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Controlling the Detrimental Effects of Humidity on a Fine Acoustic Guitar.

5th March, 2008

Sean Hancock, BDesSt



Relative humidity is a measurement of the percentage of moisture in the air.  The effects of both high and low humidity are detrimental to an acoustic guitar.  The body of an acoustic guitar is constructed from timber, which is a material greatly affected by humidity.   To protect a guitar from low or high humidity you will need to know the level of humidity around the instrument.  A digital humidity gauge as oppose to an analogue gauge is the best way to measure the level of humidity.  High humidity will swell the top, back and sides of an acoustic guitar, in some cases actually heightening the action, as the top bulges upwards.  This swelling will subside as the humidity comes back to a normal percentage (around 50%).  Low humidity is extremely detrimental to a guitar, as it will cause the top, back and sides to shrink, causing cracks in the timber.  In general, the instrument should be left in its case as much as possible during high humidity (above 70%) or during low humidity (under 35%).  Generally, a few hours of playing during high or low humidity should be okay, as long as the instrument is kept in its case while not in use.  If the instrument is exposed to low humidity, a humidifier (see Picture 1) should be kept inside the guitar until the humidity comes back to normal.  Generally, a humidifier is placed inside of the guitar via the soundhole.  Humidifiers should always be liberally filled with purified water and refilled, when dry, until the humidity comes back to normal.  A humidity gauge and humidifier can be purchased from most luthier suppliers.  The proceeding paragraphs explain these concepts in more detail.



What is Humidity?


Humidity is simply a term used to describe moisture (water) in the air.  Our atmosphere is capable of holding a certain amount of moisture depending on a number or factors including location, latitude, time of year, weather conditions and proximity to large masses of water, for instance the ocean.  Even common air conditioning will lower the humidity in a room or vehicle.  The standard measurement of humidity is ‘relative humidity’ this is a percentage of possible moisture in the atmosphere.  Relative humidity can range from 0% for zero moisture in the atmosphere to 100%, the highest possible amount of moisture that the atmosphere can contain.  As a guide, the relative humidity will be lower in dry environments like desert areas far from the ocean and will be higher near water.  The humidity level will also rise when there is a lot of rain or even overcast weather.  On average, tropical areas near the equator will have a higher humidity than areas with low or high latitudes. 



How is Timber Effected by Humidity?


Most materials, like metal and plastic, will not be affected by humidity, however, timber is extremely susceptible to humidity.  The technical term for a material that is susceptible to humidity is ‘hygroscopic’.  High humidity will cause mould to grow on many materials, especially leather.  However, timber is one of the only materials that will actually expand with high humidity and contract with low humidity.  This is due to the fact that living vegetation holds water in its cells naturally.  When the tree is felled and the timber is dried or cured the ‘free water’ contained in the cells will evaporate first.  After the free water has totally evaporated more water will evaporate from the actual cells themselves this water is called ‘bound water’.  During this stage the timber will actually shrink until the timber is totally cured and ready for use (see Diagram 1 & 2).  This timber will still continue to absorb water in high humidity and lose water in low humidity.  This absorption will mostly take place though the ‘end grain’ as the cells of the timber are open and porous at the ends of a piece of timber.















How Does Humidity Effect Acoustic Guitars?


Guitars are mostly constructed from timber.  The effects of humidity have long been known by acoustic instrument makers.  The acoustic guitar, by design leaves almost no exposed end grain.  This is accomplished by the bindings of a guitar covering the end grain of the top and back plate.  Acoustic guitars are also finished, whether by oil, French polish, lacquer or polyurethane, further preventing water from entering the timber.  Despite these preventative measures, the effects of humidity will still cause moisture to enter the timber and a guitar will swell slightly in high humidity.  In some cases this swelling will cause the action to heighten, as the top becomes more ‘domed’ and the bridge rises. 

The greater problem, however, is when the humidity is too low.  When the humidity is low, the instrument’s top, back, and sides will shrink as the moisture leaves the timber.  This shrinkage can cause the timber to crack.  These cracks, when viewed in low humidity, are open and will actually close up as the humidity comes back up to normal.  It is interesting to note that every time timber swells and shrinks a slight amount of shrinkage remains.  When this cumulative shrinking is added, cracks are inevitable.  This is one of the main reasons older instruments contain cracks.  These cracks also remain open and have to be repaired with a splice of matching timber. 

It is interesting to note that the sound quality of a fine acoustic guitar will change depending on the relative humidity the instrument is exposed to.  The tone will become more crisp and the volume slightly louder as the timber becomes drier due to lower humidity.



How do Luthiers Work with Humidity?


The detrimental effects of humidity can be prevented or limited during construction and also during the life of an instrument.  The raw material used for the top, back and sides should always be waxed or painted at each end while the timber is seasoning or curing, so that the timber does not dry too rapidly.  A luthier should always use completely seasoned timber.  It is not unusual for a luthier to use timber years, sometimes decades, after it has completely seasoned.  A fine guitar should be constructed in a humidity-controlled environment.  Around 40% to 45% relative humidity is acceptable.  The only way to keep the relative humidity level in a workshop constant is by the use of an electric dehumidifier for high humidity and an electric humidifier for low humidity.  The level of relative humidity can be checked quite accurately by using a humidity gauge.  The author has found that in general, high quality digital humidity gauges work accurately enough for instrument building.  In general, analogue gauges are inaccurate unless they are of a very high quality and therefore cost.  Humidity absorption in the finished instrument can be further controlled by lightly finishing the inside of the instrument.  When the inside and outside of an instrument are sealed, humidity will penetrate the timber slower and in a more even manner.    



How to Protect a Guitar from Humidity


To keep the effects of humidity from harming an acoustic guitar the owner needs a humidity gauge and a humidifier (see Picture 1).  These items can be purchased from many luthier suppliers.  The humidity gauge can be left inside the case of the instrument to monitor the humidity level in the case.  The gauge can then be taken out to monitor the relative humidity level in the environment that you are playing the instrument in.  If the humidity is particularly high (above 70%) the instrument should be left in its case as much as possible.  A few hours of exposure should not affect the instrument but leaving the instrument lying around outside of its case for a day could be detrimental.  A high quality case with good seals is recommended, as it will act as a humidity buffer when left closed.  It is interesting to note that as the damaging effects of humidity are becoming more well known, cases which have an inbuilt gauge and humidifier are being produced more and more.  Some of these cases are low quality and not recommended as they could give false readings but high quality cases are available.


It is most important to monitor for low humidity (under 35%), as the effect can be much more damaging to a guitar than high humidity.  Again the humidity gauge can be used to check the level inside the case and then in the atmosphere.  Again, a few hours of exposure during playing should be fine.  If the humidity in the case becomes low due to long time exposure of low humidity from the outside environment, then a humidifier should be used.  Humidifiers are devices that release small quantities of water vapour over a period of time until the water in the device evaporates.  A humidifier should be filled only with purified water or mould can grow inside the instrument.  Beware not to fill the humidifier with excessive water.  If possible remove excess water by squeezing out the water holding medium (usually a sponge).  Generally once filled, the humidifier is placed inside the guitar via the soundhole.  The humidity level will rise while the guitar and humidifier is in its case and the level can be monitored with the gauge.  The humidifier can be left inside the guitar during playing if the relative humidity level in the atmosphere is very low.  Humidifiers have little effect on the sound quality of the instrument.  Lower humidity will cause the tone to be slightly more crisp and clear and the volume will also increase very slightly.  The humidifier can be taken out of the instrument when the humidity comes back up to an acceptable level, (around 50%).  Remember, a humidifier will dry out eventually, so when in use, check that it always contains moisture.


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