Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: should cinderblocks be painted?

Learn more physics!

Most recent answer: 10/07/2012
In an Illinois winter, does a South facing cinderblock wall painted flat black give up more heat through radiation at night than the heat it recieves through direct solar radiation in the daytime? Should I paint my long South wall, white or black?
- David (age 57)
Gary Indiana USA
This is a very interesting question. I can't quite answer your first question because the infrared radiation balance depends not just on the radiation from the cinderblocks but also on the incoming infrared from all the environment. So the balance depends a lot on the precise temperatures of the outer surface of the cinder blocks and of the surroundings.

So here's my guess: precisely because the infrared radiation out should be largely cancelled by infrared radiation in, you'd do best to paint the cinderblocks to maximize the heat from the visible light.

What you'd really like to do is use a paint that reflects far infrared and absorbs visible and near infrared. There are discussions on the web about paints in which the pigments have been adjusted to give infrared reflection and visible absorption. (Here's an example, including some nice illustrations of the spectra: ) Unfortunately these paints are designed for people who want a visible pigment with as little extra absorption of solar infrared as possible, to avoid overheating surfaces. Thus their crossover from reflective to absorptive is in the near infrared rather than the far infrared. Using a black paint like that would get you some extra heat input from the visible light without costing you any extra infrared output, but because it loses a lot of the near infrared input it might not be as good for your purpose as plain old black paint.
You might write the paint manufacturers just to see if there's something closer to optimal for you.

Mike W.

(published on 10/07/2012)

Follow-up on this answer.