Chimney Breast Damp - Cowl or Ventilation

[stevs]

Hi all,

Just moved in and there is a patch of damp about 1m2 at the top of the chimney breast in one of the rooms on the second floor. Fireplaces have all been covered before I moved in. Feels wet to touch so had damp guy round, he said could either put a cowl on the chimney for £120 (none of the chimneys have been capped), or could drill a few holes and fit one of these in: https://www.screwfix.com/p/map-vent-adjustable-vent-white-229-x-152mm/7374d

Either option would probably dry out the damp enough for it to be painted, he said.

So could I just drill a few holes and fit the vent over it? Any potential problems in doing so? Or any benefit to having the cowl put in instead?

As a side note of curiosity, I see a lot of damp problems posted that are related to chimneys and fireplaces being borded up without proper ventilation - why is that? Do we know more about ventilation requirements now than before? Or were a lot of people just cutting corners when all these fireplaces were boarded up?

 

[Harry Bloomfield]

My view is to stop moisture going down a chimney and to allow some ventilation too. So vented cap at the top, check the pointing, lead and flaunching, plus make sure air can enter at the bottom to dry any condensation in the chimney.

 

[stevs]

Air can enter at the bottom. So a cowl would do the job just like a vented cap Or is there a difference?

 

[Harry Bloomfield]

Air can enter at the bottom. So a cowl would do the job just like a vented cap Or is there a difference?

Whatever will stop rain landing inside the chimney.

 

[bobasd]

Neither "rain landing in the chimney" nor lack of through ventilation is the problem.
" the pointing, lead and flaunching" is more likely the problem esp the lead flashing.

OP, you need someone to go up on the roof and photo: the stack, the flaunching and the lead flashing.
Also, you can go in your loft and examine the chimney breast for water stains.

 

[noseall]

Chimneys are notorious for misleading the owners - sneaky bu££ers. There can be numerous reasons for staining to manifest on the surface and not always from external damp sources.

As said, rule out the obvious first, then look at other culprits. We have had to render chimney breasts in the past that have been saturated with hygroscopic salts, in order to prevent stain bleed. These would be coated in SBR slurry and then with SBR added render etc. Once blocked the breast can be finished with usual plaster methods.

 

[bobasd]

what do you mean by "usual plaster methods"? After floating with sand & lime the render is typically skimmed with a Remedial finish - never use a usual gypsum finish if possible.

 

[noseall]

what do you mean by "usual plaster methods"

It could be dabbed over with plasterboards and skimmed, scratch floated and skimmed (as you said) or even coated with renovating plaster and skimmed.

 

[bobasd]

D&D is a bad remedial idea for c/breasts and damp affected walls because salts could come through the dabs and present on the decorated plaster board surface.

 

[Nige F]

Through the sbr slurry ?

 

[noseall]

Through the sbr slurry ?

No. The render is the protective coat. There would be no point in putting the waterproof render on otherwise.

 

[noseall]

D&D is a bad remedial idea for c/breasts and damp affected walls because salts could come through the dabs and present on the decorated plaster board surface.

Err....

These would be coated in SBR slurry and then with SBR added render etc. Once blocked the breast can be finished with usual plaster methods.

 

[Nige F]

Hi all,

J Or were a lot of people just cutting corners when all these fireplaces were boarded up?

That was the era of Barry Bucknell .....covering everything with hardboard. Debatably better than rippin out features. I hope you are following the thread