Guitars and other fretted instruments thrive best in the same temperature and humidity range that is comfortable to people. A temperature range of 60 to 75 degrees and humidity of 40 to 50 percent is ideal. In many areas of the country this means it is necessary to provide humidification during the winter months when central heat is in use and dehumidification in the summer.
Attics and basements with no heat or air conditioning are the worst possible places to store instruments, and care must be taken when transporting guitars, banjos, and mandolins in your car. Just as you would never put a child or pet in the trunk of a car in the dead of winter in Minneapolis or leave one in a car with the windows rolled up in the summer time in Miami, you should not subject your guitar or any other instrument to this sort of abuse. While technically speaking your guitar may not be alive, if you treat your instruments as though they were alive and provide them the same temperature and humidity conditions that are most comfortable for you, they will perform at their optimum and be far less prone to problems.
George Gruhn