Checking the green walls

Ashraf Mir doubles as a driving instructor - Photo: Tomas van Dijk

They are credited for a milder city climate, cleaner air and improved insulation. But how well do green walls really perform? Ashraf Mir (MSc) graduated on the topic.

It’s a long road for a refugee from Afghanistan to ultimately become a TU graduate. Ashraf Mir (30) once earned his money knotting carpets in Pakistan, and later by giving driving lessons in Rotterdam. Last Friday however he graduated with an ‘8’ for his thesis on green facades.

The concept of green walls is not as new as it may seem. Think for instance of the famous Hanging Gardens of Babylon (600 BC). There is now however a re-emergence of vertical green in various forms that has spawned from various ecological benefits – true or just perceived.

Mir distinguishes three basic types of vertical green: green facades (planted in soil or boxes); wall vegetation (plants naturally growing on old walls), and living wall systems (growing from substrates in or on a wall, complete with irrigation systems).

Mir tested the thermal and hydrological properties of various forms of green walls in a so-called hotbox, developed by his predecessor Dr Marc Ottelé. In this closed compartment, temperature and vapour pressure across a piece of wall can be set at various values. Sensors provide data from which the isolation values and moisture transport can be derived.

Mir found that in the case of living wall systems in particular, condensation occurs between the masonry and the insulation material. Theoretically, the water that condenses in winter should evaporate during the summer. Nevertheless Mir advises to use a vapour barrier anyway.

Even more surprising is the outcome of the life cycle analysis for the various green walls. A brick wall scores best in terms of global warming potential, human toxicity and impact on freshwater reserves. All forms of green walls have a larger ecological impact.

So, should we just abandon the idea of vertical gardening? Mir thinks not. The advantages, like a beneficial microclimate, reduced inside temperatures via external shading and increased biodiversity, make it worthwhile “to apply green walls more in future, especially in dense urban areas”.

He recommends the use of materials like hardwood or polyethene (HDPE) as supporting systems for living wall systems. Until now, aluminium and stainless steel are commonly used, which amplifies the ecological footprint.

Mir still has two subjects to finish, but after that he will be looking for a job in a construction department, where he will hope to making concrete not just durable but also sustainable.

M.A. Mir, ‘Green facades and building structures’, 30 September 2011, Master thesis supervisor Professor Michiel Haas (CEG).