Mineral Wool on Green Roofs - Compaction and Toxicity?

by Brad Garner on Saturday, September 14, 2019

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What is mineral wool, and why is it used on green roofs?

Rock mineral wool is a super-absorbent, fibrous material that holds approximately 85-93% of its volume in water, which is the highest of any material known to be applicable for use in green roof assemblies.

Mineral wool can be thought of as cotton candy made of rock. The material consists of many small, thin fibers held together in some manner. You’re probably most familiar with mineral wool as an insulation material.

There are different types of mineral wool that differ in density as well as in methods of production.

Because of its absorbent properties, mineral wool increases green roof water retention and thus improves the functionality of the roof as a stormwater volume reduction tool.

Let’s take a closer look at this interesting and useful material and critically inspect the pros and cons.

Mineral wool and green roof water retention

The density of the mineral wool matters when it comes to retention. Higher density mineral wool generally retains water more efficiently than lower density mineral wool. This is because higher density mineral wool has a higher percentage of micropores and a lower percentage of macropores. Micropores hold water more tightly. There is just so much surface area for the water to cling onto in those small pores!

One great thing about mineral wool is that it increases drought resistance. This is one of the core research topics of the independent research institution Green Roof Diagnostics (GRD). Green Roof Diagnostics is documenting evapotranspiration rates and plant health during drought with and without mineral wool.

The use of mineral wool on green roofs is also believed to increase the water-holding capacity of the soil media, something that Green Roof Diagnostics is currently researching.

In addition, the company Evolvement Pty Ltd has conducted scientific tests with Australia’s premier agency Sydney Soil Laboratories (SESL). They tested a typical low soil growing media with added 20% of Urbanscape Flocks, a type of non-toxic rock mineral wool. The water holding capacity was increased to an additional 33%. This is, of course, very useful for vegetated roofs.

Robert Griffith (BEnvSc) from Evolvement Pty Ltd in Australia has grown a number of native succulents on green roofs with as little as 15mm of growing media on top of a 40mm a layer of mineral wool. The layer is providing all the water needed for the living roof and roots grow well into the mineral wool providing a well-rooted plant for wind roof locations.

GRD is focused on testing fully assembled green roof profiles for both horticultural and biological performance, and immediately measurable stormwater performance. These tests are much more representative than ASTM tests of individual components.

Irrespective if the testing starts from wet or dry, mineral wool profiles consistently retain more stormwater than non-mineral-wool profiles. Also, plant performance has generally been improved with mineral wool. These tests are performed after compacting profiles.

Green roof media / green roof soil begins to drain via preferential flow paths well before reaching saturation, and this causes the media to drain rapidly long before reaching saturation if it ever reaches saturation. Mineral wool absorbs water with very high efficiency and distributes water laterally, which disrupts preferential flow paths, and renders the entire profile more efficient at water absorption.

Mineral wool and stormwater detention

You have probably noticed that Purple-Roof is focusing a lot of energy on detention. How does mineral wool affect detention?

Mineral wool by itself provides almost no detention. This is because pore spaces in mineral wool are so small and tight that nearly all water held by mineral wool only escapes via evapotranspiration, i.e., mineral wool is a retention material. However, mineral wool is a key component of a detention-based green roof, as restricted drainage conditions can fill substantial macropores within mineral wool.

Macropores are pores that drain out via gravity. Without restricted drainage, this emptying is nearly instant. With restricted drainage, this emptying can take from several minutes to a few hours, creating detention.

What about mineral wool compression?

All mineral compresses to some degree. Normal foot traffic has demonstrated to compress high-density 128kg/m3 (8 lb/cf) mineral wool approximately 20% both short-and long-term. This renders the mineral wool higher density, with slightly higher absorption properties, though slightly lower volume.

Interestingly, normal foot traffic might even allow the mineral wool to expand beyond its original volume, which has been documented. There is data supporting an expansion of 150% (from 128kg/m3 (8 lb/cf)) over a time period of 30 years and in the absence of any foot traffic.

A green roof is exposed to weather and wind and many freeze-thaw cycles over a lifetime. It is possible that there is a slight rebound after these such freeze-thaw cycles that could explain the many observations of mineral wool expansion.

Mineral wool is highly durable and resistant to density changes. There is abundant data that nearly no amount of foot traffic compacts mineral wool beyond 192kg/m3 (12 lbs/cf) over 30 years!

It simply is an amazing material!

The dangers of mineral wool

However, mineral wool isn’t uncomplicated. All those tiny mineral fibers need to be held together somehow, and the traditional way to do that is via a binder. Unfortunately, a toxic chemical has been used in most mineral wool production; there were no good alternative binders in years past, but there are several alternatives today.

Since green roofs are exposed to the elements, we need a durable method to hold the fibers together. Starch-based binders are completely natural and safe, but break down in the weather, so they aren’t an option.

However, “mineral wool needling” is a good option for use in green roof systems, and we’ll discuss needling in a few paragraphs.


Phenol-formaldehyde (PF) is the toxic material traditionally used as a binder, which essentially “spot welds” the fibers together at the joints and create a durable and stable fabric. Thus, if you look at mineral wool through a microscope, you will see blotches of phenol-formaldehyde rather than a fine coating.

Phenol-formaldehyde is hydrophobic but only makes up about 3% of the total dry mass and provides major structural benefits. The mineral wool, fortunately, retains its natural hydrophilia despite the phenol-formaldehyde treatment creating a durable and extremely absorbent material suitable for green roofs that can last for decades.

Phenol-formaldehyde is a plastic that incorporates a carcinogen, formaldehyde. Phenol-formaldehyde also degrades extremely slowly, its leaching rates are unknown, and the particles that are ground off are often so small that they evade almost every filter.

Many reports and studies have been published over the past years underlining the severity of our plastic pollution issue with tiny plastic particles being found in even deep-sea fish and in much of the food we eat.

Also, phenol-formaldehyde produces hazardous working conditions in factories. Factories that use this chemical should be equipped with air quality devices to ensure breathable air for workers. Warehouses and distributors who handle phenol-formaldehyde mineral wool do not normally have these air purification measures, and workers there are exposed to hazardous off-gassing.

We don’t have firm evidence of other maladies of this product, but over the past 50 years of industrial and petroleum-product-related pollution, we should have learned to exercise caution, vs. proceeding unless there is firm evidence of wrongdoing. For these unknown reasons, we should avoid phenol-formaldehyde whenever possible.

Mineral wool needling – a non-toxic solution for use in the green roof industry!

Mineral wool needling avoids all that toxicity! The needling excludes the need for toxic phenol-formaldehyde. When mineral wool fibers are pressed together to form a mat, the fibers are generally directional, or substantially horizontal in nature. Needling involves punching the fibers with needles to stitch fibers together. Since this process is entirely mechanical, no chemicals are involved, and the end-product is 100% natural: fibrous rock. Needled mineral wool from the company Urbanscape has recently been declared free of redlist chemicals providing proof of material safety.

Full disclosure: I used to work at another green roof company that uses phenol-formaldehyde-bound mineral wool, and I was skeptical of needling. But like any good scientist, good data persuades me.

Since needling has been around for less than a decade, we do not have long-term performance data, but the short-term performance data is very promising. When Green Roof Diagnostics tests the material, all testing is done in fully assembled profiles, compacted to maximum density, and subjected to routine foot traffic.

Don’t hesitate to contact us if you have any questions or comments on this or related topics!

You can find our listed partners on our main landing page: www.purple-roof.com

Don’t miss our article on green roof inoculations: Are Green Roof Inoculations Worth It?

You might also be interested in our article: The Story of Purple-Roof & How to Generate an ROI with a Detention Roof!