Humidity & Your Guitar’s Health

thermohygrometer_meade
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 topic again 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 reasonably-priced thermo-hygrometer set, 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 condenser-type dehumidifier, 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 desiccant-type dehumidifier.  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 “Dampit,” 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.)

36 thoughts on “Humidity & Your Guitar’s Health”

    1. I’ve never tried this because I’ve always had to the resources necessary to maintain optimal ambient room humidity, so I don’t know from experience how well this would work. It might be the best available option if one expects to travel with a guitar to places where humidity is known to be very high. I see some problems with this approach: (1) one would have to keep the case closed except when playing the guitar; (2) one would have to monitor the the humidity regularly with a reliable in-the-case hygrometer, and; (3) how to know when the desiccant granules are saturated and need a change. It seems to me that a small studio dehumidifier is still the best for a guitarist with a guitar at home.

    2. Debora: It has been my experience that desiccant packs are somewhat of a sand that will only absorb a certain amount of (water). I do not see how they could hurt in an enviornment of very high humidity, but I guess you’d have to change them out often. Being that they keep absorbing moisture wherever they are, I don’t know how you would keep them dry enough to do the intended job. I live in south Texas where the humidity is always 70% or higher consistently. I have several high quality guitars for many years and have not had any problems so far. The swing in humidity rates here are such that it usually evens everything out

      1. Tommy.
        Thanks for the advice. The waves are still here though now it is winter and RH is way down to 34%. There has been no adverse structural or sound effect on the guitar so I am just not worrying about it, though it does irk me aesthetically.

  1. Having read this article,I have concerns about my new guitar,steel body/mahogany neck.
    I spend alot of time living unconventionally,mobile home,no electricity and high relative humidity levels.
    I have a digital hygrometer and am using reusable,self indicating silica gel packets inside my semi hard shell case.
    Would putting my guitar in case inside a XL sealable vacuum bag offer more protection,with silica packets. inside both bag and case,and if possible dangle some inside the ‘f’ holes to protect the neck stick?
    Is difficult to maintain a constant relative humidity below 70%.
    Your comments/suggestions would be greatly appreciated.
    Regards,
    Phil.

    1. What you’re already doing would seem to be about as much as can be done in the circumstances you describe, viz. the absence of electricity, which would allow you to use a dehumidifier. Placing desiccant pellets anywhere in the closed case—for instance, in the headstock compartment—should help to protect the entire guitar from excess moisture. Keeping the guitar in a sealed plastic bag would probably be more trouble than it’s worth if you’re playing it regularly while in your mobile home.

      1. Thanks for your response PJ,
        I’m now using some crystal air dehumidifiers as well,so guess as you suggest,I’m doing all I can within limitations.
        Phil.

      2. Phil: Have you tried a product called DAMP RID ? works well in high humidity situations. Just a personal question, I dont know if Id put your steel bodied guitar in a plastic bag, because it would seem to me that any moisture in the air in the bag would stay in the bag. Jus wonderin……

  2. Phil:

    Yes, it is hard to find good info on remedying high humidity. I tried silica packs in my hard case with my classical guitar. I did not put the packs right on the guitar, but in the spaces under the guitar. I started to notice pits in the varnish on the guitar top. So I discontinued this practice figuring there was some sort of chemical reaction going on (no actual proof of this with facts). Also it did not cure the problem. As the guitar plays well and is not pulling away from the inside frame I am just accepting high humidity and the waves that it causes in the top below the bridge.

    All inquiries I put out told me to get a humidity controlled display case ($7k) or get air conditioning for my house. If you find a good solution let me know.

    deb

    1. I doubt that the silica packs are causing “pits in the varnish on the guitar top,” Deb; silica gel is chemically inert. Nor is it likely that high humidity is causing “waves…in the top below the bridge”; this is a normal result of string tension on the bridge over time. You don’t need a $7,000 case or even air conditioning in your house: if the guitar is kept in a closable room with a small dehumidifier, this will keep humidity at a very tolerable 50%-60%.

    2. Hi Deb,
      I’m now using two crystal dehumidifiers as well which soak up moisture and last about 6-8 wks before you have to change the crystals. (available online or most good hardware shops). I may get one more.
      I’m in a caravan mostly so moisture at night always a problem at this time of year. ( relative humidity often over 80% during the night/early am )
      I keep one next to the guitar case at night,with silica packets inside the case. I also wrap a ‘shammy/chammy leather around the neck.
      Without electricity,I think I’m doing all.I can now to protect my new resonator,she’s amazing by the way!
      If I come up with anything else,I’ll post on here.
      Regards,
      Phil.

  3. Hi, great article. I have an all wood instrument and took in up to the mountains in 14% humidity, very dry. Have had it here for a week. I have noticed a slite warp in the neck only, raising the action, again only slightly, but noticeable. I’m leaving to go back down to lower elevation tomorrow.
    Do you think the neck with straiten itself out with average 50-60% humidity? Didn’t notice any cracks or other issues. Will use a solution mentioned in your article next time. But a little worried about the warp.
    Thank you
    Dean

    1. Taking the guitar back to 50%-60% humidity should fix the problem, Dean. You might want to relax the string tension for a couple of days when you get there to help things along. If you “go up to the mountains” again, you will definitely want to add some humidity.

  4. Thanks PJ:

    The ‘waves’ are parallel to the strings just below the bridge. I had thought about string tension but when the guitar came 2 months ago (from sunny and dry southern California) the top was flat. I just figured that this was humidity related (70-85% ) because it showed up about 1 week after arrival. That’s good to know about the silica. Dehumidifier is not practical as I have a small very open house with all doors of the louver variety. Maybe I’ll build my own cabinet and stick a dehumidifier in it. Or just wait to see what happens in winter when the humidity goes down to 50%

    deb

  5. Thanks for the useful advice. My archtop guitar was exposed to high humidy so is showing some bowing at the s type sound holes. My home is controlled at a humidity of 50 to 55% so will the guitar return to normal or do I need to take addition action? Many thanks.

    1. You needn’t do anything else about the humidity; however, it’s hard to predict what normalizing it, as you’ve done, will change in the guitar, if anything.

  6. Hi, I haven’t read all the comments so i don’t know if anyone has asked this question. I have read that it’s not recommended guitars be hung on the wall during the winter months. I love hanging mine on the wall so that they are easy access…..and I love looking at them! 🙂 I like to be able to grab one and not have to get it from the case every time. I practice more when it’s on the wall. I am concerned regarding the lack of humidity during the winter months. I have gas heat and a heat pump. I live in southeast Va. I have 3 guitars on the wall. A Taylor 814CE…Takamine EF508C and Fender Strat. Is there a good way to humidify the room? I’m thinking about buying a humidifier and a hygrometer. Would appreciate opinions and advice. Thanks! Jo

    1. You should be able to hang your guitars on a wall in your location, Joan, if do three things: 1) Purchase a hygrometer; 2) Purchase a small humidifier for your studio, and; 3) Stir up the air in your studio with a fan—a ceiling fan is best, but a smaller fan aimed at the ceiling and run constantly will probably be adequate. #3 will drive the warmer (drier) air near the ceiling down toward the floor and help to equalize the humidity throughout the room. The humidifier needs to be mobilized when your hygrometer is telling you that the humidity is <30%. Maintaining 35% is best for your guitars.

  7. My home in Alabama often has humidity below 10 percent in the winter months…..is it even safe to play my acoustics at this level? I have a Guild Jumbo F-512 (12 string) and a Martin D-40. I’m actually afraid to pull them from their cases for an hour or two to practice.
    Any thoughts? Thnx! Tim

    1. I find it hard to believe that there’s any location in Alabama that actually >>…has humidity below 10 percent in the winter months.<< That's a near-desert humidity level. Your Comment makes me wonder, first of all, how you're determining humidity level. Do you have a thermo-hygrometer of some sort? If the humidity in your location really does get as low as you say, your guitars are definitely at risk, and you urgently need to implement one or more of the remedies suggested toward the end of the article.

  8. The rosewood fingerboard on my bass shrunk this year because of low humidity. The frets are now sticking out a little. So it looks like I will have to have the frets filed.

    My question is will the fretboard expand when the humidity in my apartment (in Montreal) goes back up? And it will.

    Other question; If I have the fretboard files all nice and everything, will it shrink again next winter? Right now I’m going anal with hygrometers and I don’t recall this being a problem back in the 70s while touring all over North America.

    Thank you for your time.

    1. Once your fret ends have been trimmed, the problem should not recur. even if the fingerboard shrinks a little next winter. I presume your instrument is an electric bass; if so, I wouldn’t be as concerned about humidity shifts as with an acoustic guitar. (I’m pretty sure humidity changes are quite extreme in Montreal due to cold winters.)

  9. I live in Southern Arizona so low humidity is my issue. The humidity levels are pretty consistent most of the year, with a few weeks exception certain times of year. My two most valuable guitars and ones I am most concerned about are a 1934 000-45 Martin and a Hernandez Classical guitar. Only recently, have I been smart enough to start worrying about this issue. Now I am suddenly panicking about what I should be doing and am feeling a little overwhelmed with all the info on this topic. Can you tell me what I should do in layman’s terms? I’m feeling a little like a fish out of water. Thanks

    1. The first thing I would do, April—if you haven’t already done this—is get a thermo-hygrometer to keep in the room where you store and play your guitars. Then you need to have a way to add moisture when conditions are really dry. If you live in a home you own, I would consult with an HVAC (heating, ventilation and air conditioning) contractor in your area to get your entire house humidified. (Your solution will probably be different from mine because your climate is so different.) If you reside in an apartment, a warm/cool mist or evaporative humidifier kept in the same place where your guitars are located should work well.

        1. In-the-case humidifiers like the Dampit can be of some help when traveling in a dry climate, but at home a more robust approach is better…at least a room humidifier.

  10. I keep my arch top guitar in its case in a room with a humidifier targeting around 40 to 50 percent reading on an electronic hygrometer. I’m curious whether it makes any difference if the guitar is in the case or not. Does the guitar access the room level of humidity while in the closed case? Is it better to leave in or out of the case in this situation?

    1. If you’re maintaining humidity as you specified, Brad, it makes little difference whether a guitar is kept in the case or outside; if it’s inside, the ambient room humidity will reach it, just a little slower. In more extreme humidity conditions, in-case or out- might make a small difference: room temperatures are typically higher near the ceiling, lower near the floor; that means humidity will be a little lower near the ceiling than near the floor.

  11. Awesome article! Been looking for something like this for a while. I have about 10 guitars (a few historic les Pauls, a PRS, a telecaster…to name a few).

    I currently have my guitars on the wall in a finished basement. The temperature has been between 64 and about 70 (usually 68) and the humidity has been between 40 and 50%. Living in the north east, I use a humidifier to keep that level.

    After reading this article, I feel much better about the condition of my guitars in their current environment. However, I have to ask. Is 64-66 degrees too low for them even if it’s not that low for very long? I’m assuming humidity is the bigger culprit.

    Thanks so much again for this awesome article!

    1. 64º-66º is well within the tolerable range, Steve. Temperatures have to get really cold—well below freezing—to be of concern.

  12. I have a selmer style guitar (koa back and sides, sitka spruce top) that is chronically dry. I have to keep two moisture devices in the case at all times, and replenish them daily — but on hot dry days, or in the winter when the heat is on it buzzes and is unplayable. Any suggestions? On the few days its happy, the sound is amazing.

    1. Your comment suggests that you have a guitar that was made in workshop conditions that were too humid, Michael. This forces you to maintain household conditions that match that high level of humidity. I can only suggest that you take it to a competent repair person whose workshop humidity conditions are more like your home; perhaps a refret to remove some fingerboard backbow could help.

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