I’m seeing a lot of activity on my Facebook feed regarding guitars and humidity, dangerously low levels and questions about the best ways to keep a guitar safe in this weather. This has inspired me to publish my notes from a 2009 GFA lecture about humidity and guitars. Following is an edited version of my lecture notes.
While I started building guitars in high school, I didn’t really start my professional building career until I was finishing my doctorate in guitar performance at Florida State University. Living in Tallahassee is a humid experience, but I was beginning to understand issues regarding humidity and managed to keep a stable humidity in my shop, most of the time around 45-50%. Around this time I sold a guitar to a student at the Manhattan School of Music, around April as I recall since the student was anxious to use his new guitar at his year-end juries. Next January I got a call from him – he was a little concerned because he said he thought his guitar had a crack.
“Where is the crack?” I asked.
He replied “on the back”
My next question was “how long is the crack?” answered by “the whole way.”
“The whole length of the back?” “yes”
“is the crack open?” “What do you mean?”
“Can you fit a piece of paper in it?” “I think so.”
“Can you fit a credit card in it?” “Yes”
“Can you fit a pencil in it?” “I think so.”
When he shipped the guitar the back to me it was so dry and so shrunken that it not only cracked so that you could literally fit a pencil in it, it actually was beginning to peel away from the back braces. I called him back and asked him if he used a humidifier. Yes, he said, his father went to a violin shop and got one that fit in the case. He filled it every month because that was what the instructions said, and it stayed in the head portion of the guitar case because that is where it fit. When asked where he stored the guitar he said near the radiator (steam style metal radiator) because the room was drafty and he didn’t want it to get cold.
I decided to replace the back, so I cut off the bindings and lifted it off the guitar. It looked like a potato chip. After 3 days in my more humid shop environment it returned to a flat state and the crack closed up completely.
On the other side of the spectrum, I received an email from an owner of one of my guitars who lives on the Carolina coast who said the action of his guitar has become so high as to be unplayable. I had already adjusted the action down previously so I was a little concerned. He shipped me the guitar and yes, the action was really high – strings almost a half inch off the frets at the 12th fret. I sighted down the neck fearing a major warp, but it was pretty straight. I put the guitar back in its case and noticed some mildew inside – curious as mildew doesn’t really doesn’t prefer to grow on artificial fibers. Then I checked the fret ends, which were completely buried into the fingerboard. I called to ask about how he was storing the guitar. He said his practice space was a basement, and that it was pretty humid. I asked him to measure it with a hygrometer, and he came back with 70%. I didn’t want the guitar to dry too quickly, so I opened the case for 1 hour a day for a couple of weeks, and then hung it up in the shop for another month before it returned to normal. Easiest repair I’ve made.
The idea for this lecture came from one of my good friends, a professional player that recently moved from the mountains of North Carolina to deserts of southern California. He has decent awareness of humidity and we talked about his strategy before he left, but half a year after he moved he sent his guitar back to me because it didn’t sound right any more. When I received the guitar I came to the same conclusion. The fret ends were poking out so I knew the guitar was dry. I hung it in the shop for a week and it sounded back to normal – just needed a good drink. I returned the guitar with a different case, a different humidifier, and all is better.
What is Relative Humidity?
Relative humidity (RH) is simply how much water (in the form of vapor) the air currently holds compared to how much it could hold if the air were completely saturated at that temperature (actually the term ‘hold’ is misleading but it works for our purposes).
This is pretty simple. For example, if the air in a given room could hold 1 cup of water (imagine for a moment that we toss one cup of water into the air and it evaporates instantly) and be totally saturated – not able to hold any more – we would call this 100% humidity. If we only tossed ½ cup into the air the humidity would be at 50%.
The trick is in the “relative” part. Air can ‘hold’ different amounts of water depending on the temperature. When we refer to relative humidity we are taking into account the fact that as the temperature of the air increases it can hold more water. As the temperature decreases it can hold less. We mostly need to know that warmer air holds more moisture than colder air.
So, returning to our room at 50% RH, if we increase the temperature of the air the relative humidity will fall below 50% because the now warmer air can hold more than 1 cup. If we decrease the temperature the humidity will climb above 50%. This inverse proportion is important to remember; if you heat up the air the relative humidity will drop.
Now consider this scenario. It is a chilly, rainy day much like many in Boston’s late fall. Since it is raining, the outside relative humidity is near 100% (the Weather Channel concurs). At 40°F and 99% RH the actual amount of water vapor in the air would be 6.58 grams per cubic meter (gm/m3). If that air with 6.58gm/m3 is heated to 70°F the RH drops to 35%. A little dry, but not terribly alarming.
But say that you are living in North Carolina, a typically warm and humid state, and in February a cold front moves in from Canada with temperatures of 25°F with 45%RH. That air heated to 70°F has a relative humidity of 9.4%! You need to get your guitar out of there or have a plan to prevent the wood from shrinking.
How does Moisture (or lack thereof) Effect Wood?
Wood is a cellular material, much like a sponge. In fact it attracts and releases moisture just like a sponge and that is the heart of the problem. We all know that if we have a completely dry sponge and drop it in water it grows. Dry that same sponge out and it will shrink again. Wood swells and shrinks with moisture too.
One of the urban legends of luthiery states that a soundboard (prior to being braced, finished or glued to a guitar) will measure as much as 1/8 of an inch less across the lower bout with a 40% decrease in humidity.
I took two soundboards, one of cedar and one of spruce, and tried it. It turns out that the spruce guitar did grow exactly 1/8th of an inch from 24% RH to 66%RH. The cedar soundboard grew slight less at 3/32nds. If that seems outrageous, think of it as slightly less than 1% smaller and the legend seems much more plausible. Shrinkage is our primary enemy as this is one of the most common causes of cracks.
So, how does a guitar crack?
If your guitar is hard to play, or doesn’t sound right that is a problem, but the most serious impact of a low humidity event is cracking. I’m sure most of us have had guitars in very low humidity environments for periods of time. Most of the guitars didn’t crack.
Cracking occurs when the strain of shrinking exacerbates some flaw in the wood: either a hidden crack or impact mark, or some flaw in the wood’s structure. The strain of the wood’s contraction being held on all sides by glue joints and braces causes the flaw to separate the wood, much like a weak link letting go on a strained chain.
It is that same strain that causes guitars to sound constricted, not open, under low or high humidity conditions.
What is the Ideal Humidity?
The ideal humidity depends much on the conditions under which your guitar was built. Most modern builders shoot for a range of between 40-50% RH in their climate controlled workshops, or wait to glue and finish until the humidity is in the appropriate range.
Keeping your guitar in conditions between 40-55% RH is good. A little higher and the action might get too high, the fingerboard may swell and the guitar might not sound quite right although most of the guitars I have owned, built by myself or others, seem to like the higher end of the ‘ideal’ range.
If the guitar is kept too long at humidity levels below 35% it will start to shrink, causing the strings to buzz and ultimately the back or soundboard might crack. Watching the fret ends is a good way to keep tabs on the long term humidity level of the guitar. If they protrude from the side of the fingerboard the guitar is being kept too dry. If the ends recede into the fingerboard the guitar is probably too damp.
Of course the best way to monitor the humidity levels is to use a hygrometer. These are readily available in both the dial and digital versions, but there are problems associated with each. Don’t assume that a dial hygrometer is less accurate than its digital counterpart. Some digital hygrometers can be off by as much as 10% in either direction.
Dial hygrometers are usually more accurate in the middle of their ranges but can be quite far off reading the extremes and most people don’t realize that they need to be reconditioned every six months or so.
When buying a hygrometer make sure you get the most accurate instrument you can afford. The Oasis company sells a digital hygrometer it claims to be accurate to +/- 1% which is quite good, although it costs a bit more than its competition.
Some guitar cases have built in hygrometers, but they are often placed in awkward locations in the case. To give reliable data free flow of air (the same air inside the guitar, hopefully) must surround the hygrometer
Without a hygrometer there is no sure way to know the RH in a particular room, but there are some clues to know when the room is too dry. Static shocks are a great clue. Low humidity creates good conditions for the isolation and storage of static electricity which is transferred on contact. If you are getting static shocks at home it is too dry – most likely under 35%. Your skin can also be a good indicator. If your face and hands are getting dry and chapped it is likely too dry in your living environment for your guitar too.
What is the Best Way to Control Humidity?
The most startling revelation about my research in this area is that it takes a long time for the air to recycle in a guitar that is not being played, but the air in the soundbox changes quite rapidly when the guitar is played.
The week before the GFA lecture I was looking for more information regarding the speed at which the humidity levels in the guitar changed. It being summer, my workshop RH level was at its consistent 45% but the basement of my house (then in North Carolina) was high, peaking at 70% in the basement bathroom. For the experiment I taped a hygrometer to a used guitar string so I could fish it in and out easily. Leaving the guitar in the bathroom, I checked my watch and returned 10 minutes later, sure the humidity level in the guitar would have increased, but there was no change. After 20 minutes, no change. After 2 ½ hours the hygrometer read 46% and after 4 hours it reached 47%. Clearly, even though there was no covering over the soundhole and the guitar was not in a case, it was taking a long time for the humidity level to equalize. I wondered if playing the guitar made much of a difference, so after 10 minutes of playing the guitar in the more humid environment the humidity in the guitar read 57%. This is good news for guitars that stay in storage, especially in a case that seals well, but it means that when we play we put our guitars at highest risk.
That is why the best way to control the moisture content of the wood in your guitar is to control the environment in which the guitar lives and plays. If you are a player that keeps your guitar in the same room in your house or apartment for the vast majority of time, you need to keep that room’s humidity controlled, which is usually a pretty easy thing. Guitars enjoy the same pleasant environment that humans do.
An evaporative humidifier is probably the best way to maintain good moisture in a room that tends to be dry. Vaporizers (either cold or especially hot) can put more moisture in the air than we want. An evaporative humidifier will stop adding moisture to the air once it has reached around 55%. Room humidifiers are available now that have their own moisture meters and you can dial in the humidity you desire.
I recently bought a whole house humidifier for my heating system because it is more comfortable to have a moderately humid environment in the winter, especially for my kids and all of our instruments. Air conditioner or de-humidifiers (which operate on the same principle) are two appropriate tools for drying the air in a room.
What about in the guitar?
Every time you play in a dry environment you have to replenish the moisture the guitar lost to air in the room.
There are many devices intended to keep a high humidity level in the guitar, and most of them are good, but they have best and least favorable uses.
Two years ago I decided to buy as many of these products as were available to find out which was best. Along the way I discovered that most of them work, perhaps some better than others in particular circumstances so I wanted to share that research with you.
I consulted with a music friendly Biology professor at Furman University to determine a good procedure for evaluating the products readily available in February of 2009 when I conducted these experiments.
We both determined that the best way to really see which products imparted more moisture into a guitar was to weigh the whole guitar before and after the product use, but there were a couple of problems. First, a scale (or “balance” as they call them in the realm of the Sciences) that was capable of accurate measurements to the 100th of a gram in the 1.5-1.8 kilogram range (3-4 pounds, or the weight of a typical classical guitar) was not available at our institution or in my budget. Second, it is very difficult to ascertain the moisture content of a whole guitar, and using the same guitar left our procedure open to too many variables of environmental humidity on successive days.
We eventually settled for a procedure that determined how much water (by weight) each product delivered to the environment under the same conditions. While this could not tell us how well a humidifier transferred water directly to the wood of a guitar it could let us know how much water was taken away from the device.
I weighed each device dry as it came out of its packaging from Strings by Mail. Next I added water (or in the case of the Humicase pods, humidifying solution) according to the directions on the packaging, and weighed each product again to see how much water (1mm3 = 1 gram) was absorbed/added. I then arranged them with equal spacing on the piano in my office and took weight measurements on each successive day at the same time for 3 days. The last weighing took place after a particularly dry weekend on the 4th and 5th days after initial charging. I also recorded the high and low humidity levels during the same 24 hour period.
The devices available to me for this experiment were the Dampit Jumbo Guitar Humidifier, the Planet Waves GH humidifier (both of these are sponge based humidifiers, the Dampit with a kitchen type sponge and the Planet Waves a type of artificial foam); Oasis “Original” (formerly known as the “blue”) and “Plus+” (formerly the brown), each using a hygroscopically porous fabric containing a water absorbing crystal; the “Guitar Humidifier” that uses a diametacious (water absorbing) stone, and the Humicase pod (a sponge based humidifier), both of which earn a pair of asterisks on the chart because they are case humidifiers rather than products inserted into the guitar.
Each product (with the exception of the “Guitar Humidifier”) performed pretty well, especially on the first day while fully charged. I am guessing that while the humidity range of Day 3 was lower than of Day 1, that the average of Day 3 was higher. The final water loss of the Planet Waves GH humidifier was greater than the amount of water that I added to the device, probably indicating that it had some moisture in the sponge already.
Dampit – This product was the most efficient at transferring water to the air, probably due to the large amount of the sponge surface area exposed directly to the air. It has the most holes, and the yellow kitchen type sponge seems to absorb and release water more quickly than the other options.
Planet Waves – Not quite as effective as the Dampit, the Planet Waves unit has fewer holes which inhibits some of the water transference. It works well though, and out-performed the Oasis products by a small margin in the short run.
Oasis – Both the Original and Plus+ versions work well, releasing almost as much water as the sponge based Planet Waves. Each product easily handled the 5 day experiment with a reserve to spare.
The Herco Guitar Humidifier and the Humicase pods are both case humidifiers, intended for placement inside the case in either the head compartment, near the heel of the guitar, or as in the case of the Humicase pods, inside spaces carved out of the case padding over the soundhole and under the head.
The Herco Guitar Humidifier is the only product I reviewed that I would not recommend to anyone for anything. While the diametacious stone provided an ample reservoir for a case humidifier, the one, tiny, meager hole in the cap allowed for just miniscule evaporation. Also, the only viable placement for this product would be in the head portion of the case, which in most cases is almost completely blocked off from the body of the guitar. Incidentally, this was the product used by my aforementioned customer from New York…
The D’Addario 2-way humidification system
Just as I was finishing this experiment in 2009, D’Addario released its 2-way humidification system, a pouch that contains a chemical gel that stores and releases moisture depending on the ambient humidity. This seems like a great idea, but I have concerns about using the product in prolonged periods of extremely low humidity. To maintain 40% humidity in my 300 square foot Chicago shop in the winter I have to put 2-3 gallons of water in the humidifier every day. Scaled down to guitar size, that means a humidifier has to put 5-7 grams of water in the air cavity of a guitar to maintain the 40% level. I am not sure how many grams of water the 2-way system contains without being recharged by a higher humidity level but it seems like a lot to ask of that system. The packs must be changed more often in very dry conditions which may be more expensive than using a conventional humidifier. In more moderate conditions it seems like a great idea. Perhaps guitarists with more experience with this product may wish to send me their feedback!
 A water vapor ‘calculator’ based on empirical data can be found at http://hyperphysics.phy-astr.gsu.edu/HBASE/Kinetic/relhum.html#c4
 Search on-line for instructions on reconditioning a dial hygrometer.
Steven Walter Guitars