Use trees to shade constructed surfaces. Planting shade trees in hot spots, such as over streets and parking lots, can significantly reduce surface temperatures. Shade trees can make buildings up to 20 degrees cooler in the summer, and can cut summer air conditioning needs by up to 50%. Click here for more information on trees for natural temperature control.
Minimize building footprints. Multistory housing structures, such as this one in downtown Ventura, reduce the building footprint of individual homes.
Minimize parking footprints. Multistory parking structures substantially reduce the overall footprint of parking areas. Downtown Ventura’s multistory parking structure has recently added solar panels on its façade.
Use cool roofs.
People who live in tropical climates usually wear light-colored or white clothing to help keep themselves cool. They know that light colors reflect heat and sunlight, whereas dark colors absorb heat and light. For the same reason, a dark-colored roof will be hotter than a light-colored roof.
Cool roofs control their surface temperatures by reflecting solar energy (high reflectivity) and discharging stored heat (high emissivity).
That keeps inside temperatures—and cooling costs—down. Compared to standard roof surfaces, cool roofs stay 50-60° F. cooler on hot days.
Cool roof coatings can also be applied by brush or roller, such as on metal roofs. Green roofs, or roofs planted with vegetation, are also being used as an effective cool roof strategy.
To be considered a cool roof under California’s Title 24 Building Standards, a roof must achieve at least a 0.75 initial emittance and 0.70 initial reflectance.
Use less pavement. The most effective way to reduce heat pollution is simply to reduce paved areas. Narrower streets, shorter driveways or two-strip “Hollywood” driveways, less pavement on schoolyards, less parking and smaller parking lot footprints, and compact urban designs that minimize the need for roads are easy places to start.
Use light-colored paving materials. Dark-colored pavements, such as asphalt, can get up to 40° F. hotter than the surrounding air. Light-colored concrete, pavers, or top coats offer a cooler alternative. If you are paving with asphalt, a white aggregate can be applied as a chip seal layer, or a light-colored surface coating such as a zinc-oxide slurry mix can be used to increase reflectivity.
Albedo, or solar reflectance, is the ratio of reflected solar radiation to the total amount that falls on that surface, known as incident solar radiation. Albedo values range from 0, for perfect absorbers, to 1, for perfect reflectors.
An albedo of 0.3 means that 30% of all the energy striking a reflecting surface is reflected back into the atmosphere and 70% of the energy is absorbed by the surface.
White cement concrete pavements have albedos in the range of 0.70 to 0.80 when new, and 0.40 to 0.60 when aged. New asphalt is very dark, so it has an albedo of 0.05 to 0.10, preferably with a reflectivity of 0.30 or higher.
Use permeable paving materials. By allowing water to soak in, rather than quickly runoff, these materials are more like natural ground cover in the way they absorb and store heat and allow for evaporative cooling.
Common types of permeable (also called porous or pervious) paving materials include:
- Conventional concrete with the fine materials left out of the mix,
making it coarser and therefore porous.
- Open grids that are filled with aggregate (such as pebbles) and
planted with vegetation. These are also called “grass pavers.”
- Paving blocks that are spaced apart.
- Decomposed granite or gravel. For parking stalls and walkways where wheelchair accessibility is not essential, decomposed granite or other compacted crushed rock can replace pavement. Gravel reflects and sheds heat better than paving and its superior porosity makes it preferable for storm water management.
