Cold air coming in through gas fireplace
I bought a house that had been added on to 10 years ago and during the build a gas fireplace was installed on an outside corner wall. Cold air is streaming into the room from that fireplace. I don't see a flue to close off when the fireplace is not in use.
First question: I've tried various and multiple methods to block the glass opening, but the cold air keeps seeping around my pieces of wood, masking tape and bubble wrap, so I'm going to put a much larger piece of plywood over the entire front of the fireplace and probably screw it in place. Any suggestions on additional steps to take, such as applying some insulation? Or is the plywood sufficient? I looked at fiberglass insulation, but for various reasons, would rather use some other product.
Second question: once warmer weather comes, how do I go about identifying and fixing the problem?