Damp areas appeared in new lime mortar indoors

[RedBrick]

Hello. I repointed a brick fireplace (c1841) and adjoining wall in lime mortar, after removing plasterboard/cement render/cement mortar. This was to resolve severe damp issues, exacerbated by flooding earlier in the year.

The mortaring was done over a period of about a month and was completed 3 weeks ago. In some places it dried overnight, others took about 2 weeks. The new mortar is much lighter than what remains of the old lime mortar which has been left in situ where it is sound. Early this week I noticed the new mortar had changed colour in a few patches mostly on the lower half of the wall. Today the patches are on the mortar above the fireplace (active log burner beneath and room is centrally heated). I tested with a damp meter to be sure that this was not a colour change and the readings are high on all the patches. The damp patches are not consistent with those areas where the mortar was filled deep and were in any case dry until this week.

There are no water ingress issues from the roof/chimney or floor above.

Anyone experienced this issue? Can it be resolved, or is it normal for this to occur?

Thanks.

 

[Cubist]

Presumably you have a steel flue from the stove connecting into the main flue mounted in the chimney stack. Do you know if this another steel or terra-cotta pipe? These can, besides getting very warm/hot at the lower levels carry rainwater down after entry through the chimney pot. That water, if it can't exit any other way out, may be findings its way into the wall of the chimney breast and reveal itself by evaporating out through the surface. Are there any signs of water having dripped on to the stove from above and then evaporating off?

I have a Clearview 750 and saw this very problem which seems to have been triggered or simply exacerbated by the recent heavy but un-driven by wind rains many have seen lately.

 

[RedBrick]

Presumably you have a steel flue from the stove connecting into the main flue mounted in the chimney stack. Do you know if this another steel or terra-cotta pipe? These can, besides getting very warm/hot at the lower levels carry rainwater down after entry through the chimney pot. That water, if it can't exit any other way out, may be findings its way into the wall of the chimney breast and reveal itself by evaporating out through the surface. Are there any signs of water having dripped on to the stove from above and then evaporating off?

I have a Clearview 750 and saw this very problem which seems to have been triggered or simply exacerbated by the recent heavy but un-driven by wind rains many have seen lately.

Hello Cubist, thank you for your reply.

As far as I know, the flue (pipe coming out of the stove top) is not extended up to the chimney stack and I'm not sure if it's steel or terracotta. When tapped it sounds like the latter. There have been no drips from above. The damp areas however, started and continue to spread, on the adjacent wall. This is an internal wall, behind which is a cavity which formed the back of a cupboard in the next room. It was boxed in (in the adjoining room) but there is a flow-through of air. I was able to check this while working on the wall. I had to remove/replace some bricks and took the opportunity to examine the cavity, which goes up to the floor of the shower room above this space. There were no signs of water from above and all seals visible in the shower area appear sound.

I've scoured the internet for this issue but found nothing so far. Oh dear.

RedBrick

 

[Cubist]

If you're confident that internal water sources can be ruled out that can only leave three other possibilities. The first would be rainwater currently entering the chimney stack OR such water having penetrated at some time in the past finally percolating/filtering down to where it is now escaping to atmosphere OR it is the product of condensation from the rooms atmosphere.

This last is the simplest and easiest to test to either confirm or reject the cause, so do the following first.

Tape a sheet of kitchen foil or clingfilm directly over the damp patch(es) you're seeing and leave it for 24hours while heating the room in your usual fashion. As you have a wood-burner directly below you should also tape the bottom edge of the sheet as well to ensure it remains in direct contact with the chimney breast. If the outer face of the foil/film is wet when you return to it then the problem is caused by condensation from within the room. If the outer face is dry but its inner face is wet then damp is being drawn through from within the wall.

Try this and get back to us.

 

[RedBrick]

If you're confident that internal water sources can be ruled out that can only leave three other possibilities. The first would be rainwater currently entering the chimney stack OR such water having penetrated at some time in the past finally percolating/filtering down to where it is now escaping to atmosphere OR it is the product of condensation from the rooms atmosphere.

This last is the simplest and easiest to test to either confirm or reject the cause, so do the following first.

Tape a sheet of kitchen foil or clingfilm directly over the damp patch(es) you're seeing and leave it for 24hours while heating the room in your usual fashion. As you have a wood-burner directly below you should also tape the bottom edge of the sheet as well to ensure it remains in direct contact with the chimney breast. If the outer face of the foil/film is wet when you return to it then the problem is caused by condensation from within the room. If the outer face is dry but its inner face is wet then damp is being drawn through from within the wall.

Try this and get back to us.

Thanks again Cubist. I'll follow your advice this weekend and report back.

 

[aockenden]

In addition to some experimenting as Cubist suggested, I’d also suggest giving things a few months to properly settle in. You only finished your pointing a few weeks ago right? Things are still changing in the lime at this point and it likely hadn’t reached its long-term equilibrium with the surrounding building fabric yet. In the meantime, experiment and observe and see what shakes out.

 

[RedBrick]

Thanks aockenden, good point. I'll do the test as Cubist recommended and see what it shows.