A less expensive and more commonly available material is foam board, being mindful of its negative qualities. The verdict on sealing vs. circulation is mixed. Generally if the painting is to be kept in a totally climate controlled environment sealing is recommended, but where there is adequate humidity to cause strange fungal growths in closed spaces, small holes to allow circulation may provide a better long term result. The only true vapor barrier is metal foil. When water comes and goes, it does not go without making changes.
Industrial air is high in damaging acids.
Cobalt salt cards are an inexpensive way to monitor humidity in a framing package. Artwork that is sealed by the more conventional method of using paper should be removed and inspected every 5-10 years. When a moisture proof seal is employed artwork should be checked every 20 years. Frames Should be chosen to enhance the artwork. A narrow molding can be given additional support by means of strainers attached to the inside rear of the frame.