What is the range of relative humidity, in percentage, that is optimal for storing a guitar, cello, violin or the like in its case?
I live in Atlanta, Georgia, USA, which is a humid environment year-round. In this climate, musicians never worry about their guitars, cellos, violins and the like experiencing damage from the wood becoming too dry, not even in winter. So I have never encountered this situation before.
I have heard that guitar, cello and violin owners in dry climates use a humidification system when the instrument is stored inside the case; a sponge to deliver water vapor, and a hygrometer to measure the humidity relative to the air temperature.
I've moved into a new ground-floor condominium in Atlanta, and here in August, my rooms are so excessively humid (85% or even higher when the temperature is 78°F or 26°C) that mold and mildew have started to appear. I've had to purchase a stand-alone electric-powered building dehumidifier to remove the excess moisture. It is now removing 50 pints (24 liters) of water out of the air every 24 hours. The relative humidity is now down to about 45% at 78°F.
Is 45% relative humidity at 78°F (26°C) going to harm the wood of my guitar and my viola da gamba (similar to a cello)? Should I purchase humidifiers for the instruments in their cases? If so, what degree of relative humidity is optimal for the instruments?