Efflorescence on chimney breast plaster?

[Reclaimer122]

Hi everyone,

My 1925 house is having some issues on a chimney breast in our dining room. When we moved in we pulled down the wallpaper in there, including on the chimney breast. The wall had been repaired before a long time ago and was uneven so I applied a skim coat of joint compound to level the area. Afterwards I painted and it looked great.

A month or two later I noticed the paint was sort of popping open in small areas. Over two years the compound has now been cracking and popping off and even exposing areas where the plaster underneath is deteriorating.

It looks like efflorescence is coming through the plaster, destroying the weak joint compound.

We've had countless roofers and chimney pros out to look and nobody knows what it could be from. Flashing looks good, roof is 12 years old and doesn't show signs of leaking, and the area is never actually wet or even damp. We have a chimney cap. The steam boiler flue is on the other side of the stack - the fireplace flue is on the side with the damage, but it isn't used and hasn't been for years. The previous owners had shoved some insulation up there which we removed about a month ago.

Does anybody have any ideas? Did I just use the wrong compound? Should we cover it with drywall and call it a day? Thanks in advance!

 

[JohnD]

It isn't efflorescence.

I don't know what materials are on that wall. The base render might be lime, or sand and cement. Doesn't look like gypsum plaster.

It looks like your skim coat is harder than the material beneath.

I don't know how (or if) the flue is ventilated

Is a steam boiler flue a metal pipe inside the chimney? Or are flue gases passing up it? Are they sulphurous?

 

[dontbelieveawordofit]

looks like there’s a lot of efflorescence to me , probably combined with using the wrong material to bodge it, and lack of or no preperation to adhere the incorrect filler to the dusty substrate. just a guess. you will have to take it all back to brick or stone and go from there.

 

[Reclaimer122]

Interesting! Thanks for taking a look.

Below are some closer up pictures including a small chip of the material that was underneath my skim coat. In my layman's terms I would say it looks like sand. When the skim coat first started cracking/popping, there was a fine powdery material underneath that looks like efflorescence to my untrained eyes. You can see it better in some of my new photos. The skim coat was an all-purpose joint compound (https://www.homedepot.com/p/USG-She...ose-Pre-Mixed-Joint-Compound-385140/202329714), looks like the spec sheet says it is a "Latex-type formulation" but doesn't give more details.

The steam boiler flue is metal. The first picture in my OP shows roughly the right half of the chimney breast, the boiler flue is in the left half and there is no sign of this issue over there. The boiler is oil fired and from about 2002.

The side of the chimney with issues houses the fireplace flue, but the fireplace has been unused for a long time by the looks of it. I can't see up it because it shifts to the right. It does have a damper.

EDIT: Photos below.

 

[Reclaimer122]

looks like there’s a lot of efflorescence to me , probably combined with using the wrong material to bodge it, and lack of or no preperation to adhere the incorrect filler to the dusty substrate. just a guess. you will have to take it all back to brick or stone and go from there.

This is what I was thinking. If we are getting efflorescence in the chimney bricks is it okay to re-plaster or simply cover the bricks with drywall? I would probably have a pro tackle it this time.

 

[JohnD]

I'd hack it all back to the brick too. Look at the base material and start again.

In UK we'd probably use a lime plaster.

In my region, people also use sand and cement render and a hard gypsum plaster skim to finish.

 

[tel765]

Presumably you are on a first floor - is there a basement or any other floor?

What your pics are showing is the face of a chimney breast - can you post pics showing the whole front face and the two side cheeks?
Is the chimney breast internal or on an outside wall?
Where the chimney breast pierces the roof is known as the stack. If safe, can you go on on the roof and examine the stack and its flashing and crown, and take pics and post them?
Its not clear where your steam boiler or its flue is located? But a pic should tell.
Are there two separate terminals above the stack capping?
Have you had a look at the chimney breast condition in the loft or any room above?

The odds are that the problem could be chemical reactions in the flue due to sooty residue from previous solid fuel fires but its best to check as above to eliminate possibilities.

Whatever, as advised above, the skirting should be removed and the plaster hacked back to bare brickwork before rendering with a 3:1 sand & lime render and a board finish.
Covering with plasterboard could make things worse.