What are the benefits of vertical planting?
Vegetation provides a multitude of environmental and social benefits including improving air quality, reducing pollutants in run-off, reducing noise, creating a habitat for wildlife and reducing flood risk (e.g. by retaining rainfall within soils and canopy). There are also well documented physical and psychological health benefits.
Vegetation can cool the air within and surrounding masonry by providing shade, evapo-transpiration (leaf respiration of cooled moist air) and also by reflecting some of the sun’s heat back. In winter, vegetation can reduce wind chill, protect masonry from rainfall and/or freeze/thaw effects and crucially, trap warmed air next to the building to save significant amounts of energy – and hence reduce energy bills!
How much energy could be saved in winter?
Recent research at the University of Reading found that the heat lost through un-vegetated brick was significantly greater than through bricks covered with foliage.
Figure 1: Thermal images of winter brick temperature with and without foliage
Vegetated walls (in the form of heated brick cuboids shown in figure 1) consumed a mean 21% less energy compared to un-vegetated walls during the first winter (mean energy 4.3 & 5.4 kWh per week respectively). In the second winter, when foliage was more extensive, a 37% mean saving was recorded (3.7 & 5.9 kWh per week).
Largest savings in energy were associated with more extreme weather, such as strong wind and/or rain, when energy efficiency was enhanced by up to 40-50%.
It is worth noting that loss of solar-gain (due to vegetation shading) had no effect on energy consumed (until early-spring); to the contrary, vegetated walls remained warmer even on days with notable sunshine hours in a UK winter. The type of planting – that is, foliage density and orientation – will influence efficacy, as will weather variables such as wind creating flux around a building, the influence of which depends on house location and construction as much as species planted. So, to maximise benefits, there needs to be careful design and species selection.
What about summer cooling?
Carefully designed vegetated walls can eliminate the need for mechanical air-conditioning, and have been found to keep wall temperatures below 24°C, which is the critical level required for human-health. Canopy cover, particularly on a south facing wall, will provide a dual-cooling effect (via evapo-transpiration and shade) which is species specific and so will vary with canopy density, plant architecture, number of leaves, leaf shape, and crucially, water availability.
Figure 2: Thermal image of brick wall covered with french-beans (Phaseolus vulgaris ‘Cobra ) in mid-summer: 7-13°C cooler than the co-located exposed brick.
How could summer benefits be maximised?
The foliage canopy structure is particularly important for summer cooling; although dense foliage provides shade, this will not optimise evapo-transpiration (et). Research suggests that once leaf cover to a wall is complete, additional foliage does little to enhance cooling, probably due to the higher proportion of leaves in shade (and reduced et). Consequently, species which can maximise both shade and et are best, for example, Roses, Honeysuckle, Clematis, and Jasmine. Leaf size is also important, since small and pinnate leaves increase shadow footprint per leaf.
Water is critical. Without it, vegetation will cease to evapo-transpire and wilt, leaving foliage gaps for solar radiation to heat the wall. So, an idealised plant might have small or pinnate leaves to optimise sunlight and et (e.g. south/west-facing), with sufficient soil moisture. The idealised plant should also possess growth that maximises both leaf/stem coverage to the wall as well as exposure to sun (i.e. leaves well distributed along stem lengths). Although for shady walls (e.g. north-facing) the trapping of cooled shaded air is preferred, as et will be limited, so dense canopy is the best option in this situation.
How could winter benefits be maximised?
Vegetation can both trap warmed air and also reduce convective heat loss through reduction in wind chill and protection from rain. Hence to maximise winter efficiency, the idealised green-wall species would provide maximum foliage density and volume – thus trapping warmed air within the canopy. In addition, the leaves should form a dense sward on short leaf-stem petioles, in order to avoid flux in high-wind conditions (allowing cooled air into the canopy). Reduction in convective heat loss is key to insulating existing housing e.g. with draught proofing. This suggests vegetation would be most effective either for buildings in exposed or northern parts of the UK, or where cavity insulation is not practical e.g. Victorian brick housing.
Other considerations in species selection for your site:
Clearly different plants will be suited to different orientations and building structures. The practical limitations include locating suitable planting areas at ground level, limits to growth height (generally 10 to 20 m), and aesthetics, including seasonal growth (for example, Honeysuckle may be desirable in summer but not in autumn).
There are numerous varieties of evergreen climber which can reduce maintenance e.g. Clematis, Jasmine, or for a strong canopy, varieties of climbing Bougainvillea or Wisteria. However, in sub-optimal conditions hardy species such as Ivy are useful. Ivy’s dense interlocking canopy is ideal for energy saving, but may cause concerns for masonry; alternatively, shrubby growth of species like Magnolia grandiflora also provide good canopy density without such concerns. Annual species may be used for temporary cover including food species if conditions allow e.g. runner-beans, but may be unreliable if plants are stressed (drought or poor soil). Even if ground-planting options are limited e.g. due to hard-paving, greening can still be achieved via containerised climbers, shrubs, and/or fruit cordons. In summary, the incorporation of site-specific varied planting is required to gain the best benefits!
R.W. Cameron, J. E. Taylor & M.R. Emmett (2013); What’s cool in the world of green facades? How species choice influences the cooling properties of green walls).
R.W. Cameron, J. E. Taylor & M.R. Emmett (2015); A Hedera Green Facade- energy performance and saving under different maritime temperate weather conditions.
Read the research
in full here.
Further information from:
Dr Jane Taylor CEnv: Green Wall and Urban Green Infrastructure Consultant. LinkedIn.