by David Fuller
This time, we investigate the performance of the façade greening systems mentioned in previous parts, with reference to a range of recent research projects.
Green façades like any applied construction material or cladding offer a degree of solar shade and improved thermal resistance. Such applications create their own microclimate through absorbing solar radiation and the cooling effect of leaf evapotranspiration (evaporation of intercepted rain water on leaves and the transpiration of water taken up by plant roots within leaves) – this contributes to the cooling of buildings and reductions in the urban ‘heat island’. For indirect greening, Perini et al (Building and environment 6, 2011) state that the air cavity of a green façade can affect wind velocity across the envelope, creating a buffer, which improves thermal performance a fraction.
Tilley et al (2012 in Maryland) conducted a trial test using four identical buildings incorporating green façades grown on trellis structures. A 4°C reduction in internal temperature during the summer months was recorded. Two of these cubic buildings (2.5 m square x 3.5 m high) were greened on the East, South and West façades the other two were left un-greened as a control group.
Wong (University of Singapore 2010) tested eight variants of vertical planter systems in Hort Park, Singapore. Various forms of trellises, modular panels, planters and layered mats, etc. were used and surface and substrate temperatures recorded. The best of these achieved an 11°C temperature difference between the surface of the system tested and the control wall.
These tests have shown that façade greening offers reductions in solar gain on façades and glazing. Living wall systems with their greater mass and ability to hold moisture perform better. A table in ‘Materials architecture design environment’ (MADE) from the Welsh School of Architecture highlights the various research results, including those above. From this it can be concluded that tropical or arid climates benefit greatly from surface temperature decreases afforded by façade greening in all its configurations with a resultant reduction in air conditioning loads.
According to the University of Sheffield (Stuart Archer – based on theoretical studies) there is a small benefit to be gained from installing greened façades on well insulated buildings in temperate climates (where heating demand is more of a concern). Poorly insulated buildings may benefit more but whether this approach is cost effective compared to other methods of upgrading or retrofitting is less certain. Reductions in solar gains in more northerly locations must be weighed up against any beneficial gains, particularly during the winter – perhaps a deciduous approach may work in cooler climates, presumably with a downside on the visual and maintenance aspect (i.e. leaf litter, bare stems and branches).
As part of the Hort Park tests described above, Wong also tested acoustic performance of the different wall build-ups by assessing sound reduction. The results indicate a 5 to 10 dB reduction for the low to middle frequency ranges for a number of the tested planter systems. Wong also tested a green wall panel internally in a reverberation chamber; his findings suggest that use of green walls internally can provide a higher sound absorption coefficient, than many other materials, a characteristic useful for speech privacy.
Further to this, Azkorra et al (2014) have tested pre-cultivated/ vegetated polyethylene modular units lined with natural material and planted with Helichrysum thianschanicum (Curry Plant). The test standards used were UNE-EN ISO 10140-2 (sound insulation) for 10 units at 2.42 sqm and UNE-EN ISO 354 (sound absorption) for 42 units at 10.08 sqm in a reverberation chamber.
For sound reduction, the weighted sound reduction (Rw) was calculated from measured R values using UNE-EN ISO 717-1 to give a figure of 15 dB. The authors indicate that the performance is poorer than many other construction materials but that better values could be achieved using acoustic isolation detailing and panel joint sealing. The measured reverberation times approximated the findings of Wong; the calculated Sound absorption coefficient (α) being 0.40. The authors indicate that green walls (of the type tested) are very effective at decreasing reverberation time and concludes that the design and selection of materials for the system used can affect the reverberation times across the measured frequencies; therefore, these need to be carefully considered, particularly if acoustic performance is a design factor.
It would appear from the above that green walls are poor in terms of sound absorption but very good for reducing reverberation times. The most obvious application for this trait would be in spaces where speech definition is required or multiple sound sources are conflicting – restaurants, offices and meeting spaces would be good examples. To this end, interior wall systems are available that include self-contained relocatable partition modules with planting either side.
Air quality benefits
Façade greening in urban settings can reduce pollution levels, with vegetation filtering airborne particulates including nitrogen dioxide, sulphur dioxide and ozone. An article by Lancaster Environment Centre suggests a reduction of 40 % and 60 % of various particulates, giving better air quality. This supports the development of ‘street canyons’, ‘filtered avenues’ and green ‘billboards’ planted with vegetation.
Clearly, individual buildings planted with this material may benefit also from such improvements and systems are being developed where intake air for air handling and cooling is filtered through a vegetated layer.
Life cycle benefits
Façade greening and green roofs are not yet considered as an ‘environmental quality restoration and energy-saving method’ (see article ‘A life cycle assessment and comparison: Sustainability impacts of five vertical greening systems; the full environment impact of five types of vertical greenery’ in Citygreen, 2013). There is a paucity of information to quantify environmental and economic credentials.
A 2014 conference paper presented by Nalanie Mithraratne (University of Singapore) titled ‘Hanging gardens in cities: Are they really beneficial as an add-on?’ compares the various benefits afforded by incorporating façade greening in buildings. The paper looks at savings in cooling loads, reductions in running costs along with biophilia and attractiveness benefits, afforded by a proprietary living wall and planters in an atrium, to users leasing an office in Singapore. Measurements indicated that initial installation coupled with ongoing running and maintenance costs outweighed any direct energy savings (e.g. through evaporative cooling) or benefits to the environment. Mithraratne states that whilst clearly offering a more calming and beneficial environment, these can be intangible and there are no measures currently for these subjective benefits.
The University of Tehran (see American journal of civil engineering and architecture, 2013) has conducted a life cycle assessment of the façade greening types described above covering environmental impacts of raw material depletion (including soil and support structures), fabrication, transportation, installation, operation, maintenance and waste. The assessment showed that most of the systems tested had a major impact. The worst was the felt system which had an environmental burden of twice the other systems – this was due to the greater frequency in which the felt layers needed replacing (up to 3.5 years) and the eco-toxicity of fresh water (output from system) used which was five times worse than that of other systems. The indirect system, which commonly uses stainless steel, also had a high impact. The direct greening system faired the best in all climates – impacting little on the overall environmental burden. The study suggests that a greater integration with the building façade, including omitting outer cladding layers and using the greening for this weathering function would improve sustainability further. With regards energy consumption in hotter climates the direct greening and living wall system proved nearly sustainable – the felt system proved an environmental burden. For temperate climates the environmental burden of systems exceeded any savings on heating, with the exception of direct greening, which proved sustainable.
The foregoing evidence indicates that there are measurable benefits to façade greening but the analysis and choice of materials used is critical for the most sustainable outcome. It is perhaps an area worthy of study to estimate what value building developers/ tenants would be prepared to put on such greening measures – particularly the less tangible and difficult to measure benefits, i.e. longevity of tenancy, greater well-being and productivity of staff.
Tilley et al have conducted a test (see thermal benefits above) of vine covered façades growing from ground level planters onto trellis work on four purpose built test buildings (each with a floor area of. 6.25 m²) in Clarksville, Maryland. The measured mean water use was 1.3 L/ m²/ day. Comparative data from manufacturers for modular planting systems, along with additional data from installations in the UK appear to parallel this usage level. Fernández‐Cañero et al (2011) suggest that an indoor vegetated mat wall has a water consumption of 3 - 5 litres/ m²/ day.
The Mark Lawrence blog gives details of annual running costs for a living wall system in Edgware Road, London. The wall is around 200 m² and suggested costs are £1/ m²/ year (dated 2013) for water, power, drainage etc., this figure excludes maintenance and other management requirements.
To promote biodiversity, broadening the variety of plants will attract a more diverse array of insects and birds.
An article by AECOM (The journal of the Landscape Institute – Summer 2013) suggests that there is a clash between keeping a striking green appearance throughout the year with the use of evergreen species and the ability to improve biodiversity; colonization and succession by native species and seasonality are desirable but this will result in brown patches and dead material. An ecologist on a project will be able to advise on selecting species that strike a balance between the need for seasonal variation to attract wildlife, and year round attractiveness.
Certain species, such as Ivy, will have a tendency to dominate slower growing competing species, therefore creating a mono-culture. Publication ‘Designing for biodiversity’ gives guidance on creating the right conditions for attracting wildlife onto buildings – an overview of the large array of planning and other legislation that affects wildlife is provided along with a programme of when, on a project, biodiversity needs to be considered.
UV light can degrade the physical characteristics of coatings, paints and plastics on façade materials. A façade covered with plant material will benefit in terms of extended durability and resultant maintenance costs.
Green walls all require maintenance to foster the best out of the planted material. Maintenance is required to manage growth that is intruding into building services or fabric and to keep the planted material in good health and looking its best. With some plants regular pruning is advised and deadheading of flowers can encourage regrowth. According to Ottele (2011) maintenance of felt systems will require periodic checking of the layers, replacement of dead plants and checking of the irrigation system.
Some manufacturers recommend two primary maintenance visits a year; these will include a calibration of the irrigation system.
The Department for Communities and Local Government (DCLG) carries guidance of fire performance of green roofs and walls in the UK on the gov.uk website. It is suggested that whilst growing medium, with the exception of organic material, is unlikely to contribute to flame spread, HDPE plastics used on modular living walls systems are capable of igniting.
The DCLG document ‘Fire performance of green roofs and walls’ states that there has been ‘no significant fire testing of green wall systems’. It suggests for fire prevention that considerations are made for:
- Increasing the non-combustible content of the growing medium
- Decreasing the organic content of the growing medium
- Preventing the system from drying out
For fire prevention the document suggests that use of grasses and mosses is avoided and plants with high moisture/ low resin content be used. A recommendation for fire breaks, supporting standards and a summary of compliance requirements to the (English) Building Regulations is offered.
Failure risk of façade greening
George Irwin, a green wall specialist writer and system patent holder, indicates in an article (2015) that there are two primary reasons for failure of installed living walls, these are:
- Material breakdown – degradation of substrate, commonest in felt and fabric walls but has occurred with plastics based systems, where the connection to the façade has failed.
- Plant failure due to:
- Under or over watering – caused by poor design and management of the irrigation system.
- Poor drainage of substrate.
- Poor nutrient supply through incorrect specification or application.
- Plant species not suitable for the site location, aspect and micro-climatic conditions.
- Exposure – plants on façades during winter are elevated, unlike other plants conventionally
The physical performance of a façade should be taken in its entirety including any applied façade greening, and its functionality optimized. Papers call for greater use of façade greening and its significant benefits but such gains must be weighed up as a contribution to the overall façade element, not in isolation. This, of course, may apply to a retrofit situation of green over-cladding but as yet little is written on this subject.
Façade greening systems are primarily designed for aesthetic improvement; it is unlikely that more holistic tests have been undertaken by manufacturers on other contributions to the façade’s performance. Where there are benefits, other materials will often provide these better, for example:
- If enhanced thermal performance is required, add more insulation as this will be more cost effective.
- If weathering of the façade is a concern, robust materials and good detailing is surely the first port of call.
Planting can offer some limited physical performance enhancement in the UK but the primary benefit is the intangible human one that relates to ecology and man’s psychological connection with nature. However, there have been notable failures, do clients really understand their commitments to the specialist maintenance and irrigation to prevent what is in essence a living ecosystem, becoming a vulnerable façade.
London’s first living wall, the Paradise Park Children’s Centre in Islington was subject to this susceptibility. A brief telecon by the author of this article with the manager of the centre has indicated that the planting on the façade, which died, has been removed and replaced with planting which is now rooted in the ground, albeit grown up onto the original mesh support panels. It would appear, at least from this example, that the idea of vertical planted walls is both an exciting and useful architectural solution but that the current means to achieving it carries a degree of risk to the building owner. Irwin states that:
“The success of such walls are the result of a well-rounded research, design, training, installation, and maintenance program. Utilizing the right plants in the right setting for the right system as well as understanding the science behind the technology are key to your green wall's longevity”.