Why you need to humidify!

This is happens to guitars that are left in dry, arid or cold environments without humidity supplementation.

Why you need to humidify! Save your guitar.

As a hygroscopic material wood absorbs moisture from the air when it’s humid, and releases moisture when it’s dry. This causes the wood to swell and shrink. And if it shrinks too much, guess what… It cracks. This is why you need to humidify!

Lack of humidity may also have other implications, affecting playability, as well as tone.

Above shows what happened when we put the Prolix Music Humidifier inside this guitar (with a sound hole cover) and after just 10 days the crack had healed so well it could be taken to a professional luthier for gluing and cleating. The amazing healing power of the Prolix Music Humidifier brought this guitar back from the crack! 

So if you ever get a crack – Just remember, you can get back!


Why should I humidify?
The wood your guitar is made from is a living, breathing material. It slowly absorbs moisture from the air when it is humid, and slowly releases it when it is dry. This causes it to expand and contract, which affects the action (playability), structural integrity and resonance of your guitar, which is why you need to humidify.

How does humidity affect my guitar?

When it is humid, the body slowly swells as it absorbs moisture from the air. Conversely, when it is dry, the body slowly shrinks as it loses moisture to the air. The amount of moisture in the wood is known as moisture content (MC).

What do humidifiers do?

Humidifiers work through the diffusion of water vapor with the air. Lower vapor pressure in the air causes water to evaporate from the humidifier and be absorbed by the air, balancing the vapor pressure throughout. This is known as the process of equilibration. Similarly, wood gains or loses moisture by equilibrating with the air around it, but at a much slower rate. This is good because it means that you don’t need to supplement the humidity all the time – you should only humidify when your guitar needs it.

What is the ideal humidity?
Most manufacturers use tone wood that has been equilibrated to a 50% Relative Humidity (RH) environment. Hence the ideal humidity is 50% RH. However, it is the Moisture Content (MC) of the wood we are more concerned about, not necessarily the humidity of the air. Luckily, wood is slow to equilibrate, making it immune to daily fluctuations of high and low humidity. As such, it is a myth that you must maintain an absolute 50% RH continuously. Instead your aim should be to maintain a long term average of 45-55% – nominally 50%.

Does temperature affect humidity?
Humidity is a relative thing, hence the term RH. It is relative to the capacity of the air to absorb moisture in vapor form. As you heat air its capacity increases, causing the relative humidity to drop. This is why, despite having reasonable outdoor humidity, it can feel really dry once the air is heated in your home.

What if I don’t humidify?

If you don’t humidify, you may be playing a game of Russian roulette with your guitar. Changes in MC will affect the action (playability) of your guitar, the resonance of the soundboard, and can even result in structural failures such as failed glue joints, 
as well as cracks in the body.

If I live in a humid climate do I still need to humidify?
It really depends. Heating and air conditioning can significantly affect indoor humidity, so you cannot always rely on outdoor humidity as your guide. Ideally you should measure the humidity in the room your guitars live in before making a decision.

Should I humidify all the time?

No, no and no! Only humidify when your guitar needs it. If you humidify all the time you risk adding too much moisture to the wood, which can be just as bad as being too dry. Think of it in the same way as heating your home. The furnace only runs when it needs 
to, not all the time.

How should I humidify?
It depends on how you use your instrument. If you keep your guitar out, play often, and it was set up professionally, then we’d recommend using the action (playability) of your guitar as your guide of when to humidify and when to stop. Be warned that if you keep your guitar entombed in a case, you must use a fully functional hygrometer and monitor it daily to prevent over humidification.


Fig. 1 – How the shape and action of your guitar indicates its condition –
without the use of a hygrometer.


About your tone wood, Moisture Content and Relative Humidity…

After harvesting, wood is typically dried in open air (although modern guitars are kiln dried which makes them much more susceptible to humidity levels or the lack of) – the dry time is nominally 1 year per inch of board thickness. During this time it loses all of its FREE WATER content, which can make up to two-thirds of its “green” weight. It then achieves an Equilibrated Moisture Content (EMC) to the environment, leaving only BOUND WATER – water absorbed by the cell walls.

EMC can vary a lot with humidity. At 10% humidity it’s typically 2% or less, whereas at 90% it can be 20% or more. That’s an order of magnitude change that essentially changes the weight of the wood by 20%, which is significant in terms of tone and shape, which is why you need to humidify!

In terms of strength – wood is generally stronger along its length than it is tangentially or across the grain. Moreover, wood tends to expand and contract more tangentially than it does along the grain.

Also, each species have different properties in terms of it tensile and compressive strengths, hardness, yield modulus and more.

As wood equilibrates to a low RH it shrinks – stressing the wood. It’s not a foregone conclusion that it will crack, but it certainly has a higher propensity to do so.

Beyond cracking, there are other less visible effects. Shrinkage affects playability by causing bad ‘action’ and fret buzz (See Fig. 1). It can cause intonation issues, where it is in tune at the first fret but out of tune an octave above at the twelfth. And it can cause a loss of the wholeness of sound. Its hard to explain, which is why we wrote a completely different article Zen and the art of guitar humidity about it here…, but that wholeness is the full-bodied sound one would like to hear from their guitar as opposed to a more objectionable, hollow sound.

About cracking guitars (makes us shiver down the spine)
Guitars can and do crack when there is a lack of humidity. Not all guitars will, but trust us they absolutely do. Take a look online… Taylor Guitars – one of the greats – has a video series on repairing an excessively dry guitar with cracks… One would like to think Taylor know what they are talking about…


Fig. 2 – How moisture content affects the bowing or sinking on your acoustic guitar.


Fig. 3 – How  Relative Humidity works – moisture wants to escape from your tone wood when the air has a low Relative Humidity.


Fig. 4 – It’s important to note that wood responds slowly to Relative Humidity levels. This is WHY Relative Humidity is NOT a good indicator to the condition of your Tone Wood.


Always consult a professional Luthier
A luthier is familiar with the physical properties of wood. As such, they know that wood is anisotropic, meaning that its material properties are different with the grain than orthogonal to the grain. Wood expands and contracts at a greater rate across the grain than it does with the grain. If wood was isotropic (in that its properties were the same in every direction), you wouldn’t see hocky stick shaped 2 x 4’s at the local big box hardware store.

Anyway, if you allow your guitar to “equilibrate” to a DRY environment, you really won’t be doing it any favors at all. You will distort the shape of the body. (See Fig. 1)

You will start to lose the arch in the top of the guitar, which will affect the action. As the body shrinks the location of the saddle will move relative to the neck which will affect the intonation. And finally, the density of the wood will change which effects its resonance and hence affecting fullness of the sound. And if you are really unlucky you will end up with cracks, which is why you need to humidify, humidify, humidify.

Never over Humidify
That said, be warned against over humidification. Always avoid over humidifying, all we ask is you just maintain the guitar at a reasonable humidity. And if you do have a cracked or overly dry guitar, use a humidifier. Ideally you want your guitar to live in a 50% RH environment. You want the wood to maintain the moisture content that it was built with, which it will do if you keep it at 50%.

If you live in a region where humidity is consistently 50% all year round… then you have no worries, but for those of us that live in areas with extreme dryness, or in areas with seasonal changes, we really need to be aware of the affects of humidity on our instruments.

Wood and humidity are important
Many musical instruments are made from wood. Wood is a naturally hygroscopic material, meaning it has the capacity to absorb water. It does so by equilibrating with the relative humidity of the local environment. Increased humidity causes wood to absorb, raising its moisture, causing it to swell. Similarly, decreased humidity causes wood to desorb, lowering its moisture content, causing it to contract. This can result in many adverse effects, such as bowing, warping, cupping and splitting of the wood. For musical instruments, in addition to structural issues, changes in moisture content can also affect playability as well as tone.

The moisture equilibration process for wood is slow, taking days to weeks, hence short-term changes in humidity have little effect. However, the effect of long-term, seasonal changes can be profound. For musical instruments, ideally the moisture content of the wood should be maintained at around 8%, equating to a humidity of around 50%. Hence, when the ambient humidity drops below that level for an extended period, humidity in proximity to the instrument should be supplemented.

Typically, this would be achieved through the use of a humidifier. For a musical instrument, ideally a humidifier would have the following features: It would be passive, meaning that there are no batteries, fans, heaters, etc.; it would have a high capacity for water allowing it to supplement humidity for multiple days between refills; it would be easily refillable; it would sequester liquid water in a wholly absorbed form, only releasing it through evaporation; it would have a large surface area that has a high porosity to water, hence allowing water to evaporate efficiently; it would not need to be attached to the instrument; it would have an outer shell that is soft and non-marring; it would have an outer shell that prevents direct contact of the absorbent with the instrument; it would be simple to manufacture in a variety of sizes and colors to suit a variety of instruments; it would be simple to use; and it would have a built-in, unmistakable indication of its state of hydration.

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