For years, actually generations plumbers and steamfitters used this chart to determine the size of radiators that were needed to heat a room. They based this on testing that was done that measured the amount of condensate produced by a radiator in certain enclosures against that which was produced by an uncovered radiator. This became the gospel that was used to promote the removal of radiator covers. If you look at the chart for what it is you would probably come to the same conclusion. Its a wrong conclusion though. As the testing was about how much 'heat' is produced by a radiator enclosed verses unenclosed. There is no mention at all about how this heat is be utilized. Is it where the people are? did it all go to the ceiling? It was a customer of mine, Bob Bonthius that turned me on to these old University of Illinois studies that turned my world (as far as radiator enclosures go) upside down. I dug up the studies and below are links to them. Pay close attention to the 1929 study as that one will ultimately instruct you on what a good radiator cover should look like. Yes, we are never to old to learn new (or antique) tricks. So the dead men who covered those radiators weren't so stupid after all!
Radiator enclosure testing on how it actually heats the rooms against an uncovered radiator.
University of Illinois Testing
Bulletin 169 was the initial study done in 1927 that determined that with but a few exceptions an uncovered radiator produced more heat output than an enclosed radiator. This study while true, had what it was implying overturned by the subsequent studies as yes an unenclosed radiator did in fact produce more heat, but did that heat output actually convert to occupant comfort or just go to the ceiling. You can access this study at http://hdl.handle.net/2142/4515
Bulletin 192 was the study from 1929 that determined that radiator covers when designed properly far exceeded the uncovered radiator in its ability to provide comfort and a certain degree of steam economy. This was the study that changed my thinking about radiator covers. You can download a pdf of this study at the following web address http://hdl.handle.net/2142/4165
Bulletin 223 was the study from 1931 that went a little farther in the study of comfort and also included wall radiators. This was also the study that determined long low radiators of the same square footage heated a room better than tall radiators of the equal square footage. Its download page can be accessed here http://hdl.handle.net/2142/4262