Key concepts
Energy conservation
Temperature
Heat
Plants
Insulation
Introduction
Have you ever seen a rooftop garden? Around the world, some rooftops have been transformed into living green areas. Besides beauty, rooftop gardens have a number of very visible advantages, including growing (very) local food. How would you like home-grown sky vegetables for dinner, or some fresh-cut roof flowers for vases in your house? Rooftop gardens also take carbon dioxide out of the air while releasing breathable oxygen. Chicago's City Hall is one famous building with a rooftop garden. But can rooftop gardens keep your house cooler and lower your energy bill on hot summer days? Try this activity to find out!
Background
Rooftop gardens, also called living roofs or green roofs, have many advantages, including providing more space for agriculture, adding beauty to the cityscape and increasing air quality. During photosynthesis, plants remove carbon dioxide from the air and release oxygen that we need to breathe.
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On hot summer days, rooftop gardens may also keep buildings cooler than traditional roofs—especially larger buildings that often have tar and gravel roof surfaces. Because they sit in the direct sunlight for many hours, the temperature of traditional rooftops tends to rise above the actual air temperature. That heat radiates back into the environment, making urban areas much warmer than rural and suburban ones. If you live in a big city or have visited a shopping center with a lot of concrete and buildings during warm months, you might have noticed the temperature difference. When heat is radiated back into the environment from rooftops, an area with many buildings, like a city, can experience an increase in local air temperatures by as much as 5 to 7 degrees Fahrenheit! This phenomenon is called the urban heat island effect.
Materials
• Two shoeboxes, photo storage boxes or half-gallon size cardboard milk cartons. The boxes should be the same size, color and shape.
• Sod, available at most nurseries or garden supply stores. Alternatively, a small amount of moist soil, some freshly pulled weeds and tape may be used.
• Exacto knife (and an adult's help when using this sharp tool)
• A sunny spot outside on a hot day
• Thermometer
• Clock or timer
Preparation
• Place one of the box's lids (or side of a milk carton) on the sod. Using the knife, carefully cut around the lid to get a piece of sod the same size as the lid. Adult assistance may be needed to use the knife. Place the cut-out sod piece on top of the box (or on the side of the milk carton).
• If you are using soil and weeds instead of sod, use tape to make a raised perimeter around the edge of the lid, then pour a thin layer of moist soil on the lid (the tape should help contain the soil) and then add several freshly pulled weeds.
• You should now have one box with sod (or soil and weeds) on it, which will represent your rooftop garden house, and one box without anything on it, which will represent your traditional house.
Procedure
• On a hot sunny day put the thermometer in the box with sod on it, close the box, and take it outside.
• Place the box in the sunny spot you picked. Leave it in the sun for 30 minutes. (You'll want to test both boxes in the same cloud conditions, specifically when it is sunny and warm out for the entire 30 minutes that each box is outside. If cloud conditions change when you are testing a box, try to retest it again later when it's warm and sunny out.)
• When 30 minutes have passed, open the box and quickly read the thermometer's temperature. How hot is it inside the box?
• Put the thermometer in the shade near the box (still outside). After it has adjusted to the shade, read the temperature. How hot is it in the shade? How does this compare to how hot it was in the box?
• Repeat these steps with the box that doesn't have sod on it. How hot did it get in the box without sod on it after 30 minutes? How does this compare to how hot it is in the shade, outside of the box?
• Overall, which box was coolest inside, compared to the temperature outside of the box in the shade? Can you explain your results?
• Extra: You could investigate how having a rooftop garden affects a building's temperature over the course of the day by repeating this activity but keeping the boxes outside the entire day and taking measurements throughout the day (including after it gets dark outside) or by using a heat lamp on the boxes to mimic a hot day (and turning the lamp off to mimic the sun going down). How does the temperature of the boxes change over the course of a day?
• Extra: You could try growing your own rooftop gardens for your box houses and explore many variables. What kinds of plants work best? Does soil depth alter the temperature results?
• Extra: You could explore how having a rooftop garden affects how warm a building stays during the winter. To do this, repeat this activity but this time test the boxes on a cooler day (or indoors) and put a layer of ice cubes on top of the boxes. (Cover the boxes first with plastic wrap to keep them dry.) You could alternatively place the boxes on top of a tray of ice (again covered with plastic wrap). Which box stays the warmest in winter-like conditions
Observations and results
Did the box with the "rooftop garden" stay cooler than the plain box?
It's thought that rooftop gardens might be able to diminish the urban heat island effect. Generally, rooftop gardens absorb heat and insulate buildings better than traditional tar and gravel roofs. In this activity, you should have seen this; while both boxes were probably warmer than the temperature in the shade nearby, the box with the sod (or soil and weeds) should have been relatively cooler inside compared to the box without the sod. As an example of how real rooftop gardens can help keep their buildings cool, measurements from the Chicago City Hall building show that on a summer day when the air temperatures were in the 90s, areas of the roof area covered in black tar rose to a surface temperature of 169 degrees Fahrenheit, while areas planted with a rooftop garden only rose to 119 degrees Fahrenheit – that's a 50-degree difference!
Cleanup
With permission, you can plant your sod somewhere, or compost it. Any clean cardboard can be recycled.
More to explore
Green Roofs: Best Management Practices, from the City of Chicago
The Living Roof, from the California Academy of Sciences
Green Roofs, from the United States Environmental Protection Agency
Rooftop Gardens: Are They a Cool Idea?, from Science Buddies
This activity brought to you in partnership with Science Buddies