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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?

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  • Living in an area where it gets very dry in the Winter, I can tell you that every guitar seller/maker in the area recommends a humidifier for all instruments. But I can't tell you the best range for instruments.
    – Ryan Kinal
    Aug 13, 2014 at 18:44

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A wooden instrument is ideally kept at about 40-60% humidity, so 45% is fabulous. This range is supposed to produce the best sound, and the least stress on the wood.

But the absolute worst thing you can do is change up the humidity frequently and quickly, because this put stress on the wood and the seams. So if your instruments spend hours a day outdoors, for instance, you might start to run into seams opening up, or even wood cracking from the wood constantly shrinking and expanding. But even this is a low risk situation, especially if you let the instrument sit in the case for a while to adjust before taking it out in new conditions.

That said, most instruments I've seen with those problems were either left in a car, or kept for long periods in an attic or basement. If you've got a lot of concerns, find a local luthier, and ask what he or she suggests.

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My understanding is that the upper limit on relative humidity is set by the conditions which promote mould growth, around 70% RH (a reference for the 70% value), and not due to things like warping/splitting that you have to worry about with low humidity.

I usually target 50% RH, year around.

Martin guitars advocates 45%-55% for their instruments.

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  • In that case, if the relative humidity in my condo is 45%, because of the need to run the dehumidifier to control the mold and mildew on surfaces, then perhaps I need to humidify my instruments to get them up to 50%, and start monitoring conditions in the winter as well.
    – user1044
    Aug 13, 2014 at 19:23
  • Unless your instruments are vintage or otherwise particularly precious to you, I wouldn't sweat the difference between 45 and 50 -- plus your hydrometer may not be that accurate anyway.
    – Dave
    Aug 13, 2014 at 19:30