ᐅ Attic Conversion Without Insulation (Insulated Ceiling Below)

Hello,
I purchased my house (built in 2008, low energy, 26 cm (10 inches) exterior insulation, triple glazing, air-source heat pump with ventilation, solar thermal) in 2013. The house has two floors plus an attic. The ceiling between the top floor and the attic is insulated. Insulating the attic itself is not advisable due to building physics reasons (no room ventilation).
However, I use the attic as storage space and would like to improve its appearance a bit. Currently, I can see directly onto the roofing underlay, which is installed with overlapping seams but is not taped. As a result, hundreds of flies gather in spring and autumn, crawling through the overlaps in the underlay.
I would like to tape the seams with special adhesive tape and then cover the roof structure with OSB panels for aesthetic reasons. I plan to install ventilation grilles in the panels.
Is taping the roofing underlay recommended, or might it create a moisture trap?
Does anyone have experience with attic conversions without additional insulation (with an insulated ceiling between living space and attic)?