A sun-facing wall in the middle of an August heatwave can get hot enough to hurt. Covering it in greenery is one of the oldest ways to fight the heat, and it is back in fashion now that air conditioning is pushing bills up. But how much does a green wall really cool a home? The measurements show a striking effect on the wall itself, and a real but more modest effect on your bill than the marketing suggests.
How plants cool a building
The cooling does not come from one thing but from three that stack up. The leaves cast shade and intercept the sun before it hits the wall; the layer of air trapped between foliage and brickwork acts as an insulating cushion; and the plants release water through their leaves, cooling the surrounding air much like a natural mister. It is the combination of shade, trapped air and evaporation that does the work, not the colour green on its own.
How many degrees cooler
The figures are telling. The surface of a bare, sun-baked wall can reach 60 °C, while the same wall under a blanket of plants stays closer to 30 °C. The air right against the facade drops by around 4 °C on average, and indoors the fall ranges from 1.5 to 5 °C depending on the aspect and the insulation. On energy use, the estimated saving on cooling lands somewhere between 37 and 51 %.
The effect on your bill
Turned into pounds, the effect needs a reality check. If cooling one room gets through something like 450 kWh over a summer, the real saving from a green wall is worth a few tens of pounds a year at the current price cap. A green wall will not pay for itself through your bill alone, but it improves comfort and cuts how often you reach for the air conditioning on the hottest days. To see how much cooling adds to your costs, it helps to start with what a unit of electricity costs, explained in our guide to coping with a heatwave.
Cheap climbers or an engineered living wall
There are two very different routes. Climbing plants such as Virginia creeper or ivy cost little, scramble up a trellis by themselves and ask for minimal upkeep: this is the affordable option. Engineered living walls, with a planted substrate and built-in irrigation, look spectacular and perform better, but they are heavy, expensive and demanding to run, with maintenance that can run to tens of pounds per square metre each year.
A complement, not a substitute for insulation
The expert view is clear: greenery on a facade works best on poorly insulated buildings, where there is most room for improvement, and it is a complement rather than an alternative to proper insulation. The fabric of the building comes first, the planting second. Anyone looking for a lasting cut in energy use will see a quicker return from the steps in our guide to home energy efficiency.