Wall shrubs and climbers can provide "significant passive cooling" with wall temperatures behind the plant canopies up to 10 degsC cooler, HTA research has found.
Experiments by HTA-funded PhD student Jane Taylor, led by University of Sheffield senior lecturer in landscape management Ross Cameron, used a replicated wall system outdoors.
It showed that during warm weather, walls screened with evergreen cherry laurel (Prunus laurocerasus) were 10 degsC cooler than the surface of bare walls.
Air next to the walls was 3 degsC cooler than nearby bare walls. On clear sunny days, walls screened by plants were significantly cooler between 11am and 6pm, with the greatest differences in mid-to-late afternoon.
Cameron said species such as jasmine, honeysuckle and fuchsia may be better "cooling" plants than ivy because their leaves are more effective at cooling the surrounding air.
"This is probably good news for the industry because the implication is that many of our highly ornamental and attractive flowering climbers are likely to have positive functional benefits to the wall too," he suggested.
Taylor said the research adds weight to the argument that plants could be used to reduce buildings' energy loads by partially substituting for artificial, mechanised air cooling. Brick was chosen as the building material to help demonstrate the potential of plants to provide summer cooling and winter insulation to older domestic housing stock, where retrofitting by other means can be difficult.
The research also emphasised that not all "green infrastructure" should be treated in a generic manner by policymakers and practitioners. A range of controlled-environment studies showed that different plant species have different capacities to cool wall surfaces, and the mechanism by which cooling occurred could vary between plant species.
Cooling due to the presence of fuchsia, for example, was strongly reliant on evapotranspiration, whereas ivy (Hedera), honeysuckle (Lonicera) and jasmine (Jasminum) cooled primarily through shading. Prunus cooled via both shade and evapotranspiration.
Although not normally considered a wall plant due to its short stature, lamb's ear (Stachys byzantina) was included in the study to assess the effects of hairy, silver leaves. This species was surprisingly effective at cooling with mechanisms being attributed to shade, evapotranspiration and an albedo effect - the silver leaves reflecting back light from the wall environment.
Another important factor in optimising the cooling effect was the thickness of the foliage and developing a uniform facade of full transpiring leaves may be more promising than developing a thick, deep canopy of foliage where the leaves self-shade each other.
Green facades lend themselves well for domestic properties. Little previous research documented cooling advantages in a temperate climate.
PhD student Jane Taylor's thesis also showed very strong positive effects of green walls' plants on providing insulation to walls in winter.