ᐅ Cold Chimney in New Construction – Water Damage Caused by Condensation?

Hello everyone,

I have a problem with water damage and since I don’t personally know anyone who can help, I’m trying here before hiring a building surveyor.

Situation:
We have been living since 2025 in a newly built house with a ground floor and one upper floor. Above that is an uninsulated attic. (Solid masonry house with a hipped roof)
The house has a chimney that currently has no stove connected.
The opening is there and is sealed with glass wool insulation.

Now to the problem:
Every cold season, around October, water has formed on the chimney.
This was visible from the outside in the part that runs through the attic (photos). The light-colored brick got darker day by day until actual water droplets were visible.
The bricks became increasingly darker and damp.

When spring came, it gradually disappeared.

This winter, we have the problem again and now it seems the water has moved one floor lower and caused increasingly large stains on the plastered wall in the dressing room (see photo, red border for the room, green for the chimney). I suspect from online research that salt efflorescence is now visible.
(more photos)

Our neighbors have exactly the same house from the same builder and currently have no problems – but they have a fireplace that they use regularly.

Where is the water coming from in our case?
The builder had the vapor barrier and the roof checked last winter; everything is tight.

Could the condensation be inside the chimney shaft – and does the chimney need to be fully insulated to prevent condensation?
(We live in northern Berlin and have had a rather cold winter so far.)

I am at a loss and worried that further water damage may occur.
Thanks in advance!

As for me, I don’t see any photo, so: is water leaking inside? Is there a cover on it?

Thanks to the heat pump, we now also have two unused ducts, which have simply been capped off...

The photos. They apparently were not attached.

By the sealed opening, you probably mean the spot where the non-existent stove’s pipe would connect to the chimney (?).

An unused chimney is essentially a duct running alongside heated rooms that itself is not heated and therefore cooler. If the chimney is sealed at the top, this only applies to the duct itself. If it is open at the top, it obviously communicates with the outside air (and there is also rain entry). But even “just” condensation is enough to make the chimney walls damp. Anyone living in a house without a chimney should either omit or seal off the chimney and ideally also remove the chimney cap, relocating this closure inside the building’s thermal envelope.

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Hello 11ant, thanks for the comment! What you described makes a lot of sense.
We would like to have a fireplace, but currently don’t have the budget for it.

How can a fireplace be sealed?
I’m actually confident enough to climb onto the roof myself to save on costly contractor fees. =)
Best regards