Lee

As other have noted, there are no hard and fast rules.  The important issue is to ensure that changes in relative humidity, if necessary, occur as slowly as possible, allowing the building and the artifacts to acclimate.  

In an older building, it is often impossible to completely stabilize both temperature and humidity because of the lack of an effective vapor barrier to prevent moisture transfer between inside and outside. In a new building, a vapor barrier can be designed into the building envelope system. In an older building, installing a vapor barrier is usually not advisable because it can end up trapping moisture or causing condensation in places where it is not good for the building's structure.  While a fully stable environment may not be possible, humidity changes can be slowed with an effective air barrier that minimizes air flow between interior and exterior spaces.

It is important to be sure that the engineers working on your project have experience detailing all of the components of environmental control systems for museums.  Simply installing a humidification/dehumidification system can cause more problems than it solves.  The ideal solution in each case must balance the HVAC system, the building envelope, and active curatorial management of the system and the artifacts.

Good luck.