Is Phenol-Formaldehyde right for the Green Roof?

by Anna Zakrisson on Friday, January 10, 2020

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Perhaps Phenol-Formaldehyde Mineral Wool Doesn’t Belong on a wet Green Roof

Phenol-Formaldehyde on green roofs might be an environmental concern.

Mineral wool is an excellent material for increased retention capacity and the resilience of extensive vegetated roofs.

Mineral wool can hold a whopping 85-93% of its volume in water, which is the property that increases green roof retention significantly. Mineral wool, with its water-holding ability combined with a living soil layer for plant health, creates an excellent setup for resilient green roofs.
However, under wet-dry conditions, most of us should hesitate to utilize mineral wool that uses phenol-formaldehyde-based binders, the binder being the glue that holds the fibers in place.

This hesitation has reasonable grounds as phenol-formaldehyde is a known carcinogen and has been shown to have negative environmental impacts.

It should also be noted that there is mineral wool on the market that is free of phenol-formaldehyde. Needled mineral wool uses needles to stitch together the mineral wool fibers and has thus no need for toxic binders. It is also readily recycled by, for example, mixing it directly with soil to be used in landscaping with no treatment needed. This type of needled mineral wool has been Declared (Declare label) Red List Free (sold by Urbanscape/Knauf).

There is also mineral wool with starch-based binders, but there have been some concerns over the durability of these products as an organic starch-based binder may or may not hold up in wet-dry conditions on the green roof.

Read more about mineral wool on green roofs in our article:
Mineral Wool on Green Roofs - Compaction and Toxicity? By Brad Garner

Phenol-formaldehyde leeching and biodegradability

There is very little data available on how much of the phenol-formaldehyde resin that ends up in the environment, especially under fluctuating hot-cold and wet-dry cycle conditions. Green roofs with phenol-formaldehyde have been especially poorly studied.
Due to the rather low concentration of phenol-formaldehyde in Rockwool (3-4%) and its high stability, it has been argued that leaching is not an issue.

However, there are a few crucial aspects that should be considered. One is that the green roof is an extreme environment — the green roof mineral wool experiences substantial temperature fluctuations. We also know from published studies that extreme temperatures and other external conditions affect the breakdown of phenol-formaldehyde significantly1,2.

Thus, before we have reliable data available, it might be wise to treat the product with caution.

Furthermore, as we aim to green our cities substantially through millions of square meters of green roof areas, maybe, we should ask ourselves if it is wise to let a considerable portion of the urban precipitation to be filtered through phenol-formaldehyde mineral wool?
With large areas, even small concentrations begin to matter.

Besides, the incorporation of the phenol-formaldehyde plastic resin may make recycling challenging compared with that of needled mineral wool. Imagine the trouble and complexity if we need to remove the phenol-based product from the roof when plants are fully rooted in the product.

Unlike needled mineral wool where the complete green roof can be ground up, reused on the roof 100% or recycled into the landscape, this phenol-based product cannot be recycled straight into the landscape and millions of square feet, 5-10cm (2-4”) thick need to be put into a landfill instead. Just imagine the pile of green roof garbage this amounts to!

We cannot pass on this problem to future generations. There are alternatives, so let’s use them!

Mineral wool is a fantastic material that makes a green roof more resilient and, thus, cools our cities more efficiently. It aids in the reduction of heat island effects and pollution mitigation as it improves the functionality of the living roof.
Let’s use Red List Free mineral wool and skip the phenol-formaldehyde. It’s hazardous and unnecessary!


1. Horadam, W. et al. Leaching studies on Novolac resin-coated proppants-performance, stability, product safety, and environmental health considerations. J. Appl. Polym. Sci. 135, 1–12 (2018).
2. Yu, Y., Xu, P., Chang, M. & Chang, J. Aging properties of phenol-formaldehyde resin modified by bio-oil using UV weathering. Polymers (Basel). 10, (2018).

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