34. Landscape Design for Energy Conservation
Here in the San Francisco Bay Area September is our hottest month of the year, and in addition to all the important things you need to be doing to conserve water, it is also a good time to think about how the design of your landscape can help you conserve energy.
Keeping Cool
Energy conservation in the landscape comes in many forms, and one big part of this is simply about the wise use of shading devices. Using plants and other elements to provide shade for the places we humans want to inhabit makes a big difference in the energy we need to keep them cool. The Bay Friendly Landscape Guidelines provide some impressive numbers on the impact of shade: “When properly placed, mature trees can reduce the interior temperature of a building by as much as 20 degrees, reducing summer cooling costs by 25-40%, and reducing greenhouse gas emissions.”
So, a good starting point is planting trees to shade your house. Blocking afternoon sun is especially important because everything has already warmed up with the air temperature, so tree planting on the west side of a structure is the best. Shading the actual air conditioning units outside the house is also a key way to save energy. But it is not just about cooling, in the winter months direct sun is a good thing, so using deciduous trees is preferable. With their leaves gone in the winter, they allow the sun to warm up buildings and reduce the need for heating energy use. A well designed home includes passive solar features that take full advantage of the winter sun to warm surfaces inside the house that re-radiate heat for hours in to the evening.
Lighten Up
Shading buildings helps with reducing what is called the Heat Island Effect. A city neighborhood has all kinds of surfaces that absorb sunlight and radiate heat, increasing temperatures as much as 5 degrees compared to a natural area and creating what is called a Heat Island in the environment.
The darker the material, the more heat it absorbs, and with black asphalt so prevalent in our cities, there is a lot of extra heat there. There are a whole host of design strategies to minimize the impact of these darker materials, starting of course with shading them. Many California cities now require parking lots to have a certain number of shade trees to keep the sun off of cars and reduce that need to crank the air conditioner when you get in. Beyond that you can select lighter colored materials with a high Albedo, or solar reflectance, which means that they absorb less heat. Just be careful not to put down bright white paving everywhere as it will create a lot of uncomfortable glare for people using the space. Anything that reduces the amount of solid paving is also worth considering, such as two strips of paving for a driveway instead of a full width of asphalt, using open grid paving types, or loose gravel surfaces. These ideas also have the overlapping benefit of reducing stormwater runoff, so they are especially valuable for the sustainable garden.
Rooftop Resource
Obviously roofs are a huge amount of the built environment that we expose to sunlight. So many houses traditionally have dark colored asphalt shingles on them, soaking up sunlight and radiating heat down into attics and living spaces, demanding more fans and air conditioning use. A low cost alternative is to switch to a light colored roofing material or coating. Even better, take that dark shingled roof and replace it with solar panels and run the meter the other way, generating energy from that great sun exposure. Another fantastic way to cool that roof is to cover it with plants, a living green roof in many ways moves the building footprint back towards its pre-development state, with a great number of sustainability benefits.
These shady strategies are only one way to design for energy conservation in the landscape. Other things to consider are energy use in landscape construction and maintenance, efficiency in landscape lighting, and what goes into manufacturing the materials that make up the built garden. In fact, the concept of embodied energy and the full life cycle of manufactured materials is one that has been investigated in detail, so we will cover that another time. One day I hope we can fly over cities and see nothing but rooftop gardens, solar panels, and leafy trees, with no paving in sight. There is no doubt in my mind that we will be far happier and better off because of it!
References:
Bay Friendly Gardening Guide and Landscaping Guidelines – Principle 5: Conserve Energy
Multifamily Green Building Guidelines, by Build it Green and Green Building in Alameda County, part of the Green Points Rating System.http://www.builditgreen.org/
Sustainable Landscape Construction, by J. William Thompson and Kim Sorvig