Remember the shocking wave of heat when you get out of your car in a freshly asphalted treeless parking lot? Now imagine building your house with that black asphalt. In Finland it’s cool, in Miami it is dumb.
A matte black surface is ideal at absorbing the sun's radiation, making it hot while a white or highly reflective surface reflects that radiation, reducing its temperature. This is discussed a lot regarding the polar ice caps. The white polar ice caps reflect solar radiation. The dark ocean absorbs solar radiation. As the white solar ice caps disappear and the dark ocean surface takes its place, radiation is absorbed by the ocean rather than reflected back into space and the atmosphere warms.
These infrared images are from my energy auditing days and this was at the home of a man who owned a roofing company. He had a dark grey roof (155°) while his neighbor had a light colored roof (112° ). Even though you don’t spend time on your roof this heat affects the way your home works.
Dark roofs or walls raise the temperature in and around your house
A dark masonry wall absorbs that heat all day and continues to radiate that heat into your house through the night, which is useful in cold climates but not in South Florida.
Dark roofs create hot attics (where your AC ducts run)
The heat on your roof surface heats your attic where your AC ducts run. Your AC has to work harder when the 55° degree air in your ducts is running through a really hot space. It often causes condensation to form on the ducts which causes moisture and air quality issues.
Efflorescence is light colored
You’ve probably noticed that dark walls often have areas that are stained with a light colored chalky substance (crystalline salts) especially where water has been on the surface. This is not necessarily a bad thing, but most folks who like the dark look don’t want a weathered look. The real problem is the poor water management that caused it.
Mosquitoes are attracted to dark colors.
This is more of a problem for walls than roofs. Mosquito researchers capture mosquitos by painting the inside of a box black and the mosquitos are drawn to that dark space. If you have a dark walled porch you know how well this works.
You may ask “what if I don't choose a black roof or walls but I choose a darkish color?” Ask your doctor “what if I just smoke a little crack?” Umm, I guess that is a little better. Just know that the lighter and more reflective your roof and walls are the less your air conditioning will need to run.