The Cooling Effect on the Atmosphere of a Green Roof

Our roof top garden is made almost entirely from post-Christmas Christmas trees. It was easy to make and practically free. But it also appears to be a benefit to the city, and even to the planet.

Recently, I suspended two thermometers from tripods about a foot above the surface—one in the middle of the half of my roof that is covered by a living green garden, and the other in the middle of the half that is covered with a silvery tar-like substance.

Then I took readings, and kept track of the results.

In general, I found the green side was always cooler, ranging from the same (at night) to 23 degrees cooler just after a watering.

Using back-of-the-napkin calculations, it appears the chemical roof was on average hotter by six degrees than the green roof when the sun was directly overhead, and between zero and six degrees hotter relative to how high or low the sun was in the sky throughout the day--assuming a sunny day.

From sunup to sundown, then, the extra heat being generated on the non-green roof appears to be 42 degrees per square foot (one degree between 6am and 7am, two degrees between 7am and 8am...six degrees during each of the hours on either side of high "noon", and so on back to sundown).

Here are the actual readings taken:

Since the green roof and the chemical roof both cover about 725 square feet, my crude calculations seem to suggest that the non-green half of my roof throws an astonishing 30,450 extra degrees of heat into the atmosphere every sunny day—something like 6,000 BTUs of heat since it takes about .02 BTU to raise the temperature of one cubic foot of air one degree.

Assuming I'm not missing something that makes my conclusions wildly off, that's a whole lot of extra heat from just my building.

Cooling Effect of a Green Roof

Plotting surface temperature differences between adjacent green and non-green roofs Aug 2 - Aug 9 by time of day.

Some variation can be accounted for by the relative cloudiness when the temperature readings were taken. Additionally, wind speed seemed to have some effect. Readings immediately following a watering were not included.

A quick look at Google maps satellite view shows my building accounting for something like a half percent of the hard surface area in this block. To make a leap: you could say the presence of non-green surfaces in my block produces extra heat roughly equivalent to running ten gallons of jet fuel through a jet engine in this block every day.

Same for the next block over. And the next block after that.

I can't help but think it would make a huge difference in the city (not to mention the planet) if there were a real effort to get gardens on our roofs.

I made my garden almost entirely out of discarded Christmas trees. Thus it was extremely cheap. It's secure, having withstood a hurricane last year, and a nasty windstorm earlier this year. It's lightweight and drains extremely well, alleviating two major reservations landlords often have about a green roof. (I hereby nominate my landlord, Pat Welch, for the Mayor's Environmentalist of the Year award for letting me try it out in the first place.)

And, I have flowers and vegetables on my roof!

At some point, I'm going to try to get a video up about how I built it, so others can use my technique if they want. But, in the meantime, I'm happy to show it to anyone who asks. Hopefully, the city government will consider tax incentives or something in the near future to encourage others to help cool the city by making their roofs green. And, who knows? If we all paid more attention to how green our city is, it might seem less black and white.

For more information, call Craig at 202 568 9448