Most guitars, especially Spanish guitars and acoustic guitars, are made from wood. The wood is still a ‘live’ entity. That means the wood is liable to change its dimensions ever so slightly. This is caused by humidity. Too much water vapor in the air causes the wood to expand; too little water vapor in the air and the wood is liable to shrink. Expand or shrink, these two movements are going to crack the guitar and ruin the sound of the musical instrument.
If you live in a climate where there are sharp rises and/or falls in humidity then you should consider buying a programmable thermostat that will regulate humidity in the room where you keep your guitar. Any good programmable thermostat will keep a home at a comfortable temperature. Judicious use of an air-con or central air system can take high humidity from the air. Unfortunately, only a humidifier can put water back into the air when it is too dry.
The Honeywell Prestige HD is one of the few programmable thermostats on the market with the ability to regulate both humidifier and dehumidifier units. This thermostat will give you piece of mind that your guitar is being kept in the right climate. The Honeywell Prestige HD thermostat, however, is a top of the range model that costs about $400.
If only high humidity is a problem a good programmable thermostat to get is the Honeywell VisionPro 8000. This model detects high levels of humidity and gives the room a quick spurt of air-con to lower humidity levels.
Finally, there is the popular Lux Products TX1500E that is only $40. It has a monitor that reads humidity in a room. This allows you to check every so often. If you think your guitar is in danger boil some water to humidify the room or turn on the air-con to dehumidify.