Capping unused fireplace chimney considerations?

[LonArcvarden]

Our home has a brick chimney with two flues for two wood burning fireplaces that are no longer used. I keep the lower dampers shut but was thinking of covering the top outlets to prevent cold air/pests from entering the chimney. Eventually, the fireplaces will be covered with insulation/drywall/etc.

My chimney looks similar to this and is on an exterior wall:


Based on this article from Fine Home Building, I was just going to place a 2x8x16 stone patio paver over each outlet and call it good. They also mention a chimney balloon but I don't think I will bother with that.
But upon further research, I see some people claiming that the unused chimney still needs ventilation and that air bricks should be installed to prevent condensation?

[princemeraj]

Why would you not use a balloon? It would really help with insulation. Couldn't you just buy an un-inflated exercise ball from a yard sale and use that?

[ayawoalhassan]

I had the same setup and found that capping the flues at the top stopped the cold drafts way better than relying on the dampers alone. I had Chimney Services New Jersey | Expert Cleaning, Repair & Inspection handle mine, and they made sure the caps were sized right so moisture didn’t get trapped. Once the top is sealed properly, closing off the fireplaces inside becomes a much simpler project.