Humidity and Your Guitar
Guitarists frequently ask me what is a safe humidity range for their guitar. For a short period of time, like about a day, the relative humidity of the air isn’t too critical but for longer periods it is.
Hopefully, your guitar was built at about 50% relative humidity. When relative humidity is between 40% – 70% the guitar should be pretty comfortable. When the relative humidity is below 40%, moisture moves out of the guitar woods into the drier air. Over a period of time, this can cause the guitar woods to crack and braces can also become loose.
As relative humidity increases, the guitar woods expand as they absorb moisture from the air. Excessive humidity of 75% and above can cause serious warping over time. At 90%+ your guitar is in a high danger zone.
When I lived in Santa Fe, New Mexico, the beautiful high desert of the South West U.S., I was over at a friend and fellow guitar builder’s house to play duets. We left our guitars in the living room and went into the kitchen for a break. It sounded like a firecracker went off! We ran into the living room. My friend’s guitar had fallen to the hardwood floor. He picked it up. Every back brace inside the guitar had exploded off the back. We had a great laugh. The guitar had been built in Baltimore, Maryland the summer before in his basement at about 90%+ relative humidity. Needless to say, the guitar didn’t respond well when the relative humidity was around 30%.
LESSON – WOOD MOVES AS HUMIDITY CHANGES
When the relative humidity drops below 40% you can use a guitar dampit or a room humidifier to increase the humidity level. When the RH goes above 75% use a dehumidifier to lower the humidity by removing moisture from the air. An air conditioner also removes water from the air. If you raise the room temperature the relative humidity will also drop. The amount of moisture air can hold is relative to the air temperature. Warmer air can hold more water than colder air. Hence the term relative humidity.
In winter months the cold air cannot hold as much moisture as it can in warmer weather. When this cold air enters your home and is heated to a comfortable room temperature the relative humidity of the air drops. A RH of 60% in cold air might be about 20% RH in the same air once it is warmed.
If you want to measure the relative humidity in the room where your guitar is kept you will need a hygrometer. I use a dial hygrometer mounted on the wall in my workshop so I can regulate the humidity as needed. Don’t mount a hygrometer above a heat vent or radiator.