Humidity and Guitars

Changes in relative humidity (RH) can affect the action of your guitar and in extreme cases can cause cracks and other damage. This article will explain in detail what happens to guitars as the RH changes and how you can keep your guitar safe and playable through  changing seasons and when traveling.

To understand the effect of humidity on guitars, you need to know a few things about wood. Wood is hygroscopic which means that it will absorb and release water molecules with changes in the relative humidity of the air around it. As it absorbs water, wood expands. As it looses water, it shrinks.

This movement has the greatest effect on what builders call the “plates”, the top and back of the guitar. Once we glue the plates to the sides, they are no longer able to expand and contract freely with changes in RH. This is compounded by the fact that wood doesn’t move equally in all directions. It expands and contracts noticeably in width  (across the grain) and practically not at all in length (parallel to the grain). This differential movement becomes problematic in guitars because they are built with braces that are glued across the grain of the plates. So if the plates want to expand, the braces and sides try to keep it the same width. The result of this tension is that the plates (which are usually domed to begin with) become more convex with increases in RH and flatter or even concave as the RH drops.

Of course this is most noticeable on tops because as the top moves up or down, it takes the bridge and strings with it which changes the string height above the frets, aka the action. The result is higher action with increases in RH and lower action with low RH.

Many guitar owners have become aware of the risk of cracks due to low humidity but few understand the risk of exposing your guitar to extremes of high humidity. One problem is that extreme moisture can soften some glues. The less obvious problem has to do with another peculiar property of wood known as “compression set”.

When the RH rises and plates can’t freely expand, the wood fibers get compressed. Think of your carpet as you walk on it. The carpet gets compressed between your feet and the floor. When you remove your weight, the carpet springs back to its original shape. But what about that place where the couch has been sitting? The spots under the legs of the couch get compressed and want to stay that way. That’s a good analogy for the compression set that happens with wood. If wood stays in a compressed state for too long, it will want to stay that way. In other words, it will want to be narrower than it once was. This can become a problem when the RH goes down again. As the wood tries to shrink, the stress will be compounded and a plate may split sooner than it would have if it was never exposed to such compression from high RH.

As a builder, there are a few different things I do to protect my guitars against humidity related problems. I start with woods that are well seasoned. I then go one step farther with my tops and heat set them. I heat the wood to 200 degrees F for 2 hours, cooking off all water in the wood. When the wood comes back up to equilibrium in a few weeks, it has lost some of its hygroscopic ability and will move less with future changes in humidity. I then carefully control the RH in my shop during construction. For most guitars, this means building with an RH of 45% which is a good midpoint for the range of humidity a typical guitar would likely be exposed to. The last thing I do is apply a seal coat of shellac on the inside of the top. The finish on a guitar won’t stop the exchange of water vapor but it will slow it down. The wood is able to accommodate slow changes more easily than fast ones.

As an owner, there are several things you can do to prevent humidity related problems. When the humidity drops below 40%, it's time to humidify. Many people in particularly dry climates keep their instruments in a humidity controlled room. Some folks even have whole house humidifiers. The next best option is to keep your guitar in it's case when not playing it and humidify the case. The Ameritage cases that are standard with my guitars have a built in humidifier. But don’t combine these with a humidifier inside the guitar unless you still can’t get the humidity in the case above 40%. As we know, over humidifying is not helpful either. And use great care with ANY humidifier to avoid leakage and water damage of the instrument. It’s also a good idea to keep your guitar in its case when you’re not playing. Some houses can experience pretty large swings in RH during the course of a day and the case will moderate those swings.

One last thing to know is that cheap digital hygrometers can be extremely inaccurate. You have to pay between $100 - $300 to get a good one. The handheld ones used in the refrigeration industry. If that's out of your price range, trust your guitar over your hygrometer. If the weather changed, your heat came on, and your action dropped, it's a good bet that the humidity is low.

And of course, feel free to contact me with any questions.