damp at base of chimney

We have a 1930's semi. I have opened the builders break on a disused chimney, but we have damp at the base. Would this most likely be from the base or from needing the chimney capped? I can get the chimney fitted with a cowl, but could it be rising damp and maybe need a chemical DPM?

Thanks for any help

You have a classic case of someone closing the stack and not venting it. So in the room, it should have a 9" square vent and the pot on the stack should just have a top lid or clay vent cowl sat on/in it. What happens is, warm air needs drawn up through the stack and out of the stack, otherwise the moisture in the stack condenses and causes damp at it's base.

https://www.toolstation.com/top-lid/p96165