Gypsum plaster, what to do

[Rosemouse]

Hello, you people are so helpful, and so I am asking another question. My poor old wreck of a house is made from stone and brick, dating to the 1820's and then the top storey was added around 1935 but built in the same way. At the same time the second storey was added, the ground floor must have been renovated because the ceiling, floors, and partition walls all seem to date to this era or near enough. And the walls are all covered in gypsum plaster.
My understanding is that gypsum plaster is hydrophilic, and that if I really want a good, dry, warm house, I probably need to peck all of this stuff off (it is already crumbling off on the ground floor and on the north-facing rooms) and redo it in lime and hemp. Does anyone have any experience with this, or know more than I do about gypsum? Do I need to bother with the internal partition walls which are also gypsum plastered - they are pretty thin and there is a bit of a problem with noise - if my family come to stay in the other part of the house I can literally hear their conversations in the main house's bedrooms. I'd love to know of some solutions/things to think about. I am going to do a course on lime and hemp plastering in March, I am under no illusion that it is a small job!
Many thanks!

 

[Kearn]

Gypsum isn’t great. At all. However, it doesn’t always cause issues.

Much will depend on the issues you have, or think you have hidden behind it. What type of stone - is it being damaged due to a harder plaster. What’s on the outside is more important than the inside, as that’s where rain will be getting behind it through the cracks and keeping your stone wet making it a great conductor not an insulator. It may be that if you have damp due to ground levels/ground make up, it’s only the lower few feet of the ground floor walls that need remedying internally. Largely internal and stud walls don’t matter*, there are other benefits of lime plaster in moisture management, warmth, health, sustainability… but sometimes you need to draw the line with your pragmatic sharpie.

*internal solid walls can still track moisture up from the ground.

Removing gypsum/tanking is normally a hugely intrusive, time consuming, messy job. Rewarding though. Getting lime back on is slow, pricey and needs some skill / trial and error (if DIY) and a lot of patience.

The thickness of your walls may negate any benefit of needing insulation, once they’re dry. If they’re sodden, that’s about an inch/month to dry out. Before then you won’t really feel the benefit and lime won’t be that feasible as it just won’t go off.
Also, hemp doesn’t like getting wet - it rots and apart from diminishing any thermal/structural benefits, it can stain through final finishes.

 

[Cubist]

Gypsum based plasters have been in use for millennia and some forms are referred to as Plaster of Paris. It was commonly used in many parts of the world and you may find that similar properties near yours also used it in their original construction. Consequently you may find that the plaster in your house is absolutely original and does not indicate a later repair or replacement. The reason that you will find many PP members here prefer and will suggest lime based plasters and mortars is that gypsum based products are considerably less permeable than these and also that lime based products were more extensively used here in the UK.

As @Kearn says above, I would not worry about your internal partition walls they contribute next to nothing to the vital breathing functions of the property. If space permits you could easily erect a further sound-proofing skin to one face or the other of these partitions to reduce the unwanted sharing of conversations.

 

[Rosemouse]

Gypsum isn’t great. At all. However, it doesn’t always cause issues.

Much will depend on the issues you have, or think you have hidden behind it. What type of stone - is it being damaged due to a harder plaster. What’s on the outside is more important than the inside, as that’s where rain will be getting behind it through the cracks and keeping your stone wet making it a great conductor not an insulator. It may be that if you have damp due to ground levels/ground make up, it’s only the lower few feet of the ground floor walls that need remedying internally. Largely internal and stud walls don’t matter*, there are other benefits of lime plaster in moisture management, warmth, health, sustainability… but sometimes you need to draw the line with your pragmatic sharpie.

*internal solid walls can still track moisture up from the ground.

Removing gypsum/tanking is normally a hugely intrusive, time consuming, messy job. Rewarding though. Getting lime back on is slow, pricey and needs some skill / trial and error (if DIY) and a lot of patience.

The thickness of your walls may negate any benefit of needing insulation, once they’re dry. If they’re sodden, that’s about an inch/month to dry out. Before then you won’t really feel the benefit and lime won’t be that feasible as it just won’t go off.
Also, hemp doesn’t like getting wet - it rots and apart from diminishing any thermal/structural benefits, it can stain through final finishes.

Thank you for this thoughtful reply, it has helped me to validate what I was thinking, that I need to concentrate on getting the exterior of the house free of concrete and then re-rendered in lime (a job for some people who know what they are doing, have scaffolding, and are not me!) and then to concentrate internal efforts on the ground floor first. I think the top floor seems pretty stable.

 

[Rosemouse]

Gypsum based plasters have been in use for millennia and some forms are referred to as Plaster of Paris. It was commonly used in many parts of the world and you may find that similar properties near yours also used it in their original construction. Consequently you may find that the plaster in your house is absolutely original and does not indicate a later repair or replacement. The reason that you will find many PP members here prefer and will suggest lime based plasters and mortars is that gypsum based products are considerably less permeable than these and also that lime based products were more extensively used here in the UK.

As @Kearn says above, I would not worry about your internal partition walls they contribute next to nothing to the vital breathing functions of the property. If space permits you could easily erect a further sound-proofing skin to one face or the other of these partitions to reduce the unwanted sharing of conversations.

I'm learning rapidly that compromise is sometimes a valid response to parts of renovation, and sometimes it's the only response that works!

 

[paulc]

The reason that you will find many PP members here prefer and will suggest lime based plasters and mortars is that gypsum based products are considerably less permeable than these and also that lime based products were more extensively used here in the UK.

I'm not entirely convinced about the "gypsum is less permeable than lime" argument. However, gypsum is water soluble and doesn't tolerate being constantly damp (it just turns in to a mush).

 

[Kearn]

Yep, and there’s the difference between people stating things are breathable because of some lab generated vapour permeability measures vs the real world where it’s actual liquid moisture that’s the challenge! It certainly mushes, but also hold moisture against substrates it shouldn’t rather than allowing to evaporate.

The thing is that often gypsum is just the tip of the iceberg, And it’s sat on top of cement / tanking…

 

[Rosemouse]

Yep, and there’s the difference between people stating things are breathable because of some lab generated vapour permeability measures vs the real world where it’s actual liquid moisture that’s the challenge! It certainly mushes, but also hold moisture against substrates it shouldn’t rather than allowing to evaporate.

The thing is that often gypsum is just the tip of the iceberg, And it’s sat on top of cement / tanking…

This is just on top of the old stone and lime-mortar wall on the inside. I know this because a large section in one of the downstairs rooms has lifted cleanly away from the wall - it's about a cm of gypsum and then wall. Upstairs seems to be holding fast but it's under a layer of 70's wallpaper so there's no doubt some wondrous surprises in store. I have exactly the same setup in one of the outhouses, only that has damp flooring and the plaster on the walls is permanently damp there. Inside the house it doesn't seem to be damp at all.

 

[Rosemouse]

This is just on top of the old stone and lime-mortar wall on the inside. I know this because a large section in one of the downstairs rooms has lifted cleanly away from the wall - it's about a cm of gypsum and then wall. Upstairs seems to be holding fast but it's under a layer of 70's wallpaper so there's no doubt some wondrous surprises in store. I have exactly the same setup in one of the outhouses, only that has damp flooring and the plaster on the walls is permanently damp there. Inside the house it doesn't seem to be damp at all.

And I've just tested a sample of that crumbled up and with warm white 14 degree cleaning vinegar poured over. There is a slight fizz, not enormous but enough to see and hear.

 

[BigTashJon]

Sounds like you're in a similar situation to us rosemouse, we moved into an 1812 cottage 6 months ago and uncovering all the abuse that has been caused by using the wrong materials, currently im running at 77 percent humidity at 18 deg c, with the windows constantly cracked slightly 24 hours a day, when the warm weather comes im going to knock off the cement based pointing someone has used to cover up the lime mortar underneath and inside knock the gypsum plaster back to bare brick on the outside facing walls, you could always do a day course on lime plastering if your a knowledgeable DIYer, i think thats what im going to do as im poor

 

[Lacan07]

As has already been stated gypsum plaster doesn't deal with moisture well regardless of breathability, but the important thing to remember about modern plaster is that gypsum plasterers are rather fond of slobbering the wall with PVC glue in order to get it to stick, (with the exception of plasterboard). This is what really damages your walls.

 

[Rosemouse]

Sounds like you're in a similar situation to us rosemouse, we moved into an 1812 cottage 6 months ago and uncovering all the abuse that has been caused by using the wrong materials, currently im running at 77 percent humidity at 18 deg c, with the windows constantly cracked slightly 24 hours a day, when the warm weather comes im going to knock off the cement based pointing someone has used to cover up the lime mortar underneath and inside knock the gypsum plaster back to bare brick on the outside facing walls, you could always do a day course on lime plastering if your a knowledgeable DIYer, i think thats what im going to do as im poor

I have enrolled in a four-day-long course in hemp and lime plastering with a woman who specialises in this and in tadelakt nearby, in March. But I'm not under any illusion that it's easy, I've watched youtube videos, it is a serious amount of work and i don't want to do more than is necessary!. But the gypsum plaster in my house is the original, it was used a lot here in France in the interiors of houses. We also have the windows open in all temperatures. It's not ideal!

 

[Stevers]

As I have mentioned in another thread PVAc is one of the most breathable polymers there is, more breathable than many paints, at least some PVA's. D4 exterior PVAc wood adhesive for example is a lot less breathable as not only is it more highly polymerised (HMW = High Molecular Weight) but it has waterproof resins added as well. Cheap PVAc sealer is totally different and is specifically designed to be permeable, try making a garden gate with PVAc sealer and see how many weeks it lasts.

Gypsum plaster although 'breathable' in all normal senses, really doesn't cope well with the damp or salts typical of old buildings so is never any better a choice than 'something you might get away with for a while'. As Toby has stated above PVAc polymers can be very 'breathable', and I for one am happy to stabilise weak and dusty substrats with dilute PVA Bond it prior to paint or plaster and have got excellent results doing that. SBR Bond on the other hand is not 'breathable', and nor is it intended to be.

In my experience pink gypsum plasters and strong OPC mixes (the devil's glue) are the particular enemies of old buildings, closely followed by Sandtex type masonry paints.

 

[Rosemouse]

Gypsum plaster although 'breathable' in all normal senses, really doesn't cope well with the damp or salts typical of old buildings so is never any better a choice than 'something you might get away with for a while'. As Toby has stated above PVAc polymers can be very 'breathable', and I for one am happy to stabilise weak and dusty substrats with dilute PVA Bond it prior to paint or plaster and have got excellent results doing that. SBR Bond on the other hand is not 'breathable', and nor is it intended to be.

In my experience pink gypsum plasters and strong OPC mixes (the devil's glue) are the particular enemies of old buildings, closely followed by Sandtex type masonry paints.

I'm taking all this in - my question I guess pertains to gypsum plaster that is not recent. I am in France, and a lot of old French houses have always had gypsum on the inside. Although I did test the plaster that is coming away in shards in my office, and it fizzed a little bit. It's been laid directly onto the stone but it isn't bonded to the stone. The ground floor of this house is 50cm thick stone wall, and dates from the early 19th century. It has a very slight damp issue which both masons have said is due to the outside being covered in concrete - I am working my heart out over the next few months to earn enough to pay someone to get all that off and re-render it in lime. But I have accepted that for a number of reasons I am going to have to redo all the plasterwork in the ground floor. The second storey however was added in 1936, it also has gypsum, much whiter and finer that the downstairs gypsum, which is brownish - if it is not coming away from the stone and there is no damp, can I just leave it? it will be a HUGE job to get it all off every single room wall.

 

[Stevers]

There is no need to remove gypsum plasters especially historic ones on principle. There is quite a lot of pink finishing plaster in my basement left behind by the damp wallies, but I have only ever removed it where it's failed - or now I think of it, where it wasn't quite straight enough for my OCD!

I've also had really good results in the UK with Thistle One Coat which is a white gypsum plaster that seems at least somewhat resistant to old building nastiness. Twenty-eight years ago (before I knew 'better') I used it to fill chases in a wall where the lime plaster was badly perished through damp. Last year I took off the rest of the perished lime plaster leaving the filled chase in place as a reference for levels. I then replastered in NHL render with a Thistle One Coat finish - after all it is my house and any failure will be my problem... The plaster in that old chase remains absolutely sound and the new plaster is fine too - hope this helps!

 

[Rosemouse]

There is no need to remove gypsum plasters especially historic ones on principle. There is quite a lot of pink finishing plaster in my basement left behind by the damp wallies, but I have only ever removed it where it's failed - or now I think of it, where it wasn't quite straight enough for my OCD!

I've also had really good results in the UK with Thistle One Coat which is a white gypsum plaster that seems at least somewhat resistant to old building nastiness. Twenty-eight years ago (before I knew 'better') I used it to fill chases in a wall where the lime plaster was badly perished through damp. Last year I took off the rest of the perished lime plaster leaving the filled chase in place as a reference for levels. I then replastered in NHL render with a Thistle One Coat finish - after all it is my house and any failure will be my problem... The plaster in that old chase remains absolutely sound and the new plaster is fine too - hope this helps!

Thank you that does help. The second storey seems to have no damp issues externally or internally - if I don't have to chip off all that old plaster that's many months worth of work and money saved!