What to do with cement render and gypsum plaster?

[JLK]

Hello forum - would welcome your views on the below, and any recommendations for lime render / plasterer / damp specialists in Cumbria in particular.

My third (top) floor, 19th century (exact build date unknown) flat has both a penetrating damp and a condensation issue on an external wall. The condensation I am dealing with (including by installing an extractor fan which has never been in place as this is a kitchen / living room). The damp is a bit tricker. That external wall has a concrete render (I know - it was there when I bought the flat, I didn't know any better at the time) which is cracked, and is gypsum plaster on the inside. From lots of reading on here it seems to me that the wall (a) can't breathe on the outside, and the concrete render is letting water in but not back out again, and (b) can't breathe on the inside either because of the gypsum.

In an ideal world (as well as addressing the condensation issue) I would take off the concrete render and replace with lime in the Spring, and remove the gypsum plaster on the inside and replace with lime, so the whole wall is breathable. However, the height of the outside wall means lots of scaffolding / possible expense, and while I'm getting quotes I don't know if I have enough money right now to replace the render (which is obviously the best thing to do). Any views on the following?

• If I do replace the external concrete render with lime, I assume I also need to replace the internal gypsum plaster with lime so everything breathes. Is that right, or can I get away with gypsum inside and lime outside?
• If I don't have the ability to replace the external concrete render with lime this year (and I know it should be done at some point), is it worth replacing the internal gypsum with lime plaster so the wall can still breathe on one side? Not perfect, but better than nothing?
• Any other solutions you've seen?

Thank you!

PS - I've had a type c damp proof course recommended on the basis that it's 'breathable', but am obviously v wary of going down that route.
PPS - I'd rather not post photos, thanks very much for understanding, and I know this means I'll get less detailed advice.

 

[aockenden]

Your question seems to basically boil down to: can I replace one side of the wall with lime and at least temporarily leave the other side as-is to minimize expenses.

The answer is yes, and you should definitely gain benefit to the entire wall from at least one side being breathable. The “all or nothing” thinking is a fallacy. The more non-breathable stuff you can replace with breathable stuff, the drier your walls will get.

You mentioned the external cement render is cracked and replacing that will be the more expensive vs. replacing the internal plaster with lime. So what about this as an approach: go ahead and replace the internal plaster with lime (going all the way back to brick or stone, as there may be cement under the gypsum internally as well). Then for the external side, get up on a ladder and use some kind of tube of modern caulking / sealant gunk to close up any cracks. This should hopefully minimize the water ingress but still allow the evaporation of whatever moisture IS in the walls outwards through the internal face which has been redone in lime (helped by your new extractor fan). Then hopefully you’ll be stable while you save up to get the external cement render properly replaced.

I would also make very sure the source of the moisture in the wall is from rain coming through the cracked external render, and that it’s not actually coming from above via a leaking chimney, broken slates on the roof, etc.

 

[Cubist]

Hello and welcome,

Some questions and an observation or two if I may.

• Is it safe to assume that you own the Freehold to the flat?
  • Are there any constraints governing what you may or may not be able to do with the exterior of the building?
  • Who is responsible for the roof and guttering?
• Does the external render coat cover only the floor your flat occupies?
  • If its a single continuous sheet covering the entire building or say just two floors your neighbours will need to be engaged and perhaps contribute to any repairs/replacement.
• The recommendation of a DPC should be regarded with considerable suspicion. A DPC is typically installed near ground level and its purpose is to block 'rising damp' from the ground below. It's highly unlikely for such damp could feasibly reach to the height of your flat.

 

[JLK]

Thanks for the quick responses and insight!

Cubist, in response to your questions (and thank you for observations):

1. Yes - I own the freehold, and no constraints to what we can do to the exterior. I am responsible for roof and guttering - I had the gutters inspected a few months ago and no issues. The damp patches are partly in line with a chimney that had lost its cowl last year - I had that replaced in July 2024 and have been assured that there's no water ingress through the chimney now
2. External render is only covering my flat, not the two below which we think are lime-rendered.

 

[alexander]

the roof can be a great source of water ingress, depending on what roof tiles you have on the roof will determine how much they leak it will also depend on what type of under felt you have on the roof as to how long it will remain viable, main areas of concern are the eaves & the verges as the felt erodes at the eaves and at the verges allowing water ingress getting under the roof tiles into the brickwork, the cement render will stop it escaping creating damp,
good luck alex