There is a chart with this information for steel pipes. You can actually calculate how much heat is being emitted from bare pipes that are now acting as if they were radiators. Which in fact if they are not insulated they actually have become. In some instances it may be desireable to have the steam or hot water pipe act as a radiator. So sometimes they are purposely not insulated. But for most cases, if the asbestos has been removed, insulation should go back on. Click on this link for an adobe acrobat version of this information that you can print out. This information is from the Hoffman Data Book from 1935 and is provided courtesy of ITT.