There is not really a recommended R-value for specific applications. It is dependent on what the insulation is for. In areas where heat transfer is to be kept to a minimum, a higher R value is desired. (I believe a common R value for insulation in homes is 16). In your home, for example, there would be no need for insulation between your living room and your dining room since they are considered in the same heating and cooling zone, with open access through doorways. Very often, a contractor has experience in what insulation is used in what application and though he may or may not have the technical know how to determine it, his experience proves he is in the right ball park. Not knowing the details of your application, its difficult to say if he's wrong or right. If it is not a cold room, as you say, and heat is not an issue, the only other reason for insulation I can think of off-hand is noise control. If you still don't trust your subcontractors suggestion, you should have someone calculate the heating/cooling loads for you. If this is strictly for noise level, and it doesn't have to be a silent room, the insulation should be adequate.