Can Paint Increase Efficiency?
The simplest efficiency upgrade may be rethinking the color of your walls and roof. The reflective quality of white and the absorptive quality of black plays a surprising role in the amount of unwanted heat your house receives. While paint can reduce overheating, the big question is—how much of a difference will these measures make?
Roofs
We must note that solar heat absorbed through your roof is a much greater issue than heat absorbed through your walls. Heat buildup occurs in a multitude of ways, such as air infiltration through cracks, poor insulation, and running appliances. However, a whopping one-third of unwanted heat comes through the roof.
Unlike walls, your roof is in direct contact with the sun during summer months. In May through August, the summer sun rises and passes east to west overhead, beating direct sunlight on the top of your house for hours each day. When the sun beats down on a black roof, the temperature is frequently 60F hotter compared to a white roof. By switching to a white surface, roof heat gain can be reduced by 50%. Ultimately, it comes down to choosing a surface that has a greater ability to reflect solar radiation. This graph shows differences in solar reflectivity for a variety of materials.
Those in cold climates may be inclined to keep their dark-colored roof, thinking it will reduce energy demand in the winter. However, even in cold climates, light-colored roofs will help conserve energy. As mentioned before, the summer sun passes over a house’s roof increasing summer heat gains. However, in the winter, the sun’s path is much closer to the horizon resulting in fewer and weaker rays hitting the top of the house. Thus, a black roof in a cold climate will do little good in the winter but will continue to generate unwanted heat in the summer.
Walls
Contrary to roofs, walls make far less of an impact in reducing heat gain. This can be attributed to the fact that walls are less exposed to the summer sun. Furthermore, neighboring houses and trees provide ample shade and reduce heat gain through walls.
We modeled two houses in sunny Los Angeles to see what the impact of light and dark painted siding would have on a poorly-insulated and well-insulated house. For the house that lacked insulation, using a light color siding reduced heating and cooling energy by 3% and for the house that had adequate insulation, it only reduced loads by 1.5%. In other words, painting your wall a more reflective color may help slightly, but it will not drastically reduce your energy bill.
While wall color has minimal impact on overall energy use, thinking in terms of heat gains and losses is the best paradigm to live efficiently. All small decisions add up and inspecting your house to see what components make reducing energy consumption more difficult is extremely valuable. Old appliances, incandescent light bulbs, treeless yards, and large air leaks can add hundreds onto your utility bill each year. By considering upgrades that address internal heat gain, you can effectively cool your home before heat builds up in the first place.
