Increasingly, North American stormwater regulations require rainfall to be managed within the property lines of any given development. In the urban setting, where virtually the entire site is consumed with structure, the rooftop is often the only available place to manage stormwater. Therefore, maximizing retention capacity of green roofs is paramount and relying on aggregate media to perform that function is – to be blunt – inefficient. Water retention within the green roof profile can be greatly enhanced with alternative materials that complement aggregate media. The aim of this paper is to provide compelling evidence that validates mineral wool in green roofs utilizing case studies and test protocols designed to simulate green roof conditions. This paper also includes some guidelines for best practices in the use of mineral wool in green roofs.
Mineral wool is a renewable resource with qualities that are highly desirable in green roofs, including high water retention, low weight, durability, dimensional stability, and excellent horticultural properties.
Mineral wool has been successfully used in German green roofs for the past three decades and continues to be used in green roofs today throughout Europe and China. Despite mineral wool’s long and successful history overseas, it remains underutilized in North America. Many green roof specifiers are unfamiliar with its use or cautious about its perceived novelty. Naturally, the change to emergent technologies has the potential to upset established market forces and trigger fear, uncertainty, and doubt. With deeper appreciation of its history and use, these concerns should shift to comfort.
Here are the white paper conclusions about mineral wool used in green roofs:
Absorbent
Mineral wool is a high efficiency stormwater retention component.
Lightweight
Mineral wool has a very low dry weight, allowing the green roof assembly to be lightweight.
Horticultural
Mineral wool is an excellent horticultural medium in green roof applications.
Stable
Mineral wool is dimensionally stable in densities as low as 8pcf, optimally 10-12 pcf.
Mineral wool retains material integrity for at least 30 years in exterior applications, likely far longer. Use of a phenolic resin binder is likely to improve dimensional stability.
Durable
Mineral wool tolerates the level of foot traffic that can be expected in green roof applications, exhibiting long-term resiliency to short-term cyclic compression.
Clean
Mineral wool is chemically stable when unbound or bound with phenolic resin. Runoff from mineral wool exceeds the EPA’s standards for drinking water.
Renewable
Mineral wool is a renewable material, utilizing dolomite or basalt – some of the few renewable rocks, and/or slag – a waste stream product.
