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A good hygrometer for guitarists.

Probably the single most exasperating problem in caring for a high quality guitar is the tendency of the wood to shrink or expand with changes in humidity.  Many expensive repairs could be avoided by understanding the effects of humidity change and by applying simple remedies available for dealing with them.

Nowadays almost all inexpensive guitars are made of plywood.  Although plywood is mediocre as a tonewood, it is reliable and durable and withstands considerable atmospheric abuse.  High quality guitars, on the other hand, are made of solid wood throughout, which is the primary reason they sound so much better than less expensive instruments.   Solid woods, however, are more vulnerable to humidity change because they shrink and expand much more than plywood.

When humidity is very high, a guitar may sound, for want of a better word, “soggy.”  It lacks volume and projection and has a dull lifeless tone.   It is also possible for a guitar to suffer structural damage when humidity is very high.  A common problem is “bloating” in the back, especially if the back is made of rosewood or some other very hard wood.  This is caused by wood expansion and, in particular, changes in the back-to-brace glue interface.  In rare instances, this condition can lead to a back-to-brace glue joint failure.

Excessive humidity can also affect a guitar’s playability.  My guitars are all built with a fingerboard that has a tiny amount of forward bow or “relief”—that’s what it’s called when the amount is just right, otherwise it’s “warped neck.”  If humidity is too high (70% or more) the expansion differential between the fingerboard and neck woods can cause the fingerboard to lose it’s relief, even develop a slight amount of backbow.  This condition can develop quite quickly after a guitar is introduced to a high-humidity environment.  The smallest amount of fingerboard straightening or backbow—too little for an untrained eye to detect visually—can cause considerable adjacent-fret buzzing.

Yet another problem that can be caused by extremely high humidity, as in tropical areas or a very damp basement, is deterioration of the glue and potentially even the wood itself.

The effects of very low humidity, which is a more common problem in temperate regions of the world, are often even more serious.  Extreme moisture loss in the tonewoods sometimes makes a guitar sound brittle or “plinky,” even though it may seem to have a peculiar, nervous liveliness.  At a critical point in moisture loss, the accumulated stress brought about by uneven wood shrinkage relieves itself by producing one or more cracks and sometimes even glue joint failures.

The key concept in understanding humidity is what meteorologists call “relative humidity” (all references to humidity in this article should be taken to mean relative humidity).  This refers to the ability of air to retain and take on water, or to dry out moisture-containing objects it surrounds.  The higher the temperature of the air, the more water it will hold before saturation and precipitation occur.  Relative humidity is the amount of moisture present in the air expressed as a percentage of the total moisture the air is capable of holding at that particular temperature before precipitation occurs.  It is humidity relative to temperature. If the relative humidity is 40%, then the air, if maintained at a constant temperature, will theoretically hold 59.999… % more moisture before precipitation occurs.  As the humidity rises, the air takes on additional moisture more and more slowly.  When relative humidity drops, the air accepts moisture more rapidly.

When humidity is very low, things (like guitars!) dry out fast.

There are a few places in the world, like parts of the American Southwest or other desert regions, where the normal everyday humidity may be low enough to be hazardous for a guitar.  However, low humidity starts to become a problem anywhere whenever buildings are heated.  If the temperature outdoors is 20° Fahrenheit, the humidity 50%, and we take this air inside and heat it to 70°, then the humidity can drop to 25% or less. The colder it gets outside, the more the humidity drops inside.

The ideal humidity level for a guitar varies from one instrument to another, depending on the humidity conditions in the factory or workshop where it was assembled.  The humidity during assembly establishes the basic dimensions of the instrument; this dimensional configuration is permanently locked into the total structure when the guitar is assembled.  Thereafter, when humidity varies, the individual components will shrink or expand unevenly, while the dimensions of the total structure tend to want to remain proportionately constant.

The humidity level in a guitar workshop or factory is extremely important.  It must be measured objectively and continuously with accurate, reliable tools and controlled within a narrow range, especially when assembling bodies. While it is possible for a workshop or factory environment to be too dry, the more typical problem is excessive humidity.  If the humidity is too high during assembly, a guitar may suffer great damage when subjected to a North American winter—severe warping, multiple cracks, glue joint failure, etc.  In my workshop,humidity is kept within a range of 45% ±2% when assembling bodies and ±3% at other times during construction of instruments.   I have found after many years of experience that these ranges assure that a guitar will perform optimally and keep its structural integrity under the widest possible range of humidity conditions. Tracking and managing humidity with the range and accuracy needed in a luthier’s workshop is challenging.  I will take up this topicagain in detail in an upcoming article—under my Just Pass It On project—called “Humidity Management for Luthiers.”

Fortunately, a guitarist need not be as scrupulous as a luthier about humidity management in one’s studio in order to adequately protect a valuable guitar.  The guitar will likely stay healthy if humidity is kept within a range of 30%-70%, provided it was made in an environment where the humidity was between 40%-50%.   If you have a “wet” guitar (one constructed in very high humidity), exposure to 30% humidity could cause serious damage.  On the other hand, with a guitar constructed in very low humidity, 70% may be harmful.  How do you know if your guitar is a “wet” or a “dry”?  When humidity is normal (ca 50%), if you see (feel)  fret ends protruding out the fingerboard edge, or the back looks slightly concave, your guitar is for sure a “wet.”  If on the other hand you hear buzzing when you tap the back—a sign certain that one or more back braces has come unglued—and you have reason to believe the guitar has never been exposed to extreme humidity (80%+) for an extended period of time,you can be pretty sure you’ve got yourself a “dry.”

In recent times the technology of digital hygrometers has improved.  One can now obtain a , consisting of two wireless-linked devices, one for the home/studio and one for case or outdoor mounting.  I have tested the home/studio devices under normal indoor temperature and humidity conditions and found them to be consistently accurate within ±2% (older digital hygrometers could vary by as much as ±5%).  This is a very good humidity monitoring option for guitarists.

It can also be helpful simply to keep an eye on the guitar itself in deciding whether to take steps to control humidity.  The very good indicator on the guitar is the back: when humidity drops, guitar backs always tend to sink in, because of a shrinkage differential between the back wood and the braces.  Conversely, backs develop arch when humidity rises. If a back becomes dead flat, you should be introducing some moisture or storing the instrument in a place where humidity is higher.  If the back develops a concave dish, you should be adding moisture more aggressively. On the other hand, if a back shows an unusually high dome, you should try to get the instrument into a dryer place.

You should be alert to low humidity conditions whenever winter weather keeps the temperature below freezing all day.  If the daily temperature range is 15° to 25° Fahrenheit, you should be adding some moisture to the guitar’s environment.  If the temperature is as low as 10° or lower, you should take decisive action to introduce lots of moisture if you hope to avoid tonewood cracks or other structural problems.

Particularly in winter, a guitar should be stored in its case, far away from any heat source, not on a stand or hanging on a wall.  The case should be kept on the floor, because indoor air in winter is quite a bit cooler (and the humidity is therefore higher) in that area of the room than closer to the ceiling.  One of the best ways to raise the humidity in winter is simply to keep the room temperature lower.  A drop in inside temperature of only 5°, from 70° to 65° for instance, can sometimes raise the humidity as much as 10%.

During periods when humidity is very high, the best remedy is to keep the guitar in an air-conditioned area; air conditioning by itself dehumidifies the air in addition to cooling it, usually to a level of 50% to 60%, depending on outside humidity.  If it is not hot enough outside for air conditioning (during a rainy spring, for instance), you should try to keep the guitar in a relatively warm area, avoiding places like cool basements.   It may be advisable at such times to keep the guitar in a room with a , especially if the instrument is being stored in an area like a basement recreation room.  A dehumidifier running intermittently can maintain a humidity level of 50% to 60%, which is an acceptable level for storing almost any guitar.

Nowadays one can also find a small .  I have had no personal experience with such a unit—such ones didn’t even exist until recently—but they’re reputed to be effective in small areas and quieter than condenser-type units.

There are several devices available to cope with excessive dryness.  If low humidity in your area occurs only in cold weather, a good quality furnace-mounted humidifier is the most effective and hassle-free option.  Installing these units can be quite technical; if you don’t have some HVAC experience, you may want to leave this to a professional.  Cool-vapor, small steam or console humidifiers with evaporation wicks, are also effective humidity sources for any season in limited spaces, and they are easy to operate and manage.

Another remedy for low humidity is a “,” a device which can be placed inside a guitar to release moisture directly inside the instrument.  I prefer a plastic food container with a damp sponge placed inside the storage compartment of the case, preferably the headstock area if there’s room.  But if it’s in the closed compartment, the moisture will slowly get out into the case and the guitar.  These devices are particularly useful when traveling.  However, during the times when added moisture is essential (e.g. severe winter cold snaps), they need re-wetting at least once a day.  One should never use a humidifying device that seals off the soundhole; such a device could create a serious risk of uncontrollably excessive, potentially damaging humidity inside the guitar.

Protecting a valuable guitar from humidity extremes, if approached intelligently, can be done with minimal inconvenience.  It also makes good economic sense.  The most expensive of the remedies suggested above may end up costing considerably less than the expense that can result from neglect, especially if there is a permanent loss of instrument value because of environmental damage.

(This article originally appeared in Guitar Review, Summer 1988.  It has since been revised and updated.)