Removing mould from Cotswold limestone windows

[sofaman]

We have several windows with mould around the limestone windows. This is worst:

PXL_20221125_132127901.jpg

• sofaman
• 25 Nov 2022

We've used biocide to kill the mould and are sorting out ventilation.

However it is very unsightly and I cannot know when new mould is reappearing. A lot of anti-mould products that contain bleaching agents state they cannot be used on limestone, as it will react. Same would be true for any acid - e.g. vinegar.

Is there either something that can be safely used on limestone that will clean/restore back to (mostly) normal colour, or can I ignore the warning re bleach? e.g. it will damage the underlying surface, but this limestone isn't well finished anyway - not like smooth tiles for instance.

If I can get it cleaned up - I'm not too whether there is a recommended sealant/preventative stuff I can put down. I've read that mould can/will just grow underneath it?

Thanks in advance,
Steve

 

[Old Salt]

What about salt? A really strong salt water solution that you could try on a small area first, say on one edge of the surround.

 

[88v8]

We have exactly the same situation, although with proper leaded lights not plastic windows....
The mould is there because the stone is cold and moisture condenses on it, rather than the glass as it would have done with the original single glazing.

Clean it off occasionally with a small amount of household mould cleaner and a toothbrush. Doesn't need anything strong.

As with all mould/damp issues. ventilation will help, but even with an open fireplace you're going to get some condensation because the stone is the coldest element in the room, and the rough surface is ideal for mould.

 

[Mr Chibs]

Perhaps this would better with insulation on it and plastered.

In a previous house I had a stone door cill which protruded in to the room by 6", most colder times of the year, this had a covering of moisture on it, as the cold was being wicked in from outside (solid walls) and the heat in the room made this the cold surface where condensation would rest.

There was always a few mould spots, periodically I used a weak bleach solution to shift it, but I just had to put up with it.

 

[sofaman]

Thanks all - does seem a toothbrush and water can shift it somewhat. Will add in some salt, weak bleach and/more elbow grease.

Yup - can imagine single glazing would shift the problem. Not sure we have enough space for even thin insulation and plaster without losing light, though it does appear it was plastered at some point. The vertical stone is also a lot softer than the cill - guess that was for cost vs structural reasons.

 

[DiyNutJob]

Have you confirmed the bleach reaction? Maybe the reaction is that the limestone will return to normal? You really have nothing to lose. So try undiluted thick bleach on a small patch on the edge and see what happens. If no damage, you can go the whole hog. Then paint it with exterior paint that has anti-mould properties. It's quite obvious what you have there is a major food source for mould. You can deny that food for them using a barrier of suitable paint.

 

[Silverbantam]

I’ve tried putting thick bleach on a small area of my mouldy stone window sill and this is the result. What should I do
Now?

 

[DiyNutJob]

I’ve tried putting thick bleach on a small area of my mouldy stone window sill and this is the result. What should I do
Now?

Use it some more? I can see you did not even get it on the silicone or the wall.

 

[Silverbantam]

Isn’t the yellowing a problem? How would I get rid of that?

 

[DiyNutJob]

The PVC, silicone, and paint will not yellow. The UV coming through the window will keep them bright white. Your bigger problem is black, and not yellow.

 

[Silverbantam]

The PVC, silicone, and paint will not yellow. The UV coming through the window will keep them bright white. Your bigger problem is black, and not yellow.

But how can I get the yellowing off the stone?

 

[Mr Chibs]

Isn't the yellow the original colour of the stone... i.e. mould/grime removed?

Bleach the whole thing.

I think you'll need to insulate these if you want to stop moisture condensing on the cills. The moisture turns to mould if left.

But how can I get the yellowing off the stone?

 

[DiyNutJob]

But how can I get the yellowing off the stone?

You, kinda, paint it white? If you are a modern kiddo, maybe you prefer a blackout. Basically, paint it to whatever colour you want. White will be the easiest because it will blend with the seals.

I suggest redo the seals where there are open cracks. Cold air leak from outside will lower the temperature of the stone/whatever. When warm air hits that, water is formed.