Too little or too much humidity can devastate a wooden instrument. Visit any guitar manufacturers Web site, or those of independent luthiers and you’ll see information on how critical it is to keep your guitars, mandolins and other stringed instruments at the proper humidity level. Instruments need to be kept between 45-55% relative humidity. Simply putting them in their cases during particularly wet or dry months will not suffice.
Most acoustic instruments have spruce tops. Here’s what Taylor Guitars has to say about the impact of humidity on spruce:
Our factory is climate-controlled to maintain a temperature of 75 degrees and a relative humidity of 47 percent. This consistency causes the wood to equalize at a specified moisture content. As the wood’s moisture content changes, so does the size of the wood. Spruce, in particular, shrinks and expands a tremendous amount as it gains and loses moisture. For example, let’s say we condition a spruce top in a room that is 47 percent RH, and then cut that spruce to a width of 16 inches. If we then were to lower the room’s RH to 30 percent, that same piece of spruce would shrink to 15.9 inches in width — shrinkage of almost 1/8 of an inch! If, instead, we were to raise the room’s RH to 60 per- cent, the spruce would swell to 16.06 inches, an expansion of almost 1/16 of an inch.
While these changes are a fraction of an inch, they are a major change, and a major problem for musical instruments. Damage caused by too little or too much humidity will not typically be covered by most manufacturers warranty plans. Fixes can be expensive and visible. On vintage instruments, certain repairs can reduce the value of the instrument by as much as 50% even after the repair has been flawlessly executed.
What do other guitar builders have to say?
Your guitar is made of thin wood which is easily affected by temperature and humidity. This combination is the most important single part of your guitar’s surroundings. Martin keeps the factory at a constant 45-55 percent humidity and 72-77 degrees Fahrenheit. If either humidity or temperature get far away from these factory conditions, your guitar is in danger.Martin Guitar Company
Martin also suggests keeping your guitar in the travel case where the small space is more easily humidified.
Fully 80% of the guitars returned requiring repair have damage which is the result of severe dehydration. Damage due to dehydration is not covered under warranty because it is owner-preventable environmental damage.Larrivee
Too little humidity can result in wood cracks (top, back, sides, bridge, fingerboard, etc…) In dry conditions, the top and back can sink in – pulling at the center seams – sometimes causing them to crack open. Also, as the bridge drops lower, it lowers the action and can cause strings to buzz as shown in the diagram below (courtesy of Taylor Guitars):
The inward flexing of the top and back can also cause braces to loosen, and in some cases cause the bridge to loosen as well. Top and back repairs can be expensive, and re-gluing center seams makes for a visible repair that will reduce the value of your instrument.
Dry conditions can also affect the neck – sometimes causing bowing and sometimes necessitating a truss rod adjustment. Metal fret wire doesn’t shrink like the wooden fretboard. When the fretboard shrinks in dry conditions, you’ll feel the sharp metal ends of the fret wire when you run your hand along the neck. This is particularly noticeable on unbound necks.
Dryness is a problem year round in high altitude and desert locations, and is a problem almost everywhere during winter months. Heating a room forces the humidity down to levels that are dangerous for musical instruments. Whole house humidifiers may make you a bit more comfortable, but they sometimes do not put enough humidity in the air to properly care for wooden instruments.
Too much humidity can also create problems for your instrument. When an instrument has too much humidity, the wood swells and can put additional pressure on side seams. The top and back of the instrument bow away from each other as shown in the graphic below (courtesy of Taylor Guitars):
As the bridge flexes up with the top, it raises the action on the guitar and makes it hard to play. Just as with dry conditions, braces and bridges can loosen. And in some cases, binding can break away from the body. The main concern for high humidity is when high heat is a factor. In the case of high humidity and high heat, the glue joints (anywhere glue is used) can come apart.
Some particularly damp locations run relative humidity of greater than 90% for much of the year.
What is Relative Humidity?
We and other firms that make humidified cabinets use Relative Humidity to measure how much water is in the air inside a guitar cabinet. However, Relative Humidity is NOT an accurate measure of how much “water” is physically in the air. Cold air holds less water than warm air. Relative Humidity a measure of how much water is in the air at a given temperature. That’s why you will see a 100% humidity day in fall or winter at 48 degrees, and yet have so little humidity in the house at 70 degrees that your instruments can get damaged.
It’s important to understand this principal when you are concerned about humidifying your guitars. It’s also important to keep in mind when dehumidifying them. “Dehumidifiers” that simply raise the temperature in a cabinet are not reducing the amount of water in the air inside the cabinet. They simply raise the temperature, so that the Relative Humidity is lower. There can still be enough moisture in the cabinet to damage your instruments. To truely dehumidify the air in the cabinet, the water needs to be removed from the air in side the cabinet.
The following information is from Utah State University
Relative Humidity : The ratio of the actual amount of water vapor in a given volume of air to the amount which could be present if the air were saturated at the same temperature. It’s commonly expressed as a percentage.
Since warm air will hold more moisture than cold air, the percentage of relative humidity must change with changes in air temperature. The graphic below illustrates this relationship. Again we have three parcels or containers of air. The number of water vapor molecules is the same in each container. At 40° air temperature, the parcel is saturated and will hold no more molecules of water vapor. The relative humidity is 100 percent. If the temperature of that air parcel is raised by 20°, it will hold about twice as many water molecules to reach saturation. Thus, the new relative humidity is now 48 percent. If the temperature is raised another 20°, it will again double its capacity to hold water vapor molecules. The relative humidity is only 24 percent.
Relative Humidity vs. Temperature
Relative humidity decreases as temperature increases
The importance of air temperature to moisture is obvious. At 80°, the air has a relatively low humidity and is relatively dry. As it cools, the humidity increases, reaching its saturation point at 40°. Now the air is very moist, and clouds will form. The dew point of the air is 40° in all three containers in the illustration.
The example in the graphic below shows a relatively moist airmass with a dew point of 60°. A relatively dry airmass has a dew point of 38°. This illustration shows that a cool, dry airmass may actually have a higher relative humidity than a warm, moist airmass. Relative humidity alone can be misleading when comparing atmospheric moisture conditions.
Most vs. Dry Air Mass
A cool, dry air mass may actually have a higher relative humidity than a warm, moist air mass.
Rule of Thumb : Relative humidity doubles with each 20 degree (Fahrenheit) decrease, or halves with each 20 degree increase in temperature.
Generally, as temperature goes up, relative humidity goes down and vice versa.