Let’s get straight to the point
I’ll defer the typical technical explanations about humidity until the end.
The real reason why you should care about humidifying is that failure to do so can cost you practice time, money, psychological torture, and possibly a successful performance.
Why? Because your guitar may end up cracked and sitting in a workshop like mine, undergoing repair.
Every winter when the air is especially dry, I receive a barrage of phone calls and emails. Panic-stricken players who failed to humidify properly contact me to fix their cracked soundboards or backs.
The good news is that once repaired, the cracks will likely not change the sound of their guitars.
What will change are the players’ bank accounts, time spent with their guitars while they’re being repaired, and the instruments’ value. It’s a stressful experience.
Adopt a practical approach
As a devoted guitarist, it’s a given that you need to have your guitar out of its case for often hours at a time. But when you’re done with your practice session or ready for a break of 30 minutes or more, close your guitar in its case with a humidifier that is working.
No cheating here. Your job is to ensure that the humidifier is always filled with fluid and your case’s seal is adequate (we’ll talk more about cases in the next blog installment).
The inside of a case also absorbs moisture from the humidifier and acts as a large moisture reservoir for your guitar when it’s inside. When your guitar is out of the case, close the lid with the humidifier inside to preserve this buffer.
Which humidification device to use
There are several good commercial humidifiers with different designs available from online string dealers.
I favor those with sealed, large-capacity reservoirs over those having low capacity (such as sponges) because they circumvent our laziness toward constantly having to add water when they run dry.
The D’Addario product has two advantages: 1) you never have to add water because you simply replace the fluid packs when they get dry, and 2) the fluid packs not only release moisture when the air is too dry, but also remove moisture when it’s too wet.
If you live in a very dry climate and you practice for many hours at a time, consider buying a room humidifier.
Lastly, the technical details
What’s too dry or too wet?
From your guitar’s perspective, any change in humidity compared to what it was when your guitar was glued together will make the parts move to some extent.
The reason is that wood behaves like a sponge — it loses water and shrinks when the air is relatively dry, and absorbs water and expands when the air is relatively wet.
Once the wooden parts are glued together, their movements — shrinkage or expansion in different directions — can cause trouble, such as a change in your guitar’s action.
A hallmark of dryness is the protrusion of fret ends as the fingerboard shrinks (no, your frets are not growing!).
In the extreme case, dry conditions can lead to severe wood shrinkage, as shown in the first photo above. Here, a split formed along a grain line because something had to give.
Buy an in-case digital hygrometer to tell you the actual humidity level.
The same string dealers sell them. The scale will read in percent relative humidity (RH) — that is, relative to full saturation of water vapor in the air (100% RH means it’s raining).
Luthiers typically control the RH in their workshops between 40–50% in order to match the average conditions their instruments will experience in the real world. If the RH in your case reads higher than 50%, you can safely remove the humidifier until drier weather arrives.
In general, small humidity decreases away from 40–50% are far more dangerous than even large increases because of the high cracking potential under dry conditions.
At high humidity such as beyond 75% RH, wood expansion tends to break glue joints, rather than the wood itself. One sign that the humidity might be getting too high is the appearance of rust on your frets.