How to close an unused fireplace?

Hello!

Last year I had a house built. We also had a chimney installed. It is a single-shell insulated chimney system from Schreyer. Inside it is lined with fireclay. At the top there is a rain cap installed. At the bottom there is NO condensate drain but only a layer of concrete recycling. Now, at our connection point in the living room at the bottom, we do not yet have a fireplace (not even in sight). The "chimney connection hole" was simply left open by the construction company. Since it was obviously drafty, I stuffed plastic foil in it and later added a loose cover cap in front of it. Now, when rain occurs combined with wind, I occasionally hear drops hitting the foil... it is driving me crazy. I don’t know how to do it correctly now. Remove the foil and seal the hole airtight with a proper cover? But then the chimney would have no draft, just like it doesn’t now because of the foil. I’m afraid that water will eventually run out of the flap at the bottom or that moisture will cause damage to the lower part of the chimney. How do I do it correctly?

Try to close the chimney at the top. For example, place a concrete slab on it or something like that. This way, no more water will enter the chimney. Then it is best to stuff the hole with foil and use insulating material like mineral wool or something similar for thermal insulation.