I am often asked about the need for installing foam under a concrete floor.  If the building will not be heated other than occasionally, the benefit will be very minimal.  There may be a slight benefit against condensation since the foam will be a barrier between the cooler ground and the floor, but even then, it may rarely be noticed.  The greater reason to install foam is when it is used under a heated structure.  It still insulates the floor from the consistently cooler ground but even more beneficial is that it minimizes the loss of heat through the floor which in turn keeps the floor at a much more comfortable temperature.  This effect is very evident with in-floor heating systems which enables much higher efficiency.