Rooftop greenhouses can supply city residents with locally grown vegetables

Yizhi Zhang is a doctoral student at the Department of Biosystems and Technology at SLU. 

She completed a master's degree in architecture with a focus on sustainable building design and performance in Dublin in 2019, followed by a second master's in Lund in energy-efficient and environmental buildings – which she studied for during the pandemic.

It was during that period, while being isolated in her student flat, that she decided to write her thesis on rooftop greenhouses.

– I started thinking that if we could just rely on ourselves to supply cities with food and help the environment at the same time, that would be very beneficial, says Yizhi Zhang.

Since 2024 she has been employed as a doctoral student at SLU, continuing the work she began in Lund. 

She is investigating how rooftop greenhouses can be designed in a resource-efficient and sustainable way. It is an interdisciplinary doctoral position with many layers, spanning building technology, building energy and cultivation – a good match for her broad range of research interests.

– Rooftop greenhouses are complicated, she says.

Resource-efficient vegetable production

Rooftop greenhouses offer several advantages from the perspectives of sustainability and resource efficiency. They take up no more additional land, compared to conventional ground-level greenhouses. They can also help save energy in the building.

– The rooftop greenhouse acts as a thermal buffer that insulates the building, she says.

Locally grown vegetables also mean less importing, shorter transport distances and less packaging material. 

– The greenhouse can also be used to collect rainwater for irrigating the vegetables, or even to meet the water needs of the building itself.

Simulating potential energy savings

In the project, Yizhi Zhang compares different design choices against potential resource savings and economic viability.

She uses simulations to analyse how much energy the greenhouses can save for the building, and how many resources go into the physical materials and the cultivation itself. She conducts a life cycle assessment to evaluate the total environmental impact of the greenhouse and will also look at total life cycle costs, including maintenance costs. 

The investment required for a rooftop greenhouse is far greater than for a conventional one, she explains.

– The installation is highly technical. In addition, the roofs were not originally intended for greenhouses, so adjustments have to be made to make them suitable. An individual risk assessment will be needed for each building, and lifts need to be installed to reach the greenhouse level.

The biggest design challenge she currently faces concerns the ventilation system.

– We will connect the greenhouse and the building through a ventilation system so that we can supply the plants with carbon dioxide and manage waste heat from the building.

Sweden's first rooftop greenhouse?

Since rooftop greenhouses require flat roofs, Yizhi Zhang sees great potential for them on buildings from Sweden's Million programme (miljonprogrammet), which was a large-scale public housing initiative running from 1965 to 1974.

– It could also work well on large retail buildings, like Ikea. As the energy savings are mainly concentrated on the top floor, low-rise buildings such as warehouses are likely to achieve higher relative energy savings than multi-story residential buildings.

Whatever the location, the goal of her project is clear:

– I hope to see Sweden's first rooftop greenhouse. Then we can continue the experiments in reality, not just in simulations.