Most of the time you can get excessive moisture in your case by leaving it open during rehearsals and where high humidity can be a problem. I use a humidity control called Desiccants. They are activated clay packets that can eliminate mildew and rust damage. The bags come in various sizes to protect a designated amount in cubic feet.

A list of various bag dimensions:

• 3 x 3 x 1/8″ and protects .14 cubic feet
• 3 x 3 1/4 x 3/16″ and protects .28 cubic feet
• 3 x 3 1/2 x 3/16″ and protects .42 cubic feet
• 5 x 3 1/2 x 1/4″ and protects .83 cubic feet
• 5 x 4 3/4 x 3/8″ and protects 1.67 cubic feet
• 5 x 6 x 1/2″ and protects 3.33 cubic feet
• 5 x 8 x 1 1/8″ and protects 6.67 cubic feet
• 5 3/4 x 10 x 1 1/2″ and protects 13.33 cubic feet
• 8 3/4 x 12 1/2 2 1/4″ and protects 66. 65 cubic feet

You can find the Desiccants from a company called:

ULINE-950 Albrecht Drive-Lake Bluff, IL 60044

1-800-295-5510