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I assume you have insulation in the outer roof, above the upper floor.
I would seal all the outer walls and the outer roof with plastic, completely tight and carefully.
I would consider sealing in the intermediate floor IF I suspected I wanted to keep various vapors and exhaust fumes away from the upper floor. If there were to be some form of living space (e.g., guest room, office, youth room) or if I suspected I would be using a lot of chemicals, solvents, etc., that might potentially harm what I store up there.
Otherwise, I wouldn't seal with plastic.
With plastic, you should be a bit more careful with ventilation, so you don't get increased humidity on the upper floor, and/or overpressure on any of the floors.
I assume you have insulation in the outer roof, above the upper floor.
I would seal all the outer walls and the outer roof with plastic, completely tight and carefully.
I would consider sealing in the intermediate floor IF I suspected I wanted to keep various vapors and exhaust fumes away from the upper floor. If there were to be some form of living space (e.g., guest room, office, youth room) or if I suspected I would be using a lot of chemicals, solvents, etc., that might potentially harm what I store up there.
Otherwise, I wouldn't seal with plastic.
With plastic, you should be a bit more careful with ventilation, so you don't get increased humidity on the upper floor, and/or overpressure on any of the floors.