Lloyd Alter


I don't own an air conditioner. The house immediately to the south does it for us, completely shading the south side of our house. What it misses, a huge ancient maple in its front yard gets, so in winter I get a lot of sun in my window, and in summer I am always in shade. A tree is as sophisticated as any electronic device around; it lets the sun through in winter and grows leaves in summer to block it.

Geoffrey Donovan studied it in Sacramento, and calculated the savings.

"Everyone knows that shade trees cool a house. No one is going to get a Nobel Prize for that conclusion," says the study co-author, Geoffrey Donovan. "But this study gets at the details: Where should a tree be placed to get the most benefits? And how exactly do shade trees impact our carbon footprint?"

His recommendations:

- Placement of a tree is the key to energy savings. Shade trees do affect summertime electricity use, but the amount of the savings depends on the location of the tree.

- Trees planted within 40 feet of the south side or within 60 feet of the west side of the house will generate about the same amount of energy savings. This is because of the way shadows fall at different times of the day.

- Tree cover on the east side of a house has no effect on electricity use.

- A tree planted on the west side of a house can reduce net carbon emissions from summertime electricity use by 30 percent over a 100-year period.

He found that summertime savings totaled about $25.00 per year, but that is in Sacramento, where everyone probably owns air conditioning. In more northern areas, it might mean the difference between having AC or not.

And wait, there's more: Not only do the trees provide shade, they cool. Oikos writes:

"Trees provide a cooling bonus. To keep themselves cool, trees pump water from the ground into their leaves. As this water evaporates from the surface of the leaves, it cools the tree. This "evaporative cooling" cools the surrounding area, too."

They estimate that the energy savings can be as high as 50%. (In our case, where we don't own an air conditioner, they probably are.)

So before you invest in fancy hardware, invest in a tree. The payoff will probably be faster and last a lot longer.