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The plastic vapor barrier should be used in all insulation projects. It is even more necessary when using organic materials like cellulose or cotton because they are more prone to feed mold and mildew and rot out if water vapor condenses inside them. (Mold does not feed on fiberglass). The purpose of the plastic sheet is to keep the moisture/humidity in the warm, inside air away from the insulation. Even though the temperature can pass through it, the vapor cannot.
Fiberglass is the standard for insulation because it is the most cost efficient. It also does not collapse and lose its insulating properties over time as much or as quickly as cellulose or cotton can. Denim will cost more and probably do a fine job, though.