When humid, wood absorbs moisture and expands. When particularly dry, wood loses moisture content and contracts. Wood that is too dry is susceptible to cracking amongst other structural issues. Wood that is too wet can affect the tone and responsiveness of an acoustic guitar. Acoustic guitars should ideally be stored between 40% – 60% relative humidity. Use a hygrometer to test the moisture content, inside your hard case and a humidifier to control the moisture content in the surrounding air.
Why Humidity Affects Acoustic Guitars
Wood is ‘hygroscopic’. That means it increases or reduces moisture content, based on the relative humidity of the current environment.
Moisture is a factor for all products manufactured from wood. Essentially any part of your guitar that isn’t metal is subject to change based on the moisture levels of the surrounding air. Changes in relative humidity can cause the cells in wood to expand, if especially humid, and contract if dry.
Equilibrium moisture content
But it’s not quite that simple, the age of the timber also comes into play. Typically wood will start losing moisture from the moment it is lumbered. It will continue to lose moisture until it reaches equilibrium moisture content, meaning it matches the moisture content of the surrounding air.
Once this occurs, the moisture content of the wood will change based on the relative humidity. Up until it reaches equilibrium moisture content it will continue to lose moisture for the most part. This is one of the reasons, amongst others, people often credit older guitars with having better tone, as the moisture content is reduced the guitar becomes more responsive and has greater clarity, at least up until a point.
Humidity and Guitars
Just how much the relative humidity affects your guitar depends on many things, including, as we have already mentioned, the age of the guitar along with the wood it’s built from and how the different types of wood relate to each other (e.g. soundboard and back and sides) concerning how they handle moisture. The construction methods used also play a large role.
While still a consideration for solid-body electric guitars, changes in humidity have less impact on the majority of electric guitars due to the thicker lacquer finish seen on most electrics.
Short Term Humidity Issues
In the short term, humidity can affect tuning stability and intonation as changes in moisture content cause the wood to expand or contract. An acoustic guitar comprises wood and metal e.g. frets and tuners. As the wood expands or contracts, metal objects can loosen.
It also has an impact on how the guitar plays e.g. changes in the characteristics of the wood can loosen the frets, affecting the feel of the neck and affect the action (string height), and general straightness of the neck.
Long Term Humidity Issues
Over the longer term, humidity can cause bigger problems, structurally. Especially if you are taking the guitar from one environment to another e.g. if you bought the guitar overseas or happen to be a touring musician or you live in an area with large fluctuations in humidity between seasons. For instance, if you live in America’s Southwest, humidity can be as low as 20% during winter and as high as 80% at times in summer.
Structural concerns include changes to the neck relief which affects the action of the guitar e.g. bowing inward or outwards and becoming more difficult over time to adjust. This is especially a problem in classical and children’s guitars as they often do not have a truss rod, meaning the neck cannot be adjusted.
There is also the potential for cracking, particularly the bridge, which can also lift away from the soundboard if the glue seal becomes compromised.
Structural damage is more likely to occur where different sections of wood are joined e.g. the bridge, neck joint, back , sides and top, and headstock. Particularly if the tensile and flexible strength of the joining timbers is different as the wood will handle tension differently.
This is one of the reasons guitars often have a binding strip running around the body of the guitar. Firstly, it protects the hard edges where the timbers join but also reduces moisture absorption along the open grains where the edges meet. Lastly, being a more flexible material, the binding allows for some movement at the joint caused by changes in humidity.
Keep in mind, acoustic guitars are already under strain due to the tension required to tune the strings and resonant abilities of the soundboard so how you store your acoustic guitar concerning changes in humidity is important.
Humidity and Tone
Humidity also affects guitar tone. This is especially the case with acoustic guitars which, without the luxury of amplification (in most cases) rely far more on the materials and methods used in its construction for the sound it produces.
It’s not a case of one being more favorable than the other either.
Guitars that are excessively ‘dry’ may sound brittle and thin, at least to many people’s ears, and muddy and less articulate when exposed to high humidity environments as the wood cells expand, causing the guitar to sound duller as the wood becomes less responsive.
Tone is arguably the biggest issue for guitars that are too ‘wet’ in my experience, but guitars that are too dry are a bigger threat of being damaged.
Hopefully, I’ve done a good enough job of convincing you why addressing humidity is important, especially for acoustic guitars. In the following section, we’ll dive into some solutions, both for measuring humidity levels and controlling them.
How to Address Humidity when Storing your Acoustic Guitar
So, now that we have identified the problem, what can be done about it?
Well, the good news is while humidity is something you should be aware of, it’s also relatively (no pun intended) easy to control.
Firstly, consider the area you live in and if the guitar ever leaves home. For instance, if you live in a relatively stable environment with little fluctuation in relative humidity and haven’t noticed a problem with your guitars in the past, humidity is probably not something you need to worry too much about. However, also keep in mind if you use central heating or have an air conditioner controlling air temperature in or near the room you store your guitars, and your guitar is not confined to a case humidity is something you should plan for.
Secondly, consider the materials your guitar is made from. If the guitar is made completely from solid wood e.g. not using laminated timber of any kind it is more susceptible to humidity.
Thirdly, Consider your storage area. Generally, speaking guitarists, who prefer not to leave their guitars lying around on the couch or stashed into a corner have the following options:
- Display cabinet
- Hard case
- Wall Hanger
- Guitar Rack
- Guitar Stand
How you store the guitar will likely depend on how often you play the guitar. Is it your ‘house’ guitar, or one you rarely play?
I wouldn’t recommend using a guitar stand for anything other than while you are playing the guitar, anything more and it’s only a matter of time before it gets knocked over.
Otherwise, a hard case (with the lid closed) is arguably the best option as it’s easier to control the humidity within the confined space of the case. A display cabinet with a controlled environment is going to be a great option, but for most people, a hard case is more realistic.
Otherwise, wall hangers and guitar racks (I use both) are also good options but you will need to measure and control the humidity of the entire room rather than just the case.
Measure the Relative Humidity
Your first step should be to measure the moisture content of the air in the area your guitar is stored in. To do this you will require a hygrometer, which is an instrument used to measure moisture content.
Acoustic guitars should ideally be stored at between 40 – 60% humidity, although closer to the 40% mark is more preferable.
Where you place the hygrometer depends on where you store the guitar. E.g. if you store the guitar in a hard case the hygrometer should also be placed in the case. Several available hygrometers are small enough to place within your guitar’s case and work equally well outside of the case.
If however, you have multiple guitars, it may not be practical to keep them all in cases. In which case, If you store your guitar in a room e.g. using a storage rack or wall hanger then the hygrometer should be placed in the room, near where the guitars are stored.
I’ve listed two of the most highly reviewed hygrometers below. Keep in mind accuracy is the biggest factor when choosing a hygrometer.
Controlling the Humidity
Measuring the relative humidity is of course only half the battle. Next, we need to control the relative humidity of either your case or the room itself to ensure the surrounding air does not become too dry and increasing the risk of damage to the guitar.
To do this we need a humidifier.
When it comes to guitars, humidifiers come in three different types:
- Sound hole humidifier
- In case humidifiers
- Room humidifier
Sound hole humidifiers
As the name implies. Soundhole humidifiers sit in the sound hole, nested between the strings of the guitar.
In case humidifiers
In case humidifiers generally come as a sponge in enclosed case or in the form of a tube that hangs inside the sound hole of the guitar. In some cases using both are a good option with the encased option better suited to the neck and able to be stored in the pick compartment of the case, while the tube protects the body of the guitar. The D’Addario Humidipak for instance comes with packs for the body and neck.
Room humidifiers, because they are required to control the humidity of a room compared to a case are often larger and use electricity.
Humidity is something all guitarists should consider but unless living in an area with large fluctuations in humidity or extremely dry conditions, it’s not something you should obsess over.
I’ve heard guitarists express concerns because they have left their hard case open overnight for example, and this isn’t warranted. While you may record a different reading on your hygrometer as the guitar acclimatizes, within a few short hours of the guitar being placed in the closed case humidity levels will return to normal.
For the most part, if you measure and as required, control the relative humidity of the room or your case, the majority of the time you really won’t run into any major issues. It should also be noted that measuring and controlling humidity is a simple task if you follow the advice above. It also doesn’t need to cost a great deal of money.