Humidity and Guitars

Changes in relative humidity (RH) can affect the action of your guitar and in extreme cases can loosen braces and cause splits. In this article, I’ll explain what happens to guitars as the RH changes and how you can keep your guitar safe and playable.

To understand the effect of humidity on guitars, you need to know a few things about wood. Wood is “hygroscopic”: It will absorb and release water molecules from the air around it. As it absorbs water, wood expands. As it loses water, it shrinks.

This movement has the greatest impact on what builders call the “plates”, the top and back of the guitar. Once we brace the plates and glue them to the sides, they are no longer able to expand and contract freely with changes in RH. So if the plates want to expand or shrink, the braces and sides try to keep them the same width. The result of this tension is that the plates (which are usually domed to begin with) become more domed 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 and changes the string height (action) above the frets. The result is higher action with increases in RH and lower action with low RH.

Many of you are aware of the risk of splits 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. One analogy is carpet. When you walk on carpet, it 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? With time and pressure, the carpet under the legs of the couch gets compressed and wants to stay that way. That’s compression set. This happens in the guitar plates when exposed to very high humidity over time. The braces and sides constrain the expansion in width and over time, the wood “sets” in this compressed condition. 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. There is a graphic explanation of this concept on the left side of page 83 of Bruce Hoadley’s book, “Understanding Wood”.

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 dry and well seasoned. 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. Finally, I 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 and help avoid extremes.

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 dry climates keep their instruments in a humidity-controlled room. Some people have whole-house humidifiers. The next best option is to keep your guitar in its case when you’re not playing it and humidify the case. My favorite way is with a soap box and a sponge like this.

But Don’t Trust Your Hygrometer!

Even expensive hygrometers need to be re-calibrated occasionally, but the cheap digital kind become terribly inaccurate over time. Fortunately there’s an easy solution. Here is an inexpensive hygrometer that you can calibrate.

This comes with a packet to calibrate it at 75%, but I have seen plenty of hygrometers that are accurate in a certain range and off in others. I recommend buying the 49% packets and calibrating it with those. You may have to select 49% on the menu when you order.

When all else fails, trust your guitar. If the plates look flatter and the action is lower, it’s time to humidify. If they are bulging upward and your action is high, it’s time to dehumidify. Following this advise will keep your guitar safe and playable as the humidity changes.