Radiation– white vs black house, hot or cool?

Question

In my book's thermodynamics chapter, it says that an

"object that radiates heat faster also absorbs heat faster. This means that an object that is a more efficient radiator comes to equilibrium with its environment more quickly. With this in mind, is it better to paint your house black or white?"

I am confused which it would be. The book says white because

"in summer, you house is cooler than the environment and white reflects away the heat. In winter your house is warmer than the environment and white radiates away the heat"

Answer 1

There are two main ways that a house (or indeed any other object) exchanges heat with its surroundings: convection and radiation. As a general rule, at everyday temperatures convection is faster than radiation so it's the dominant mode of heat transfer.

With convection it doesn't matter what colour you paint your house. Convection heating and cooling is mostly by the wind blowing against the house walls and exchanging heat with the walls by conduction. In particular at night when it's cold the house will lose heat at the same rate whether it's painted white, black or yellow with green spots.

But the radiation from sunlight has a temperature of about 5,700°C so it is very good at transporting heat, as indeed you can tell just by standing in sunlight for a few minutes. Painting your house white (or better still silver) will reduce the absorptivity because it reflects away a large proportion of the sunlight, so it will reduce the rate of heating by the sunlight while not affecting convection.

So painting your house white will reduce the amount it heats up during the day but will not affect the amount it cools down at night. The end result is that it will keep the house cooler.

In winter the sunlight is often very weak or it's cloudy, in which case convection dominates and the colour of the paint has little effect on the internal temperature. It's true that on the rare sunny days in winter the white paint will reduce how fast the house heats up, but in hot climates this is a price worth paying for keeping the house cool in the summer. I suppose ideally you'd repainting the house twice a year so it was white during the summer and black during the winter.

Answer 2

The First Law demands the emissivity and absorptivity of a body be identical at a given wavelength. Most bodies that absorb in the visible radiate in the IR. A green leaf absorbs in the visible and it fluoresces in the IR to stay cool. Look at IR photos of foliage in sunlight. Bright!

To warm in winter you want walls that are black in the visible and white in the IR. To cool in summer you want walls that are white in the visible and black in the IR. Winter sunlight is attenuated. The summer solution then predominates. Iron oxides (red tile roofs) are a compromise,

http://aram.ess.sunysb.edu/tglotch/TDG23.pdf

Consider the leaf solution. The longest pump absorption wavelength is the shortest IR fluorescence wavelength (Stokes shift). NIR fluorescent dyes or pigments will be deep blue, You need an emitter in the mid-IR. It must also survive ten years of direct sunlight, arguing for an inorganic ceramic. This looks like a job for lanthanoids,

http://upload.wikimedia.org/wikipedia/commons/4/48/Commercial_laser_lines.svg