A new study by the Royal Horticultural Society (RHS) and University of Reading has found that ivy is the most effective plant cover for cooling buildings and reducing humidity.
The paper, recently published in the Building and Environment journal, determines the impact of green facades and vegetative cover on the temperature and relative humidity within buildings.
There have been concerns that these green walls may increase relative humidity and cause damp issues on the walls, but this research suggests this is not the case.
This paper looked at the impact of three plant species (Hedera helix – ivy, Parthenocissus tricuspidata – Virginia creeper, and Pileostegia viburnoides – climbing hydrangea) on temperature and humidity when grown as building cover. The experiment used model mini buildings and the study was carried out in the summer and winter.
All plant species reduced the air temperature internally and externally during the summer day times by at least 1⁰C compared to bare ‘buildings’.
The evidence showed that the best plant for summer cooling was provided by ivy. It was able to reduce the internal and external wall temperature by 7.2⁰C and 5.7⁰C respectively. Not only did ivy provide the best summer cooling for buildings, but it also demonstrated the greatest summer reduction in daily variation of relative humidity (RH).
During warm winter afternoons, RH was 5.7% lower inside ivy-covered compared to bare ‘buildings’. This means that ivy covered walls would keep buildings less humid in winter months.
Dr Tijana Blanusa, RHS principal horticultural scientist says: “Our research is an important step forward in growing our understanding of green walls. Many people with limited space are turning towards vertical gardening as a way of greening their homes or workplaces. We are encouraged by the findings that all the plants tested provide summer cooling benefits without causing humidity issues.
“The RHS has been keen to encourage new ways of gardening and the development of technically simple vertical green walls using affordable climbing plants has proved to be a welcome addition. I hope that this research will help people decide which plants they want to grow.”
Dr Tijana Blanusa worked on this new paper alongside Dr Faye Thomsit-Ireland, Dr Emmanuel A. Essah and Paul Hadley of the University of Reading.
In spring 2021, the RHS is opening its new Centre for Horticultural Science and Learning at its flagship RHS Garden Wisley.
