The right relative humidity for a solid wood classical guitar is between 45 and 55 percent. Keep it there all year, avoid sudden changes of temperature, and most of the problems guitars suffer from never happen. Here is what each season asks of you.
Why humidity matters
Wood absorbs moisture from the air and gives it back. When the air is damp, the wood swells; when it is dry, the wood shrinks. A thin, solid top held under string tension feels these changes more than anything else in your home. Small, slow changes are harmless. Large or sudden ones cause cracks, lifting bridges, sharp fret ends and a sound that goes dull or thin.
Measure, do not guess
The single most useful accessory for a fine guitar is a hygrometer, a small instrument that shows relative humidity. Keep one in the room where you play and, ideally, one in the case. The air often feels very different from what it is: a warm, heated room in winter can easily fall to 25 or 30 percent.
Winter: the dry season
Heating dries the air, and this is when most cracks happen. Keep the guitar away from radiators and heaters. Use a room humidifier or a humidifier made for guitar cases, and check the hygrometer regularly. Signs of dryness are sharp fret ends sticking out at the edge of the fingerboard, strings sitting lower than usual and a slight hollow behind the bridge.
Summer: the damp season
In humid weather, the wood swells. The action rises, the guitar feels harder to play and the sound loses its brightness. Keep the instrument in its case when you do not play it, with a humidity-control pack, and avoid leaving it in a damp room or near an open window.
Spring and autumn: the changing seasons
When heating goes on or off, humidity can change by twenty points in a few days. These are the weeks to watch the hygrometer most closely and to adjust your humidifier.
On the road
- Never leave a guitar in a car: in summer it can overheat, in winter it can freeze.
- After coming in from the cold, leave the guitar closed in its case for an hour or two before opening it.
- When flying, take the guitar into the cabin whenever the airline allows.
- Use a good hard case: it protects the instrument from knocks and slows changes of temperature and humidity.
A simple yearly routine
| When | What to do |
| Every week | Check the hygrometer; keep the guitar in its case when not playing |
| Heating season | Use a humidifier; keep away from radiators |
| Humid months | Use a humidity-control pack in the case |
| Once a year | Have a luthier check the action, saddle and frets |
For more practical advice, read Caring for your Arrius.