Two chimneys removed about six months ago, question about moisture

[Rollerball]

My bungalow's two chimneys were removed about six months ago - one hadn't been used for over twenty years and the fireplace for it was (and of course still is) blocked off in the room.

The other one is from a small fireplace in my lounge.

I've just been up to the attic for something unrelated and had a look both of the remaining chimney stubs (which end just a few inches below roof level) - the one that's connected to the still open lounge fireplace has some moisture apparently on the underside of the new white roofing 'felt'. First time that I've noticed that but I've not checked for months and we have had a lot of damp air due to the lousy weather.

There is also a vent tile above this chimney (as there is for the other one) so the airflow is good.

Am I correct in assuming that this moisture on the underside of the felt is condensation because of moisture/warm air flowing up from the open fireplace in the lounge and then condensing on the cool/cold-ish felt? Of course there's none on the underside of the felt above the chimney with the sealed fireplace.

 

[cross thread]

Sounds like warm air rising is the issue