ROOFTOP GREENHOUSES CLIMATE MANAGEMENT
By David VOLK (EBF) and Guillaume Morel-Chevillet (ASTREDHOR)
IDENTIFICATION OF THE MAIN GOALS AND WISHES OF AN RTG PROJECT
Prior to choosing the right greenhouse or the best plants, a perfect understanding of the project’s goals, target consumers, and stakeholders’ wishes is necessary (cf. chapter II.11). In fact, the more complex technical conditions are, the more difficult they are to be used by the public for social or educational activities.
If the priority objective is food production, specific technical choices are required. Limited resources restrict the possible outcomes. Clear objectives make a project more rewarding. The diagram aside summarises this idea.
ANALYSIS OF THE ENVIRONMENTAL CONSTRAINTS (BUILDING, WEATHER, SHADOW, HEIGHT, ETC.)
A comprehensive analysis of the project itself, based on environmental understanding, is a key to success for crop and climate management. Shadow from surrounding buildings or trees or even from the greenhouse structure and equipment (columns, braces, or pipes) will have an important restrictive effect on the yield if plants are not chosen carefully. Weather conditions such as prevailing wind, rainfall, or winter and summer temperatures will have an impact on the technical choices (greenhouses, equipment, plant selection). The building itself must be taken into consideration (roof lift, freight lift, security, opening hours, etc.) (see technical feasibility).
GREENHOUSE TYPE
Choosing a greenhouse style is heavily dependent on the planned production, available space, and accessibility to the building.
Tunnel
A tunnel greenhouse is generally the cheapest greenhouse style. It only consists of an arched light-weight steel frame usually covered by PE (polyethylene) film. This leads to a low weight profile and makes the tunnel applicable for rooftops (but mind the wind factor). On the other hand, the tunnel greenhouse offers restricted room for currently available aboveground growing techniques. Due to its very simple construction, technical equipment, insulation, and ventilation are limited.
Chapel
The chapel (or multi chapel) greenhouse is usually made of a strong steel structure covering several meters with a gabled roof. Due to this shape, the greenhouse can be anchored into the structure of the host building. As a chapel greenhouse has a larger roof, its ventilation properties are reduced, unless they are compensated for by sidewall or forced ventilation.
Venlo
The Venlo greenhouse style is comparable to the chapel style, with several smaller “chapels” giving the greenhouse a “saw-tooth” look, adding more possibilities for ventilation. Since the Venlo style greenhouse is the industrial standard, it is comparably cheap, and all available greenhouse equipment is well adjusted in size and connection. Please note that placing a greenhouse on a rooftop will lead to smaller-scale projects than for on-ground greenhouses. This decreases the financial interest for a greenhouse construction company unless the wall is already built.
Lean-to
The lean-to greenhouse is based on a classical design that has been found in England since the Victorian Age and is commonly used to this day in China, with heavily insulated side walls and an arched transparent opening. The lean-to style has a lot of advantages as to the energy demand and the extension of the growing period. However, it needs to be facing exactly south, and this reduces opportunities on a rooftop. Additionally, while the lean-to style greenhouse is cheaper to operate, it comes at a higher initial investment cost and brings a higher load to the rooftop except if the wall is already built.
ORIENTATION
Installing a greenhouse on a rooftop comes along with some restrictions in placement and accessibility that are important to examine before construction can start.
The greenhouse will need constant sunlight exposure to offer the required growing conditions. In an urban context, higher buildings or trees may generate shadowing and reduce the light flux into the greenhouse. Furthermore, the orientation of the greenhouse may be not ideal and lead to income losses, which must be considered (cf. Bankruptcy of Urban farmer report). A north-south orientation optimises production and minimises shading effects.
SIZE AND SCALE
The size of the greenhouse is an additional factor to be considered. Identifying buildings designed and built according to the north-south orientation constitutes a relevant prospective list of buildings on which a rooftop greenhouse (RTG) can be installed with effective performance. One of the biggest advantages of urban farming is the locally produced food directly distributed in the surrounding area. Therefore, the RTG must supply the local market or demand to meet the needs of local residents. Too large a greenhouse relatively to local needs may lead to overproduction and failing to sell all goods or having to export them further away from the production site. Moreover, it is difficult to expand surfaces in an urban context. Each RTG has to be viable on its own or until it becomes part of network of RTGs.
ACCESSIBILITY
The roof accessibility is also important (cf. Execution phase, construction of rooftop greenhouses). Greenhouses are under constant influxes and outfluxes of materials that need to be transported daily from the roof to the ground and the other way around.
Building a large greenhouse on a large rooftop with no accessibility will lead to a bottleneck operation.
COVER MATERIALS (GLASS, RIGID PLASTIC, FILMS...)
A few covering materials are available from the greenhouse industry. The covering material itself is key for generating heat in the greenhouse, keeping it inside and creating a shell to keep climate conditions inside stable.
GLASS
The traditionally used material in a greenhouse and also on the building facades is glass. It is a well-known material used both in the construction and agricultural sectors. Glass is comparably expensive but can provide the best insulation for a traditional transparent covering material if it is double or multi glazed (usually 6-8mm). It has average transmissivity properties that reduce the number of required layers. It has a long lifetime, but if it gets shattered the crops underneath cannot be sold anymore. Furthermore, glass is the heaviest greenhouse covering material. In the case of greenhouses occupying an entire roof, breakage implies a risk of broken glass falling around the building, i.e. a risk for users. It is preferable to leave an area free of traffic all around the RTG to eliminate this risk and to allow for the replacement of the glazing as easily as possible.
POLYETHYLENE
The roof accessibility is also important (cf. Execution phase, construction of rooftop greenhouses). Greenhouses are under constant influxes and outfluxes of materials that need to be transported daily from the roof to the ground and the other way around.
Building a large greenhouse on a large rooftop with no accessibility will lead to a bottleneck operation.
POLYCARBONATE
This material comes in rigid sheets that are often used in greenhouse coverings and also on building facades. It is a well-known material used both in the construction and agricultural sectors. The sheets can be made of multiple layers and can achieve good insulation standards while still generating good transmission. They have an average lifetime but are easily replaced.
ETFE
ETFE is a modern film material that is more and more used in the greenhouse business. The film is highly transparent and durable. Due to its UV-transmission properties, it has a high lifetime of over 20 years. As a single layer it has basically no insulation properties, but it can be installed as a double layer or cushion for average insulation properties. The greatest disadvantage of ETFE is its high price (up to 15 €/m²), which may evolve over time.