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Guitar Guru – Humidity

October 01, 2014

Dana Bourgeois contributes the monthly Guitar Guru column to Acoustic Guitar Magazine. If you a question you’d like Dana to address in an upcoming article, please submit it to

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Don’t Need No Steam Heat – You may get out of the comfort zone—just don’t take your guitar there

Q: I keep my three acoustic guitars on stands in my living room. In the winter I use soundhole humidifiers plus I run a small humidifier in the room. If moisture in the winter is good, wouldn’t a little in the summer be OK? I just played outside in Philadelphia, where it gets hot and humid. A guitar friend said it’s too ‘sweaty’ outside and moisture is not good. I don’t get it. Could you explain? DAVE DARTNELL HAVERTOWN, PA

A: Most acoustic guitars are built using well-seasoned woods and assembled under “average” climate conditions. I keep the assembly area of my shop at around 70 degrees and at 45- to 50-percent relative humidity—conditions that are probably typical for most shops.

Guitars assembled under average conditions will remain relatively stable in climates that are either somewhat dry or a little humid, but will develop problems when taken above or below their individual comfort zones. A guitar’s tolerance to dryness or humidity will vary depending on design, woods, and most importantly, age. Older guitars are typically more stable than newer ones.

Tops and backs of acoustic guitars are usually glued to arched braces, creating slightly domed surfaces. When a guitar dries out, woods shrink, domes flatten, and bridge, saddle, and strings drop below their original positions (relative to the plane of the fretboard), causing them to rattle and buzz. Worse, a top or back will crack after it can flatten no further.

When an acoustic guitar is overhumidified, the dome of the top increases, lifting strings, and making for stiffer playing action. Extreme humidity can cause glue joints to fail, finish to delaminate, and mold to grow. Such damage, however, usually requires prolonged exposure to moisture, as might occur when a guitar is stored in a very damp basement.

Your friend is right to be concerned about hot and humid conditions. Consider that sides are bent using a combination of heat, moisture, and pressure. Though the effects of humidity alone are usually reversible, the addition of heat and string tension can cause permanent top distortion. Excessive top “bellying” eventually necessitates expensive neck resets. Glue joints fail more quickly under hot and humid conditions than in the presence of humidity alone. And a loose brace or bridge will accelerate top distortion and add to the repair bill.

In situations such as outdoor music festivals, if you monitor the temperature—which can usually be controlled—you can afford to be less concerned with humidity. Remember that temperatures inside a guitar case, under direct sun, or inside a closed car can greatly exceed ambient outside air temperature.

I highly recommend acquiring a batterypowered thermometer/hygrometer and checking it daily. Keep it in your case during any winter or summer outings, and when you’re not traveling, it can live in the room where you keep guitars. Take appropriate actions whenever conditions exceed what you have previously established as safe and normal. And always remember that whenever you are uncomfortable, your guitar is probably uncomfortable, too.