How many inches and what type (blown in or batts) do you have?
The insulation will probably go on the floor so it will not reduce the heat in the storage area. You will also need to put in flooring for the storage.
Do you have foil on the roof panels?
I would check the tempeture in the attic to the outside tempeture in the summer and see how much of a change do you have. It may not be a lot if you have good insulation now.
A fan maybe better at reducing the tempeture for storage but if you have air leaks it could pull air-condition air from the home into the attic.
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