Saturday, May 5, 2012
More on the Atmospheric R-value
The R-value used to rate the insulation quality of construction materials and clothing is pretty simple. R=delta T/delta Q where T is the temperature, Q is the heat flow and delta is the difference of rate of change in either. If you have a room you want to be at 72 degrees with an outside air temperature of 32 degrees and only what to use 12.0 MBTH, then R=40/12=3.33 minimum R-value for the space. 3.33 is about the R value of a 3/4" airspace with some of that radiant reflective barrier stuff added. Not a very high R-value, but that was just an example. The atmosphere can have an R-value from one point to another. Since there are seldom dead air spaces, it is pretty low. It is like a house with no walls or roof, pretty hard to air condition. Adding greenhouse gases is supposed to increase the R-value of the atmosphere. If there were dead air spaces, the GHGs would do a fine job. Without dead air spaces, it is s touch more difficult to figure out how good a job they will do. Part of the R-value calculation is delta T. Since we have pretty good temperature data for the atmosphere at differing altitudes, we can get part of the R-value information.