1
$\begingroup$

which room will stay warmer for long time in winter night. The room with inner painting black or white?

$\endgroup$
2
  • $\begingroup$ Black is a good absorber and bad radiator of heat. White is a good reflector of heat. Now you decide what should the answer be. $\endgroup$ Oct 25, 2015 at 12:41
  • 1
    $\begingroup$ If lights are off (night), then the color of the paint in the visible domain is pointless. Do you know which of those two is the most reflective in the infrared ? :-) And with the light on we would need more knowledge, since direct and reflected light can leaks from the windows while energy of heated walls can leak via its connections to the exterior (and insulation might be settle at different possible steps). So all data related to all possible leaks are needed. $\endgroup$ Oct 25, 2015 at 12:47

2 Answers 2

3
$\begingroup$

It will make no measurable difference.

Heat in a room is lost by conduction and convection - not (in significant amounts) by radiation.

Radiative heat transport occurs between two objects at different temperatures, so the surface of the wall would have to be colder than the rest of the (objects in the) room for radiative transport to take place.

Assume a black sphere in the middle of the room at temperature $T_1$, and a wall at $T_2$. If the walls are "black" (maximum emissivity at the wavelengths of interest - note this is not just black in the visible since most thermal radiation is in the far IR) we can compute the rate of heat loss as

$$Q = \sigma(T_2^2-T_1^2)\pi r^2$$

For a wall at 0C and a sphere at 20 C, this results in heat transport of about 100 $\rm{W/m^2}$ from the object to the (perfectly black) walls. In reality there are not walls on all sides - so the solid angle is smaller and the rate of heat transport is less - and furthermore the walls are not that cold. Because if they were, there would be significant heat transport from the air.

For this you can refer to this engineering toolbox link. With a very modest rate of air flow you will have a heat transfer of about 20 W per square meter per degree of temperature difference. This would quickly result in a heating up of the wall surface to "something close" to the temperature in the room - at which point the radiative losses are even smaller.

It may already be obvious from the above but let me spell it out: radiative cooling would (very slightly) affect objects in the room; but the air has very little radiative interaction.

At "normal" temperatures, and with a house that is somewhat insulated, you can ignore the radiation component. This is not necessarily true if you have a flimsy tent - you may have come across the aluminized Mylar "emergency blankets" that provide radiative insulation. They do make a difference when that's all there is between you and a cold dark clear sky. Not the same thing at all.

$\endgroup$
2
$\begingroup$

The scientific fact is that white reflects the radiant energy rays of the sun and black absorbs them. Most major paint manufacturers can tell you the Light Reflectance Value (LRV) of any color paint chip. White reflects 80% of the light, black 5%. So if you want to have a warmer room paint it black since more of the radiant energy of the light will be absorbed to keep the room warmer.

$\endgroup$
2
  • 1
    $\begingroup$ At night it will not absorb heat from outside, rather inside heat is absorbed and transmitted outside via wall.so inner painting should be white. $\endgroup$ Oct 25, 2015 at 13:58
  • $\begingroup$ @Rajeshkumar: The problem is deceptively simple, with the stress on the word "deceptive" (this has connotations for the politics of global warming because it leaves the door open for plenty of false explanations of radiation forcing). The important parameters for your problem are the radiation temperatures of day and night. The radiation temperature of "day" is around 5600K, while that of night is around 300K. What matters are the "colors" of the paint at the peaks of the Planck curves for these two temperatures. $\endgroup$
    – CuriousOne
    Oct 25, 2015 at 14:40

Not the answer you're looking for? Browse other questions tagged or ask your own question.