Manufacturers warn owners to control the humidity of the guitar’s environment but many players are still either confused or ignorant to the challenges that can take place.
Wood is like a sponge, as it absorbs moisture it expands and as it gives off moisture is contracts.
When it expands, it has to go somewhere. This could lead to warping, twisting and in the case of a soundboard, especially flat tops, it could invert, in other words bend inwards, causing a change in your action and a whole lot of domino effects, like buzzing, going out of tune to mention a few, so, the wood in a guitar must be dry, but, not too dry. As a result manufacturers dry their wood to an “optimum” moisture content of about 6% – 8% and some are able keep their manufacturing facility at about 45% relative humidity. These conditions will result in a guitar that is stable under normal conditions. However, wood is still an organic material and susceptible to changes in climactic conditions and can adapt to various humidity conditions. In conditions of low humidity the wood will continue to lose moisture and shrink. Conversely it will gain moisture when conditions are damp and this will cause it to swell. The consequences can cause sometimes serious challenges.
The greatest danger is low humidity. Any part of the country where there is a high heat season (or where there are desert conditions) is likely to see low humidity. Another one is air-conditioning, as this can result in very ‘dry’ air, ie low humidity
The principal danger from low humidity is cracks in the top. Soft woods like spruce are more prone to humidity problems than hardwoods, but there are exceptions. Backs and sides can also crack quite easily. Some guitar woods, such as Brazilian Rosewood, are more susceptible than others. Another likely problem is shrunken fingerboards, resulting in frets protruding from the edge of the board.
If you leave your guitar in a car for too long in the sun, the car can heat up to some serious temperatures, especially if it is a hot day. This will result in a very quick loss of moisture, the wood to dry out too quickly, causing shrinkage and at the weakest place can crack or split, making for a very difficult repair
So, low humidity can damage your guitar. How to prevent this? First, be aware of the humidity level where your guitar ‘lives’. Humidity gauges are available and these may not be accurate enough for scientific purposes but for the guitar player they are fine. The goal is to keep your home/guitar room at about 45% relative humidity.
If the humidity is too low, a room or home humidifier is one solution, but often not enough. In most winter climates at somewhere about 30% relative humidity the moisture in the air will condense on the windows and the room will not get more humid. At that point a guitar case humidifier is in order. There are a few options available. They should be checked regularly. If you want to keep the guitar hanging on a wall, then the humidifiers which cover the sound hole will be adequate. Case humidifiers also are an option for dry conditions. If your humidity is moderately low (25 -35%) one in-case humidifier will be enough. If you over humidify, then swelling of the wood will occur.
Even if precautions are taken, wood being what it is, still tends to sometimes do its own thing regardless of what you do.