manufacturers of mineral wool green roof systems consider how the mineral wool will interact with underlayment layers and surface layers to engineer appropriate air to water ratio, drainage, nutrient availability and wind resistance.
Although mineral wool is an excellent water retention and rooting media, mineral wool possesses negligible cation exchange capacity (CEC), a measure of the material’s ability to absorb nutrients and make them available to plants. Typically mineral wool should be utilized in conjunction with components that provide CEC or used with plants that require little nutrient. As plant roots penetrate the mineral wool, organic matter may accumulate within the fibers and provide increased CEC as the system matures, as shown uncovered mineral wool in Appendix H of the report.
The current US wind-resistance standard, ANSI/SPRI RP-14 is primarily written around aggregate-based green roof systems, as the standard is based primarily on ballast or dry weight of the media. Manufacturers of mineral wool based green roofs may have wind resistance ratings based on actual wind testing versus a calculated ballast rating. Specifiers should understand the manufacturer’s wind rating and specify accordingly.
Select plants that will thrive in a mineral wool green roof. Mineral wool provides greater water availability to plants than aggregate-based green roofs, which can be a blessing – potential for broader plant palette – or a curse – potential for high weed pressure. Low maintenance green roofs should be designed to provide a certain amount of “stress” to plants to minimize competition and slow species succession. Stress within aggregate-based green roofs is predominantly created via very dry conditions. Stress within European mineral wool green roofs has traditionally been created by utilizing an ultra-thin profile. EcoCline utilizes a harsh media in conjunction with mineral wool in order to suppress weeds and minimize species competition, even within deep mineral wool profiles designed to manage high volumes of stormwater. Mineral wool green roofs are typically around 1 to 3 inches thick, yet support Sedum and similar vegetation that would otherwise require approximately 4 inches of media in a comparable climate zone. As system thickness increases, without some other constraint, higher maintenance plants can easily outcompete Sedums in surprisingly thin profiles.
A variety of sedum species coexisting on a living roof
Use a mineral wool with a density of at least 8 pcf, preferably 12 pcf if the roof will be subjected to much impact. Ensure that the mineral wool utilizes a binder that has demonstrated stability, or material that has proven stability without use of a binder. Binders technology is likely to change and improve over the next few years.
As with any green roof, avoid vegetating areas subject to frequent foot traffic; frequent may be defined as more often than once weekly. Design pedestrian access routes and pedestrian spaces such that the weight of the pedestrian is not transferred to the mineral wool in order to provide stable, level pedestrian surfaces and prevent unnecessary compression of the mineral wool. Mineral wool has adequate compressive strength to support the occasional maintenance worker, but not enough to support frequent pedestrian impact or to level pavers.
When installing, wear protective clothing, similar to insulation installers. Mineral wool may cause itching, which is often best managed by wearing long sleeves and showering soon after handling. Mineral wool is lightweight and easily handled dry, but is difficult to handle when wet, so keep it dry until it is in place. Once installed, minimize construction impact over the installed mineral wool, and cover as soon as practical. Ballast the same day as installation.