Wood shrinks and swells with humidity changes and this can damage your guitar. Many expensive repairs could be avoided by understanding the effects of humidity change and by applying simple remedies available for dealing with them. Nowadays almost all inexpensive guitars are made of plywood. Although plywood is mediocre as a tonewood, it is reliable and durable and withstands more atmospheric abuse than solid wood. Solid woods are more vulnerable to humidity change because they shrink and expand much more than plywood. Humidity is an issue for both types of instruments though and neither should be neglected.
The effects of low humidity are far and away the most common problems for wood instruments. Even moderate moisture loss in the tonewoods can make a guitar sound brittle or "plinky," even though it may seem to have a peculiar, nervous liveliness. At a critical point in moisture loss, the accumulated stress brought about by uneven wood shrinkage relieves itself by producing one or more cracks and sometimes even glue joint failures. When humidity is high, an instrument loses tone, volume and projection. It is also possible for a guitar to suffer structural damage when humidity is very high. A common problem is "bloating" in the back, especially if the back is made of rosewood or some other very hard wood. This is caused by wood expansion and, particularly, changes in the back-to-brace glue interface. In rare instances, this condition can lead to a glue joint failure. Another problem that can be caused by extreme humidity, as in tropical areas or a very damp basement, is deterioration of the glue and potentially even the wood itself.
The key concept in understanding humidity is the concept of "relative humidity." This refers to the ability of air to retain and take on water, or to dry out moisture-containing objects it surrounds. The higher the temperature of the air, the more water it will hold before saturation and precipitation occur. Relative humidity is the amount of moisture present in the air expressed as a percentage of the total moisture the air is capable of holding at that particular temperature. It is humidity relative to temperature. If the relative humidity is 40%, then the air, if maintained at a constant temperature, will theoretically hold 59.999... % more moisture before precipitation occurs. As the relative humidity rises, the air takes on additional moisture more and more slowly. When relative humidity drops, the air accepts moisture more rapidly.
When relative humidity is very low, things (like guitars!) dry out fast. Low relative humidity starts to become a problem anywhere whenever buildings are heated. If the temperature outdoors is 20 degrees Fahrenheit, the relative humidity 50%, and we take this air inside and heat it to 70 degrees, then the relative humidity can drop to 30% or less. The colder it gets outside, the more the relative humidity drops inside. The ideal humidity level for a guitar varies from one instrument to another, depending on the humidity conditions in the factory or workshop where it was assembled. Generally, though, 45% humidity at 75 degrees is a reasonable goal.
The humidity during assembly establishes the basic dimensions of the instrument. This dimensional configuration is permanently locked into the total structure when the guitar is assembled. Thereafter, when humidity varies, the individual components will shrink or expand unevenly, while the dimensions of the total structure tend to remain proportionately constant. The humidity level in a guitar workshop or factory is extremely important. It must be measured objectively at frequent intervals with an accurate, reliable tool and controlled within a narrow range; especially when assembling bodies. While it is possible for a workshop or factory environment to be too dry, the more typical problem is excessive humidity. If the humidity is too high during assembly, a guitar may suffer great damage when subjected to a North American winter—severe warping, multiple cracks, glue joint failure, etc.
There are several ways to keep track of humidity in your guitar's environment. Several are complex and require more work than what most guitarists are interested in expending. The quickest and simplest way to monitor your room or your instrument case is to buy a simple digital hygrometer. Lynch Music now sells a combination guitar tuner and hygrometer that is reasonably accurate. This allows you to monitor the levels of your case which, after all, is where your guitar lives most of its life.

You should be alert to low humidity conditions whenever winter weather keeps the temperature below freezing all day. If the daily temperature range is 10 to 25 degrees, you should add a little moisture to the guitar's environment. If the range is as low as minus 15 to 10 above, you should take decisive action to introduce lots of moisture if you hope to avoid tonewood cracks or other structural problems. However, the temperature conditions which point to a need for more moisture will vary from guitar to guitar, because of differences in humidity conditions at the time when the guitar's body was assembled. This is why you need to keep an eye on the guitar, especially the back, as well as on the weather. Particularly in winter, a guitar should be stored in its case, far away from any heat source, not on a stand or hanging on a wall. The case should be kept on the floor, because indoor air in winter is quite a bit cooler (and the humidity is therefore higher) in that area of the room than closer to the ceiling.
There are several devices available to cope with excessive dryness. If low humidity in your area occurs only in cold weather, a room humidifier for your music room may be appropriate. Furnace humidifiers do very little to raise the humidity level of your home in any significant or consistent manner. A cheaper remedy for low humidity is a "Dampit," instrument humdifier. This can be placed inside you guitar to release moisture directly inside the instrument. The trick with these is that you have to keep it damp. They don't work if you never wet them down. This is a challenge for most players as we like ot play as soon as we get the guitar out we want to play. Then we play until 5 minutes after we were supposed to be someplace so we throw the guitar into the case, replace the dessicated humidifier while promising ourselves that we'd wet it next time. Right. The remedy is to simply do it "this time".
Protecting a valuable guitar from humidity extremes, if approached intelligently, can be done with minimal inconvenience. It also makes good economic sense. The most expensive of the remedies suggested above may end up costing considerably less than the expense that can result from neglect, especially if there is a permanent loss of instrument value because of environmental damage. (This article originally appeared in Guitar Review, Summer 1988. It has since been edited.)

Qty.	The Oasis Humidifying System $16.95 for Guitar The body of Oasis™ is made from a specially designed fabric that allows water vapor (but not water) to pass through it. As the vapor transmission takes place, a vacuum is created and Oasis™ shrinks to compensate for the loss of water. By watching the Oasis™ shrink, you can determine when it is ready for refill. When refilling, you don't have to guess when it is full. Simply unscrew the black cap, fill with water to the top of the container and replace the cap. MSRP $19.95
Violin Dampit $8.50 Qty. Guitar Super Dampit $16.95 Qty.	Dampit Humidifiers Available for Guitar and Violin Protects instruments from damage due to excessive dryness. A soft rubber sleeve encloses a special open-cell sponge. Simply soak in water, wipe off, and insert through F-hole or 3rd and 4th strings. Instructions and handy humidity guage included. The Dampit humidifier provides long term humidity protection and guards against cracking and warping.
Qty.	Planet Waves Humidifier $10.99 for acoustic guitar The moisture reservoire suspended inside the soundhole of the guitar body releases moisture evenly and slowly to maintain the proper humidity level. The System suspends itself between the strings and never touces any part of the guitar body. MSRP $14.99

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