Steve Morril is the E.U. Wurlitzer Music & Sound Guitar Service Manager, a 20 year veteran technician and builder of fretted instruments. You can get in touch with Steve by calling the Massachusetts Avenue Store in Boston at 617-738-7001.
BOTANY
In the beginning, your guitar was a tree; parts of several trees actually, but for the sake of discussion, let's consider just one tree. As a tree, the wood was part of a water system.
The cambium layer is a single cell layer responsible for growth. Bark forms on the outside of this layer and sapwood is formed on the inside. The inner bark is responsible for the movement of food from the roots to the leaves and all parts of the tree. Sapwood transfers sap from the roots to the crown of the tree and also stores food. As the tree grows, season by season, ring by ring, the sapwood is moved from the actual growing area of the tree and dies...becoming heartwood. Heartwood no longer transfers sap and is solely responsible for the support of the tree. Heartwood is the useful part of the tree for building, it is what your guitar is made of. Heartwood differs slightly chemically from sapwood. It contains deposits called "extractives" that contribute to properties like color, odor, taste, and resistance to decay. At all times of the year, the sapwood and heartwood are saturated with water.
Humidity
So what? I mention this only to establish the long relationship between your guitar and water. Water and the average wooden guitar are old partners. It can take 12 years or more of air drying to separate the water from the useful heartwood cells before the wood can be used to build a musical instrument.
So it shouldn't be surprising to find that your guitar is sensitive to the changes in climate and humidity. Some instruments are more sensitive than others. But all wooden musical instruments exhibit changes in shape and sound due to the fluctuating humidity of the seasons. Here in New England humidity varies drastically season to season. Warm damp summers and cold dry winters can wrench your guitar back and forth with annoying effects.
The ideal condition in which to build a guitar is 50% relative humidity. It's the midpoint in the range of 40% to 60% which is considered safe. Relative humidity refers to the amount of moisture the air can hold at a given temperature. Warm air holds more moisture than cold air. Above 60%, molds can form and glue joints fail. Below 40%, you can expect cracking of fine instruments caused by shrinking as the wood dries out.
Summer
During warm summer months humidity can be high. Average range in New England is 60% to 80%. Warm air can hold a lot of moisture. Remember this past summer? Your guitar will acclimate to it's surroundings and take in moisture. Water molecules occupy space in the wood cells so your guitar swells. The neck can bow changing the action. The top of an acoustic guitar may belly out around the bridge area also changing the action. The bridge, bridgeplate or braces can loosen. So it's not surprising that your guitar goes a little wacky with the heat. And not surprising that your guitar need adjustment in the early summer. Air conditioners and dehumidifiers can regulate indoor humidity. Remember, if you're comfortable, your guitar is comfortable.
Winter
On the surface winter sounds like a piece of cake. According to Climates of the United States, by John L. Baldwin, average relative humidity is 67%. So, what's so dry about winter? To understand this is to conquer a great misconception about humidity. Relative humidity refers to the percentage of moisture in the air compared to how much moisture the air can hold at that temperature. "At that temperature" is the key here. Cold air can't hold as much moisture as warm air. So 67% means 67% of a smaller potential because the temperature in January is likely to be, oh say, 19 o F. - remember last January? But wait, there's more.
Question: What is the first thing the average human does when it's 19 o F ?
Answer: ...no, not go surfing... Crank up the heat ! Heat the indoor air to a comfortable 70 o F. and the relative humidity is well below 40%. The heated air sucks the moisture out of everything. Doors that were sticky and jammed during the summer, now swing freely. Your favorite rocking chair is loose and kind of squeaky. Your hands are dry your lips are cracking, your guitar is crying "help me".
Low humidity will do more damage to your guitar than high humidity. Dryness causes cracking and though cracks are repairable and don't interfere with structural integrity or inhibit sound, once repaired they always show. If appearance is important, just knowing it's there can be annoying. For some players it can be enough to make them sell a fine instrument. The better the instrument, the finer the wood, the more sensitive it's likely to be.
All heat is drying during the winter: wood stoves are the driest source of heat. Room humidifiers will help stabilize the humidity. There are many types available. They hold anywhere from 1/2 to 5 gallons of water. Misters, steamers, and evaporators put moisture back into the air. House plants also humidify. Anything you can do to re-introduce moisture into the atmosphere will be helpful. Try keeping the guitar in it's case when you aren't playing. It's easier to humidify the small area enclosed in the case than the whole room or house. There are several small guitar humidifiers available; Damp-It, Guard Father, and Onetek, to name a few. They all seem to work about the same. They consist of a porous material like a sponge or a porous stone that you soak with water and leave either in the case or even inside the body of the guitar, as in the case of the Damp-It. Most important is that you use a humidifier that keeps it damp all season. You have to remember to soak it periodically, and that can mean every couple of days. A dampened sponge in a ziplock bag with holes poked in the bag will also work nicely. Just remember to moisten the sponge regularly and don't put any humidifier where it might do water damage to the wood or finish. The commercial guitar humidifiers are fitted with all types of clips, mounts, and covers for easy access and protection.
Temperature
Temperature itself isn't the danger for guitars with the obvious exception of the extremes. That is, a hot car trunk in the summer can literally bake your guitar into a guitar kit or guitar puzzle. I'm sure you've heard that cold cracks finish. There is some truth to this.
Cold alone won't really crack or check a finish. Modern finishes are capable of withstanding extremes of cold. The real problem is that cold makes matter contract or shrink. Heat makes matter expand or swell. Remember science class? The finish and the wood expand and contract at different rates. So, if you bring your guitar into a freezer or say your car in January's 19 o F., it will cool off. But this cooling will take a while. It wouldn't instantly drop to 19 o , but slowly cool until it's temperature matches it's surroundings. As long as the cooling happens slowly, all is likely to be fine. Even staying cold for long periods of time won't do any damage. But the reverse is also critical. If you bring this 19 o F. guitar into a 70 o F. room and quickly expose the finish to 70 o F. surroundings, it will begin to warm and expand. The finish begins to warm while the wood is still cold, so, as the finish is expanding, the wood is still contracted. Something's got to give. So, if you bring a cold guitar into a warm room, don't open the case right away. Allow the guitar and the case to Warm Up just as they cooled off. Slowly, the case will act as insulation and slow the warming process so that the rates of expansion of the materials the guitar is made from, will be more even. The chance of "finish checks" will be minimal.
Obviously, it's best to to leave your guitar in extreme cold. But remember, guitars are shipped to all parts of the world in all kinds of weather conditions. They aren't shipped under climate controlled conditions. The vast majority of the time they make the trip without problems. So when you turn on the heat, make sure you humidify the guitar. If you're comfortable, your guitar will be comfortable. Be sure to have your guitar serviced regularly so that small problems don't become expensive repairs.