Guitar Care – Temperature & Humidity


So you’ve just bought a brand new all wood hand crafted guitar.  Surely after your investment you will want to take care of your instrument as well as possible.  Read on to find out how to best take care of your guitar.  If, after reading this, you have any further questions, don’t hesitate to contact me.

The main dangers to your instrument other than direct physical impact damage are temperature and relative humidity.  Excessive temperature and relative humidity readings, either high or low, could pose a threat to your instrument.  But lets take a look more closely at how these factors could adversely affect your instrument.

In a nutshell, as a general rule of thumb, if you are comfortable, so will be your guitar.  A quickly stated guideline is to keep your guitar at 50% relative humidity (plus or minus 5) and near 20 degrees Celsius (70F – plus or minus 5).  Also, as much as possible avoid extremes and quick fluctuations in temperature and humidity.

Guitars should never be left near a heat source; baseboard heaters, wood stove, forced air ducting – these are all the bane of hand-made instruments since they tend to dry out the air and often guitars stored in proximity suffer from dryness and cracking.

Guitars should never be left in direct sunlight.  This can heat the surface of the guitar and can cause excessive or uneven patina development, or in worst case scenarios can cause glue joints to fail.

Guitars should never be allowed to get too warm.  Between 10 to 30 degrees Celsius (50-85F) should be fine, and even a bit more or less.  But above 35 or 40 degrees (aprox.100F), and glue joints can begin to soften which can lead to damage described further below.  The further the guitar temperature goes below “comfortable”, the greater the thermal shrinkage will be.  This can result in cracks or finish “crazing” described further below.

Guitars should NEVER be left in the trunk of a car, in any season.  In summer, the cabin area of the vehicle can get extremely hot while parked and the AC is not running.  In winter, the cabin can get too cold when parked and the heat is not active.

Guitars SHOULD be left in their cases for a period of time to acclimatize to ambient room temperature if they have gotten too cold.  It is rarer that guitars will have gotten too hot, and the risk is lower when opening a hot guitar in a comfortable room temperature compared to the risk of opening a cold guitar in an ambient room temperature.

A more detailed understanding of the concerns about which we should be knowledgeable in order to prevent easily avoidable issues will take some more time.

Relative humidity (RH)

Wood is a hygroscopic material, meaning its porous nature will absorb and release moisture to reach equilibrium with the surrounding air’s relative humidity.  As wood absorbs moisture, it expands.  As it releases moisture, it contracts.  Wood will expand and contract more across its grain rather than along the length of its grain.  This means that the wood faces that will experience the greatest expansion & contraction due to changing RH will be those that are the widest measured across their grain – the back and the top.

The great thing about a well built guitar, however, is that it is built with expansion joints in mind.  Since the top and back are built with a slight dome or curvature, they can flex outward (perpendicular to their faces) and grow “more convex” or “less convex” as needed.  They are built this way to avoid exploding the seams of the top-to-side joints and back-to-side joints as relative humidity rises.  Also, as the top & back shrink when RH sinks, rather than immediately splitting the softwood top or hardwood back along a grain line, the convexity lessens to give more safe crack-free headroom for low humidity periods.

With this expansion and contraction in mind, many builders aim to build a guitar near 40% relative humidity.  A good target RH for regular storage of the instrument is a humidity that is just slightly higher than that in which it was built.  Hence, it is fine to aim for a relative humidity of 45% to 50%.  Up to 55%  should be fine, too, but higher than this and some guitar owners report a duller tone from their instruments, and much higher than this and you can start to see geometric changes with the instrument that might even require alteration in the guitar’s setup to ensure optimal playing comfort.  If the RH drops too far below that at which the guitar was built, this can often surpass the ability of the dome to recede which leads to one or more splits along the grain in the top and/or back.  Excessively low humidity can also affect the geometry of the instrument which would require a change in the setup to return to optimal playing comfort.

* If your guitar requires a setup to return it to optimal playing comfort due to high or low RH, it is usually best to return the guitar to its optimal RH!!

For people in areas requiring winter heating in their homes, it is a good rule of thumb to begin humidifying once heating season begins, and stop humidifying when heating season ends.  Often, in many temperate parts of the world, this is all that is needed.

However, there are some locations that suffer extremely high summertime RH, or extremely low RH even without central heating.  So, how should we assess and control this variable?  The answer starts with a properly calibrated hygrometer which measures the relative humidity of the air.  There is a lot that can be said about hygrometers.

– Dollar store analogue mechanical hygrometers – These can be fine!!  Look at half a dozen, and avoid purchasing ones that show a reading far from the median or average.

– Amazon has many different electronic hygrometers available.  Many of these sell for only 5 or 10 dollars.  Some can be calibrated, some cannot.  Any of these can be fine, but we want to make certain we get an accurate reading.

– Calibration – Boveda produces and sells a hygrometer calibration kit.  The dollar store analogue hygrometers can easily be set by hand when using Boveda’s calibration kit, and even if the hygrometer shows an increase/decrease that is not precisely accurate, it will still tell you when you are near your target RH and when you are way off your target RH, so these hygrometers will do just fine for guitar maintenance purposes.  Electronic hygrometers that cannot be set will still work fine, but if the reading is 15% off, then you will always have to calculate plus or minus 15% when doing your readings.  If you buy an affordable non-calibrating hygrometer from Amazon, I recommend you buy a few, test them with Boveda’s product for accuracy, and use the ones that give an accurate measurement.  Hygrometers that can be calibrated should be tested and set.

Your hygrometer should be kept where you store your instrument, and it is a fine idea to have both case and room hygrometers to understand your guitar’s environment more clearly.

What do you do when your relative humidity is too low?  Buy a guitar humidifier or room humidifier.  Guitar humidifiers come in many styles.  I recommend two types.  The cannister humidifier and the Kyser soundhole humidifier.

Cannister humidifiers – Buy these if yo store your guitar in its case.  These have a fairly large reservoir that holds and releases a good amount of humidity.  They clip into position between the strings and are held in place in the soundhole.  These humidifiers will humidify the guitar from inside its body.  One thing to note is that many guitar cases create 2 independent air chambers which are separated and sealed by the soft foam and/or material of the neck chamber.  With cases like this, it can be a good idea to wrap a second cannister humidifier with a bandanna and leave it under the headstock to ensure the fingerboard is properly humidified.  Do NOT attempt to humidify the whole guitar by leaving one or two cannister humidifiers in the headstock area of the case.  The bulk of the humidity will NOT enter the body chamber in many styles of guitar case.

Kyser soundhole humidifier – For people who like to keep their guitars on a stand or wall hanger, this humidifier fully covers the soundhole and keeps the humidity inside the guitar body when not playing.  It should be noted that these will NOT humidify the fingerboard, so the fingerboard should be oiled regularly with lemon oil (at least every month in dry conditions, perhaps 2 or more times per month).  Even so, fingerboard shrinkage may result.  The Kyser soundhole humidifier comes in 2 sizes for steel string and for classical/flamenco.  It has a redundant plastic ring on it that I find only gets in the way, so if you buy one, feel free to cut it off and discard as I have done with mine.

What do you do when the RH is too high??  Buy a room dehumidifier or a case dehumidifier.  There are several silica bead products on the market which will suck humidity from the air.  My preferred product is a 40g aluminum silica bead cannister (about the size of a medium smartphone) that can be used similarly to a cannister humidifier, only in reverse.  In a well-sealed guitar case, this dehumidifier will drop the RH by about 15 to 18%.  The silica beads change color when saturated, and can be recharged (ie: the moisture content dispelled) by heating on low in the oven for an hour.  Using your hygrometer, use one or two of these dehumidifiers as is needed.

It should be stated that Boveda makes a two-way humidification-dehumidification product that, while I was initially sceptical, after testing found out that it does indeed work as advertised.  This product is NOT rechargeable like the above suggested humidifiers and dehumidifiers, so requires regular purchase of the “humi-packs”.  This option may be desirable and the perfect solution for some people – just keep replacing them as needed and don’t bother even thinking about your RH!  However, other people may prefer the “buy once and use for life” solution of cannister humidifier/dehumidifiers.

* A few points for special situations!!  People who live in consistently high relative humidities, if they cannot control their ambient room RH, should seek a builder who builds at a higher RH than 40%.  For example, if your ambient storage RH will be 70-75% RH, then a builder who makes instruments at 60-65% RH is likely a better idea for you.


As stated in the introduction, extremes in temperature should be avoided.  Far too hot, and glue can soften resulting in glue creep (when the glue becomes like putty and joints move from their initial position) or in worst case scenarios glue joints can separate.  Finish at glue joints can sometimes break, causing aesthetic distress for the owner.

Far too cold, and cracking can occur due to differing shrinkage rates of wood.  As the guitar approaches or exceeds zero degrees, it will experience thermal contraction to an extent that can lead to wood cracking.  A common cold shrinkage symptom is top cracks beside the fingerboard.  Also, certain finishes are prone to “crazing” – which is a broken eggshell pattern across the finish caused by the finish experiencing a greater degree of thermal shrinking than the wood to which it is applied.

Many guitars can in fact withstand fairly low temperatures – well below freezing.  But some will crack due to minus temperatures, and some will experience finish crazing.  Also important to understand is the necessity of allowing time for your guitar to acclimatize to room temperature before opening the case (and shipping box, if it is a guitar you have just received).  A guitar that is at minus 5 degrees Celcius (aprox. 25F), for example, may survive just fine with no damage.  However, if that guitar case and/or shipping box is opened in a room temperature of 20 degrees Celcius (70F), the thermal shock and differential of sudden expansion rates can be enough to cause a few different types of cracks on your instrument.  Let the guitar sit in its case (and/or shipping box) for as long as you can resist – usually a few hours is fine – to allow the guitar temperature to slowly return to room temperature.  Then you can open the case safely.

Shipping Guitars

Guitars should be shipped with a trustworthy carrier, and should ALWAYS be shipped in a case AND insured.  To be eligible for insurance, many shippers will require the guitar case to be packed by themselves in a proper shipping box, or they will require an inspection of the packing box interior before it is sent.

Guitar strings should be loosened by a few whole steps.  This is one situation where the best option is to add one or two Boveda “humipacks” to the case.  Steel string guitars in particular can benefit by adding soft, non-finish damaging material around the headstock to prevent a headstock snapping off in the case that the box is dropped on its front.

I personally avoid shipping when day and night-time temperatures will drop below -5 degrees Celsius (25F) to avoid potential for cracking.  As well, it could be stated that it is never a bad idea to avoid shipping a guitar during a heat wave in high-temperature areas.  Don’t forget, most delivery services do not guarantee temperature or humidity controlled environments.  While your instrument is sitting in the delivery truck, if it is -20C outside (-5F), it is quite possibly -20C inside the truck’s cargo compartment.  That perfect nitro finish that your seller confirmed with photos is now crazed like crazy!  If it is +35C (95F) outside and sunny, the cargo compartment may be as high as 55C (130F) degrees or more.  Now that lovely hand made guitar needs all its back braces re-glued, its bindings repaired, and its French polish touched up at the seams.  Also remember, if the guitar will be shipped by air, even though most cargo holds on a plane are heated somewhat, it is impossible to say how short or long the guitar will wait in the wagon on the tarmac in extremely high or low temperatures.


If you are comfortable with the temperature, so should be your guitar.

Avoid both extremes in temperature, and abrupt changes in temperature.

Avoid extremes in relative humidity, and get a hygrometer, humidifier, and dehumidifier.

Be careful when shipping.


Now, off to enjoy practicing on your lovely hand crafted all wood instrument!!

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