Humidity issue – GreenBuildingAdvisor

I recently completed a full renovation of the second and third floors of my home, including closed cell spray foam insulation and all new windows. The space is now extremely airtight. I have radiant heat for the winter and Mitsubishi ductless mini-splits for air conditioning.

Over the past month, the indoor humidity has consistently remained in the mid to high 80% range. The mini-splits quickly satisfy the temperature setting, but they do not run long enough to remove enough moisture. Dry mode has also failed to bring the humidity under control. Oddly, I have the opposite problem during the winter, when the indoor air becomes too dry.

I am looking for advice on identifying the cause and finding a permanent solution, such as a dedicated dehumidifier or ventilation system.