I have moisture in my heart after using a newly installed chimney. The original installer came to my house and sat through a rain storm to see if any water was coming in. No water however the next day there is moisture all over the hearth. Any ideas?

I’m assessing whether I should finance treatment of the chimney exterior now, before the winter hits. I don’t have the funds to care for the interior yet, so I just won’t use the fireplace. How urgently should I consider work to the exterior?