Insulating off unused masonry chimney

[dougstove]

Hello;
I have an unused masonry chimney extending from my basement, with exposed masonry through the house (just behind my current air tight woodsove) and out through the roof.
It caused a draft, and was in poor condition, and caused some leaking through the roof.

So: Last summer, I demolished the masonry chimney down to below the roof line, and capped over the hole (as part of a re-roofing that was done anyway).
When I demolished the chimney, I went down as far as I could reaching down from the top, but it is still 29"-15" above the level of the insulation in the ceiling.

My goal was to convert the in-house portion into a heat sink for the woodstove.

I left the stub extending into the attic for about a month, to bake dry, and then 2 weeks ago, I crawled up and wrapped plastic/aluminized foil bubble wrap insulation over the top and around the sides, getting about 80% coverage.

Then, yesterday, I was up again, and noticed condensation on the insulation, on the side facing the masonry.

So: I am wondering if the old chimney is still drying out, or whether it is venting warm moist air to the cold attic, causing condensation. Or both.

I could crawl up and demolish some more masonry, down to nearer ceiling level, but it would be fairly gruesome work.
I could also seal off the ash dump in the basement better, to decrease updraft.

Any suggestions or experience with this situation? Writing it out is making me think of things to do in any case.
cheers, Doug

 

[branchburner]

You are going to get condensation due to temp differences, but that does not mean you are losing a lot of heat. But to avoid mold I think you want that stub to breathe. Maybe you could box it in, leaving some dead air space.
I would seal up the bottom to reduce any convective circulation within the chimney.