I recently placed some year sets into capitol plastic holders. Is the holder sufficient to protect the coins, or should I add something more than an enclosed safe with desiccant?
Capital plastic holders are fine & they are expensive. Sometimes the coins can be loose & shift in the holders but care in handling is all they require. Just recognize that they are not air-tight. Like all your coins, they should be kept in a cool dry place where the temperature does not change daily or seasonally. Avoid storing coins against an outside wall or in an attic. Can you post some photos of the sets you mounted in Capital plastic?
Collect89 is giving you good advice. I have had coins in Capital holders since the early 1960's with no perciptable changes. Yes they are expensive, but my experience tells me they are worth it.
Yup, the Capital holders are nice and strong. They are pricy but makes a good display and protects for your coins. Also if you like toned coins, coins in Capitol holders tone pretty nicely.