From the July/August 2022 issue of Acoustic Guitar | By Martin Keith
For those living in an overly humid environment, acoustic guitars can take on serious damage from the air around them. Here’s how to take care of guitars in an extremely humid environment.
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I live at the eastern end of Long Island, New York, near Montauk, sticking out into the ocean and bay. I have had several steel-string guitars that, when new, were a joy to play. But slowly the action worsened, and my most recent purchase, a Martin 000-15M, is doing the same thing. The neck appears to be straight, and the bridge is not lifting up or coming unglued, but the body of the guitar below the bridge has begun to belly out, raising the bridge and lifting the strings further off the fretboard. I’ve been told to keep my guitars humidified with the various in-case products out there. However, a musician I know suggested that the added moisture may be the problem, somehow softening up the wood in the body below the bridge, making it more susceptible to bowing up from the tension of the strings. I’ve also been experimenting with loosening the strings, putting weight on the bridge, and letting the guitar sit like that for a spell. What do you recommend? —Randall Parsons
Humidity is a daily consideration for those of us in the guitar business—I have multiple hygrometers in my workshop, whose accuracy I check regularly with a laboratory sling psychrometer. (Special thanks to my high-school physics teacher for my residual knowledge about how to use it!) This is all to compensate for natural fluctuations in ambient humidity, as well as for the artificially dry winter conditions caused by most heating systems. I keep hide glue mixed and ready all winter long for crack repairs, which regularly come in between November and March/April, and often have to re-humidify guitars for a week or more before working on them, to let the wood return to its original shape.
However, cracks are not the only humidity-related issue I come across in the repair shop. It’s quite common to encounter action and playability issues during the dry months. Though individual cases can vary, the most typical problem is to have low action in drier conditions and raised action in the summer, when humidity is high. This is mostly due to movement in the guitar top, which flattens (or, in some cases, even sinks to concave) when it is dry, and takes a more convex shape when humid. I have several clients for whom I have made “summer saddles” and “winter saddles,” to keep the action roughly consistent throughout the year as the humidity fluctuates.
Most of my customers need a little help and encouragement (and occasionally a stern talk!) with keeping their guitars humidified. How- ever, on some occasions, I’ve had to rein in somebody who was a bit too enthusiastic with their humidification—and, like everything, too much humidity can also be a problem. Though I don’t personally think that excess humidity is causing the wood to soften, it certainly would result in a guitar top bulging forward, with the attendant rise in action and deterioration in playability. In an extreme case, it could possibly soften glue joints between the top and its braces, and this could result in deformations that would not resolve with environmental corrections. Most flattop guitars are built with either yellow wood glue (such as Titebond) or hot hide glue, and both can be susceptible to reduced strength in high humidity.
Experienced luthiers have developed some techniques that can help resolve badly bulging tops. For example, many (including myself) have adopted TJ Thompson’s method of using carefully shaped metal plates to heat and reset the glue joint between the top and the bridge plate (the thin slice of hardwood inside the guitar that receives the ball ends, under the bridge). Doing this properly involves removing and then regluing the bridge, and this can be a potent combination when done carefully. I have restored many badly bulged tops to near flatness with this method. However, it requires both specialized tools and experience and is generally only necessary for older instruments, or those that have suffered badly from improper storage or maintenance. A well-built modern guitar such as yours should not require this kind of treatment for at least a decade.
Since the issue has presented itself on multiple instruments, I would be inclined to suspect an environmental condition, as it’s very unlikely that any construction issue would present itself the same way on several guitars. My first recommendation is to gather some data on the humidity in your environment. Buy a couple of digital hygrometers and place them in various spots throughout the house, as well as in the instrument cases. These devices are not always perfectly accurate, so I prefer to buy a handful of cheap ones and compare them all to each other, rather than trusting a single data point. The popular middle range for humidity is 45% at a comfortable room temperature. Given your proximity to the ocean, the ambient humidity may be higher than what’s normal, even in the winter months, so it’s possible that your guitars may not need supplemental humidity. If the instruments sound dull, heavy, or muffled, this can be another symptom of over-humidification.
Particularly if your heating system is very drying to the ambient air (forced air and woodstove heat are known to be particularly rough on guitars), consider keeping the instruments in a separate room that is closed off from the main airspace. Humidifying the room rather than the case is always preferable in my opinion—it minimizes shocks as the guitar transitions from a humid case environment to a dry playing environment, and reduces the potential for accumulation of excess humidity inside the case. I have seen mold inside guitars and cases as a result of overzealous in-case humidification—room humidification is much less prone to these kinds of pitfalls.
Rather than simple in-case humidifiers, consider a two-way system such as the excellent D’Addario Humidipak. This product is based on a clay filling that will add or remove humidity as needed to maintain a healthy level. It is low-maintenance and will adjust its effects based on actual conditions, particularly valuable in the spring and fall seasons, where conditions can swing from bone-dry to damp and steamy, sometimes within a few days.
I will recommend against putting weights on the bridge or top. The load on a steel-string guitar bridge is actually a twisting force, as the strings attempt to rotate the bridge. This force pulls the top upward behind the bridge, while forcing it downward between the bridge and soundhole. This downward caving-in can be just as problematic for the guitar’s long-term health as the bulging area. Putting a dead weight on the top imposes a simple downward pressure load, which would not helpfully counteract the twisting force of the strings and could accelerate top failure between the bridge and soundhole.
I’ve seen some people who use clamps and straight pieces of wood to attempt to force down the bulged area of the top, but I also recommend against this approach. It can cause unsightly dents or pressure marks in the lacquer and is unlikely to have any lasting helpful effect. Putting pressure on the soundboard in this way also stresses the glue joints between the top and its braces and can easily result in a loose brace—which only makes things worse!
Your Martin is certainly well-built under good conditions, and you should reasonably expect it to play well for many years. Learning what’s going on with your storage and playing environment should go a long way towards ensuring its long-term health and ready playability.
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