Covering the top with aluminum foil or aluminum foil tape is a must. I think you already said you do that. Cypress mulch is the best I've found, but even in Florida which is very humid, the humidity only holds for a couple weeks. Don't bother misting with water. Instead, when your humidity drops pour a couple cups of warm water over your substrate and mix it in very well with your hands. Much more effective than misting.

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