Effects of Humidity on Acoustic Guitars

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Percentage,
Relative Humidity
Short Term Exposure (1-3 days) Long Term Exposure (3+ Days)
90% - 100%
Relative
Humidity

EXTREMELY HIGH RANGE

Guitar body will appear swollen and sound quality will be diminished, playability will decrease. Back braces may come unglued as the wood expands. Action will get very high quickly.

All Glue Joints will loosen, Top and Back braces will loosen. Bridge may shear come off the top, the guitar top will expand and belly (become convex) both in front of and behind bridge, if the glue joints do not delaminate then the guitar will be unplayable. Fretboard from the 14th on will appear raised. Mildew may form inside the guitar. Guitar will very likely de-construct itself.

85% - 90% 
Relative
Humidity

HIGH RANGE

Guitar body may appear swollen, sound quality will slightly diminish, playability may decrease. Action will become higher.

Braces will come loose after a few weeks, top and back will appear very bellied (convex), bridge may loosen or come off, playability will be affected. Fretboard from the 14th on will appear raised. Mildew may form inside the guitar.

70% - 85% 
Relative
Humidity

MODERATELY HIGH RANGE

Sound quality may be diminished. Soundboard may appear swollen. Action may be slightly high

Top and back will appear bellied (convex), playability will be affected. Fretboard from the 14th on may appear raised. Guitar will start to have a musty smell after a couple of months

55%-70% 
Relative
Humidity SAFE RANGE

No major problems should occur with limited exposure.

Top and back will probably appear slightly more convex than normal. Playability may be affected.

42%-55% 
Relative
Humidity

SAFE RANGE

No problem will occur in this range.

No problems should occur in this range.

35%-42%
Relative
Humidity

MODERATELY LOW RANGE

No major problems should occur with limited exposure.

Fret ends may feel sharp, soundboard may appear slightly collapsed (concave), action may lower slightly, bridge wings will appear concave, the guitar may develop a buzz.

25%-35% 
Relative
Humidity

LOW RANGE

Fret ends may start to feel sharp, top may become slightly collapsed (concave)

Fret Ends will feel sharp, Soundboard and back will become flat or collapsed (concave), action will feel lower, guitar will likely buzz, Bridge wings will appear concave, after several months the bridge may “lift” or shear off.

20%-25% 
Relative
Humidity

LOW RANGE

Frets will feel sharp, top may begin to collapse (concave), Action will lower very quickly, guitar may develop a buzz

Fret Ends will likely feel very sharp, Soundboard and back will become flat or collapsed (concave), last six frets of the fingerboard will likely sink into sound hole, the action will lower, and guitar will buzz, the Bridge wings will appear concave, cracks in the soundboard may develop especially from the bridge to the butt of the instrument, the bridge may shear off (come unglued).

10%-20% 
Relative
Humidity

VERY LOW RANGE

Frets will likely feel sharp, top and back will likely become collapsed (concave), Action will lower very quickly, guitar will likely develop a buzz. Soundboard may develop a crack running from the bridge to the butt, bridge may come unglued.

Fret Ends will feel very sharp, Soundboard and back will be collapsed (concave), last six frets of the fingerboard will sink into sound hole, action will be lower, and guitar will buzz, Bridge wings will appear concave, a large crack in the soundboard will likely develop from the bridge to the butt of the instrument, and the bridge may come unglued. Rosette rings and tail wedge may be visibly raised.

0%-10% 
Relative
Humidity

EXTREMELY LOW RANGE

Frets will feel sharp, top and back will become collapsed (concave), Action will lower very quickly, and guitar will develop a buzz. Soundboard may develop cracks especially running from the bridge to the butt, bridge may shear off.

Fret Ends will feel very sharp, Soundboard and back will become collapsed (concave), last six frets of the fingerboard will sink into sound hole, the action will be extremely low with buzzes up and down the fingerboard, the Bridge wings will appear concave, cracks will develop in the soundboard especially from the bridge to the butt of the instrument, and the bridge will shear off. Rosette rings and tail wedge will appear raised. Braces which do not shear off may push out the binding of the instrument Braces will be visible as high spots on the top and back.



Acoustic guitar care, courtesy of guitaradoptions.com

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