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Maury's Blog

Wednesday, November 04 2015
Humidifying Acoustic Guitars

Humidifying your guitar is, without a doubt, the most important upkeep you can do for your acoustic guitar. Sure, we all remember to do things like change the strings to keep it sounding nice or polishing it to make it look pretty, but proper care in terms of humidity is what is going to keep your instrument safe and playable for years to come.  

What is relative humidity?

So we know that our guitars should be kept at between 45% and 50% relative humidity. And we know that humidity is the amount of water vapor in the air around us. So what does “relative” humidity mean? Relative to what?

To answer that first we should know that there are 2 ways to measure humidity, Absolute and Relative. Absolute humidity is the mass of the water vapor in a given amount of air divided by the mass of the air itself. If the two masses are equal then the Absolute Humidity is 100%. If the water vapors mass is half of what the air is then the Absolute Humidity is 50%. The amount of water vapor that air can hold depends on the temperature of the air. The warmer the air the more water vapor it can hold and therefore a higher potential Absolute Humidity is possible. Relative humidity is the ratio of the current absolute humidity compared to the highest possible humidity. As Relative Humidity nears 100% the chances of rainfall increase. That’s why on a very dry day you’re probably not going to see rainfall but on a particularly muggy summer day you can almost bet that a thunderstorm is right around the corner.

This is all a very brief and incomplete summary of what relative humidity is but, for our purposes anyway, it gives the general idea of what we’re dealing with. Now the question is how does it apply to our instruments?

Why does humidity matter?

When most people think of wood we tend to think of it as dry. Sure, a freshly cut green branch has water in it, but once it’s cured and ready to make lumber out of it’s easy to forget that there is still water present. An acoustic guitar that is properly humidified will have roughly 25 grams of water in it (This number varies depending on the size of the instrument, the type of wood it’s constructed from and the age of the instrument). As the relative humidity around an instrument changes the amount of water inside of the wood will change, leaving you with either a guitar that has too much or too little water in it.

Most acoustic guitar manufacturers, Martin included,  recommend a humidity level of between 45-50% relative humidity.

A guitar that has too much water in it is commonly referred to as over-humidified. This excess water causes the wood to expand and swell, In the short term this can lead to the top swelling up, the action raising to an uncomfortable height and negative impacts to the sound of the guitar. As I’m sure you can imagine, “wet” wood doesn’t really want to vibrate as easily as “dry” wood. Over-humidification will leave a guitar sounding bloated and dead. All of these issues are, luckily, reversible. A guitar in an area of too much humidity for an extended period may start to experience loose braces and lifting bridges but this generally takes such a long time that this more permanent damage is avoidable.

The bigger threat to a guitar is low humidity. For those of you who live in desert environments this is a year round concern while for the rest of us it’s something that needs to be thought of as the cold winter months grow closer. As humidity drops water will leave the wood of your guitar. This causes problems because as the water content of the wood drops certain parts will begin to shrink faster than others. This can lead to cracked tops and shifting neck joints, serious problems that can be at least costly to fix if not impossible. One or two particularly dry winters in a row can take an instrument from perfect to unplayable.

What do you do?

To combat low humidity and keep their instruments in good working order, players need to use guitar humidifiers. These can take the form of systems that control the humidity of an entire room or systems that fit inside of a guitar case. Let’s take a look at each.

Here at Maury’s Music we take advantage of a humidity control system built into our building. It monitors the humidity level in the room and keeps it within the relative 45-50% we need to ensure that our entire stock of guitars stay safe. Admittedly this system is costly and beyond what most players are going to be able to spend in their own homes. For players who have designated music rooms, a floor model humidifier can do the job, keeping a room at a steady humidity level at all times. All the player needs to do is remember to fill the water tank in the humidifier daily and they’re good to go. This is a great way to go if you have a lot of instruments or if your guitars simply don’t travel during the winter months.

For players whose guitars travel with them or who simply don’t have that one designated room to keep them in, smaller in case humidifiers are a great way to go. Traditionally these consist of a sponge material that is manually dampened and then set inside of the soundhole of an instrument while the case is closed. As the water in the material evaporates it will fill the case and give the guitar plenty of water to absorb to make up how dry the outside environment is. These products do come with one or two drawbacks that need to be mentioned.


Common in-case sponge humidifier.

The first concern with these traditional case humidifiers is that the player needs to be vigilant in their upkeep. A normal sponge humidifier will dry out after a few days at best. This isn’t a big deal if the guitar is being played daily and the sponge is being checked just as frequently. However, what happens if you take a trip without your guitar or, if during the hectic holiday season, you simply don’t play the guitar for a few weeks? Your sponge dries out and your guitar is now unprotected against the low humidity of the house. So if you decide to use one of this style of in case humidifier, remember to check it frequently and dampen as necessary.

The other issue with these sponge or clay based humidifiers is that the player has no control over how much humidity they release. So if the humidity starts to rise while the humidifier is in place you can end up over humidifying the guitar and hurting the tone of the instrument. Admittedly, as mentioned above, this excess humidity is nowhere near as dangerous as low humidity and ends up being more of an inconvenience, but it should be mentioned. You can keep track of the humidity in your case with a humidity gauge but you still won't be able to fine tune the levels. 

There is one more newer option for in-case humidification. The D’Addario Two-Way Humidification System consists of 3 packets, roughly 3”x4” that are filled with a saline solution. Two of these packets will hang inside of a guitar’s soundhole while it’s in the case while the 3rd will rest underneath the headstock (If any of you are cigar aficionados you may recognize a similar type of product also sold in smoke shops for cigar humidors). What’s cool about the system is that unlike other methods that can only release moisture, these packs can both release it during dry weather and take in excess humidity during humid weather. They are also balanced so that they only take in or release enough moisture to get the inside of your guitar’s case to the 45-50% humidity that manufacturers recommend. They also last for months which takes the danger of forgetting to remoisten your sponge or refill your humidifier tank completely out of the equation.

The downside of the D’Addario system is that they will need to be replaced periodically, making them more expensive than the sponge type humidifiers. I’ve been using them for about 3 years now and, personally, I find the added cost to be completely worth it. Each refill kit is only about $14 and I’ve found that I will go through 2-3 refill kits a year, depending on weather. In my opinion, $45 a year is a small price to pay for what so far has been a pretty foolproof solution.

Check out Maury's Music's selection of in case humidifiers Here.

The reason I stress the importance of humidifying guitars so much is because, I have seen first hand exactly what a lapse in care can do to an instrument. During college there was about a year where, for a few different reasons, I really wasn’t playing guitar much so I lapsed on humidifying. Unfortunately for me this year also included one of the driest winters we’ve had in a long time. One day I took the guitar out of it’s case and instead of being the beautifully playing instrument I remembered I found a guitar with an action so high that it was barely playable. The neck joint had shifted because the dry wood shrank and I was left with a guitar that needed a neck reset and was out of warranty because I hadn’t taken proper care.

Learn from my mistakes, people.  Share your humidification war stories below.

Posted by: Angelo AT 08:30 am   |  Permalink   |  4 Comments  |  Email
Beware that if you live in a very dry climate, as I do in Colorado, you will have to change out the D'addario humidipak type humidifiers much more often than 2-3 time per year, sometimes as often as once a month. Although they are great, that gets very expensive.
Posted by Neil Shropshire on 11/04/2015 - 03:26 PM
How to the Oasis soundhole humidifiers compare?
Posted by Don Walker on 11/05/2015 - 05:18 PM
Neil : You are absolutely right. The longevity of the D'addario packs is very much dependent on your local climate. A user's own local conditions may meay they go through the D'addario packs faster than I have been. Don: I can't say I've used an Oasis humidifier personally although I do have at least a passing familiarity. I can't speak for how often they need to be filled but I can say that all of my points about the possibility of over-humidifying the guitar are still applicable. - Angelo @ Maury's Music
Posted by Angelo on 11/16/2015 - 01:40 PM
Fyi, I humidify and display guitars at a uniform 45% to 50% RH inside a Musik Tent. It's an Instrument Humidor that I purchased at a guitar show earlier this year. Seems to work great. --Bill
Posted by Bill G on 12/05/2015 - 08:23 AM

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