The Importance of Humidity Control in an Acoustic

Today I want to go into the effect that humidity has on acoustic guitars.  I live in Southern California and contrary to what people might think, humidity is just as much a factor in guitar longevity as it is on the east coast.

Acoustic steel strings can be categorized in two ways, plywood tops and solid tops, with either plywood backs and sides or solid.  The first are usually entry level guitars that are typically priced below $200.00.  This does not mean they can’t be set up to play well and sound quite good but they will never live up to their solid wood counterparts, all things being equal.  They are made up of three or more layers of wood, usually spruce but sometimes maple or other hardwoods.  Some of the curly maple veneers used in these guitars can look pretty flashy but don’t be fooled by the glam, you pay a premium for these guitars and your money would be better spent on a solid top.

Most of these plywood guitars are battleships (not to be confused with dreadnaughts) that are great to take to the beach, mountains etc.  While the effect of humidity is present, these guitars are much better at handling changes.

The next step up would be a solid top with plywood back and sides.  This is where we must start considering how we’re treating the instrument in regard to moisture.   The solid top is a mere 1/8″ and can move a great deal with temperature and humidity. Usually these guitars are in the medium price range but not always.  Some custom builders like the extra stiffness laminated sides make and consider them superior.

The third is the all solid wood guitar.  These guitars can have a huge difference in price.  From about $700.00 to as much as you want to pay.  An anniversary Martin can run over $90,000.00.

Of course there are other things to consider: wood choices, kind of bracing, finish, etc.  A $700.00 Guild GAD is a much different animal than a $20,000.00 Somogyi but they all have something in common, they need to be treated with care.  Most manufacturers and builders suggest 45% humidity.  This is simply a middle ground that guitars are built at.

Guitars can also take moisture better than they can give it up.  When a guitar starts at 45% and goes to sa, 60% the wood will swell but not usually crack.  The opposite is true for giving it up.  If you’re at 45% and the humidity drops to say 15% ( this can happen in a matter of minutes in Southern California if the Santa Ana’s roll in) there is a good chance it will split.

Here’s a good chart that Kevin Ryan of Kevin Ryan Guitars developed:

Dear Friend,
Here is a chart I have made for your reference.  If you become familiar with these numbers and are conscientious about proper humidity for your instrument, your guitar will remain in pristine condition through many, many years. 

The figures below represent RH (Relative Humidity):

100%; You shouldn’t really be playing your new guitar out in the rain
95%; This is dangerous for your instrument; glue joints are compromising right now and the thin wood plates are highly stressed and buckling due to their swollen condition 

90%; This is far too humid for your instrument; the action of the strings is very high; maybe it’s time for some air conditioning for both of you? Bad things are possibly going to start happening to your instrument 
85%; Too humid; your wood plates are beginning to swell with the moisture; this isn’t good 

80%; A little too humid I think (plus, aren’t you getting uncomfortable?); soundboard movement is starting to affect the action (making it higher over the frets) 

75%; Probably getting too humid; if it keeps up you may actually notice the soundboard movement; sort of OK for awhile 

70%; OK for awhile but don’t let the guitar get too warm; more wood movement with the soundboard bellying out somewhat perhaps 
65%; A little too humid; there might be a small bit of wood movement but don’t panic
60%; Still sort of OK
55%; Not too bad
50%; OK  


40%; OK 

35%; Time to think about humidifying your guitar; the soundboard is starting to sink in; probably will be OK for a few days so don’t panic (yet!)
30%; It is really time for humidifying your guitar, (a few days might be ok); action starting to get low; maybe you can start to feel the ends of the frets beyond the edge of the fretboard (which has shrunk back due to moisture loss); install the Planet Waves Guitar Humidifier when you are not playing the instrument
25%; Time to be really concerned; time is not on your side; take corrective action now; use the Planet Waves Guitar Humidifier and put the guitar in the case until the dry conditions are over; cracks are planning their assault; frets are hanging over the edge of the fretboard now
20%; Danger Will Robinson!!! You are living on the edge now; huge stresses are building up in the plates of your expensive instrument and; cracks may start to appear at any moment; the soundboard is sunk in and you have string buzzes
15%; Give me a call and we can discuss a time slot for your repair
10%; Now we need more time to fix all those cracks and glue the thing back together
5%;   It’s over!


There are several good humidity control products by several different companies.  Planet Waves, Oasis, Martin, Crafter are a few.  They all work about the same, a sponge or other media, such as silica, that absorbs water to give it up gradually.  They can fit in the case or in the guitar soundhole.  Even a plastic container with a sponge and a lid with holes poked in will work.

Some of these even include the hygrometer ( I recommend these).  If you don’t have one you should get one.   You can’t evaluate what action to take if you don’t know the % humidity.

I know this is a rather dry subject but important.

Cheers ’till next week