Though we do use natural gas to heat on cold nights in the winter, our rooftop greenhouses use considerably less than similar ground-level greenhouses. We offset all of our natural gas consumption by never refrigerating our produce and by growing near our urban customers rather than shipping our produce to cities hundred of miles away.
Here's a more complex answer in four parts:
First, the heating demands of greenhouses occur almost exclusively during the night. Nighttime temperatures in cities tend to be much higher than in the country due to the thermal mass of city buildings and roads and due to the heating of city homes and offices.
Second, we employ energy curtains. These semi-transparent curtains, which are automatically deployed on cold evenings, help insulate the greenhouse and reduce heat loss at night. This results in a significant energy-use reduction.
Third, our customers are urban residents who live and work close to our greenhouses. Since we deliver to drop-points around the city, harvest our produce on the same day it's delivered, and minimize packaging, we drastically reduce the energy that is traditionally used to package, ship, refrigerate, and store the produce that you'll find in an average supermarket.
Fourth, the combined transpiration of the plants in the greenhouse–collectively an enormous evaporative surface–cools the air and helps to reduce the heat island effect created by the typical black tar roof. This lowers the energy needed to cool the building underneath our greenhouses during hot summer days.
This is just the beginning. Future Lufa Farms greenhouses will be much bigger than our 31,000 sqft prototype in Montreal, which will magnify the benefits outlined above. We also plan to implement biomass heating systems, which will generate heat from recycled plant and wood fiber. And our team is continually developing new and innovative ways to save energy in future greenhouses.