If you are going to do skirting, need to think about framing it. Support from bottom, sides and top. Do not want wind to blow against a solid wall just hanging. In spite of weight, given leverage, needs bracing against ground/posts or something so that movement does not twist rim joist. If post are on leading edge than could create a box frame between posts. If the more common cantilevered front edge of deck, then could put in new posts into the ground (with footings) and build frame between the new posts and deck. If the new posts are not structural to the deck (not needed to support the deck), three posts should be adequate with a box frame and some diagonal cross bracing. Wind load is probably at least as important as dead weight in this large a flat surface.

Furthermore, underside of deck needs ventilation. So whatever pattern of boards you use, leave a lot of gap somewhere for air to pass through to keep condensation from forming on underside of deck boards. Also keep ends of IPE off the ground. It is not suitable for ground contact(moisture will cause it to rot)