What is the acceptable range of RH for a laboratory? 

Specific requirements are determined by tolerance is specified in test methods being performed and or regulatory requirements depending on the test or calibration sector.  The two primary factors in creating an ideal laboratory condition are temperature and humidity. Note that often with the use of air conditioners, there is a drying effect and relative humidity is reduced. 
Laboratories should be equipped with suitable climate and ventilation control. The laboratory must look at risks – personnel comfort and risk to validity or results. The starting point is that the temperature and humidity must be kept within limits for the proper performance of each test performed and according to the manufacturer’s specifications for the proper operation of equipment.

Certain organisations, such as the FDA in the USA, have guidelines for general conditions. For example, a comfortable working environment is considered 20 to 25 Degrees Celsius with a relative humidity, depending on geographical area, of 35 to 50%.

For more information, see the Advisera ISO 17025 toolkit Facilities and Environmental Condition Procedure at https://advisera.com/17025academy/documentation/facilities-and-environmental-condition-procedure/

For more information and links to Advisera resources, have a look at a similar topic Environmental conditions for testing and calibration laboratories at https://community.advisera.com/topic/environmental-conditions-for-testing-and-calibration-laboratories/