is lime plaster all its cracked up to be?

[plasticpigeon]

Sorry in advance for the long post. I want to challenge the love affair that many conservationists have with lime plaster. I've been experimenting around now in my experimenting room, and I have tried 3 different types of lime plaster on top of lime render. I have tried two types of sand, one very fine and one sold as plastering sand, so sharp, quite fine and graded with hydraulic lime, and one ready mixed lime putty that I picked up from Matthew C at the weekend (many thanks Matthew by the way!) I got a pretty good finish with the putty, and I think on my next try I could get it as smooth and flat as modern plaster. However I accidentally bashed it a couple of days after applying and made a little mark, and it is impossible to repair!!!

I understand the reason that people like lime is because it has vapour permeability and also it is not very strong. I am sold on that idea for mortar for brick and stonework and as external render. I have seen the damage cement causes to the bricks on my property, both from trapping water in bricks causing spalling on freezing (it is very visible how much longer the cement pointed bricks stay wet after a shower than the lime pointed ones) and from structural movement, so I have used Nhl 3.5 for all repairs to brickwork.

However when it comes to plaster I'm not convinced that lime is any more "breathable" than gypsum. I've tried an experiment and sprayed a load of water at a plasterboard wall with a skim of gypsum and a lime plastered wall and the gypsum wall absorbs the water more easily and dries out more quickly. This makes me think it is both more vapour permeable and more able to absorb liquids than lime plaster. I know a criticism of gypsum is that it doesn't deal with being wet very well and tends to disintegrate but I would imagine that the wetness levels seen inside a house aren't going to be enough to cause gypsum to disintegrate (cornices and other ornament were always gypsum anyway). However the two areas that gypsum really scores in is that it is tougher than lime so deals with bumps and knocks better, and also as it has no sand in it, it is much easier to patch up if it does get damaged. It seems a trend these days to paint everything with PVA before using multifinish which obviously makes a waterproof barrier, and makes the plaster easier to apply, but wetting the wall would have the same effect and get rid of the PVA barrier.

I'm not advocating board and skim, as I believe that the paper on plasterboard is also pretty impermeable to both passage of liquid and vapour which is why it is so easy to skim onto, and I don't like hollow sounding walls. Also I don't like the idea of browning as it is probably stroner than the mortar if not the bricks, and I can see getting it off if need be an impossible task and I object to making permanent alterations. However a thin gypsum skim onto a lime render sounds a perfect combination to me. Lime against the brick which is weak enough not to damage the brick and also if there is water ingress from outside should deal with it better than gypsum. Gypsum skim for strength against knocks, repairability and soaking up condensation if there is any. I haven't tried multifinish on lime, so I don't know if it sticks. I wish I had tried it in my experimenting room. Breathable paint would be a must though. What do people think?

 

[skier-hughes]

The longer lime goes the stronger it gets, our hundreds of year old lime is pretty darn solid, so we are living with our knocks and scratches and just paint them. We live in our house and aren't too worried about the odd knock and chip, well not any more, I used to be, but I find carrying out other more important work like another room is far more important than trying to repair a couple of knicks in the lime plaster.
I have no idea how gypsum over lime works and I don't yearn for a glass smooth finish to my lime plaster, it's not higgledy piggledy either, but it has character.

 

[johnobirches]

Hi plasticpigeon,

I'm very new to this so have no predetermined stance. I think it's always good to challenge conventions and I look forward to hearing what the more experienced folks have to say.
I've formed the opinion from my recent reading that gypsum plasters were not breathable but your tests seem to suggest otherwise. Maybe I'm confusing gypsum plasters with cement renders and to be honest I don't really know what the differing properties of gypsum and cement are.

One thing I can think of though is that as the lime mortar and plaster will allow movement [however small] then a skim layer of gypsum on top may crack very easily and because it's so thin may then flake off.
Cheers for now
John

 

[Lime]

As has been said, lime plaster continues to harden over time and any dents or damage to a new surface may be repaired with pollyfilla (or the like) very satisfactorily.
IME modern plasters are stone-hard and usually cold to the touch.

I would not wish to purchase an older house decked out with modern plaster.
In every minor instance I have done so, without my knowledge, that part of the house has suffered damp problems particularly condensation.
My gut feeling is if you want to buy a barret box don't buy old or ruin an older property please.
The modern quick-fix is usually unsuitable over the long term.

 

[Flyfisher]

There's a chap on the green building forum who says that gypsum plaster has about the same vapour permeability as lime plaster: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1040 (second post)

As also noted in that discussion, the final vapour permeability of a wall can depend a lot on the additives used during plaster application. The modern approach seems to be to use lots of PVA when plastering with gypsum plasters. Perhaps a plain gypsum-based plaster with no PVA or similar additives would have basically the same vapour permeability as lime plaster?

 

[Jellyfire]

Whilst agreeing that gypsum plaster seems to soak up water and dry out generally, my experiences in our house suggest that there's more to I than that. We have a mixture of walls that have been plastered with gypsum over the last 20 or so years at a guess, and some that are still lime plastered. Wherever I've found modern plaster I've found damp and rotten timber, wherever I've found lime it's bone dry and the timber is in perfect condition. Not sure of the science behind it, but it suggests that 500 years of lime plaster has worked perfectly and 20-30 years of gypsum has caused damp and rot. I'm replacing everything with lime!

 

[JoceAndChris]

I really don't know much about this - I'd say cement render on the outside is a whole lot worse than gypsum plaster on the inside. Where exterior cement render has cracked on the outside water seeped in, was unable to escape, and caused damp on the inside. As most of you will have seen we've hacked off big patches of the exterior cement render back to stone and have a very sad looking house (but have got rid of our problem).

On the inside, we've a a blend of gypsum and lime. I take the approach that if it causes problems, repatch in lime, and if it doesn't, leave it. If all the plaster fell off I'd want it redone in lime though.

Here's an exterior wall, facing the gentle east, in 30 yr old gysum:

As you can see the plaster is delaminating and slightly damp. Still not sure what to do about it, whether it's bad enough to warrant hacking off.

Here's an exterior wall facing the south west, which seems to be the direction most of the weather comes from. This is old lime plaster, and when we moved in it was so damp the wallpaper was hanging off. But as the picture shows, the wall plaster is in good condition for its age, and hasn't delaminated like the gypsum in the other room, despite the weather being much worse this side of the house. The picture shows it after a two year drying out period.

The old lime on reed ceiling had taken a real hammering when the roof leaked in the 70s. But as others have said, the plaster itself is actually rock hard despite the cracking. Specialists in lime were able to do a really good job of patching in lime and pinning up,after I'd done several weeks of gentle soaking off the woodchip and underlying papers, and the ceiling is (nearly) as good as new. Will gypsum ceilings survive at 200 yrs old? Maybe so, but if there's lime on your house, I think it should be preserved and patched up as far as is possible to give a sense of the history of the building, and an atmosphere of things being not quite as smooth and perfect as they are in a new-build.

 

[TheForge]

Gypsum's a naturally occurring crystalline material, it is naturally water permeable and slightly water soluble before heating. As a result pure Gypsum plasters will not bode well in wet conditions, tending to fail in constantly wet locations. However there's evidence of gypsum having being used to gauge lime plasters for many centuries, even in major buildings such as Uppark and Prior Park. In areas near gypsum deposits/mines theres evidence of gypsum plaster being used internally far back beyond its rise in popularity in the 19th century. Gypsum is often present in limestone and may have also inadvertently have become an impurity in lime mixes.

Many historic Parisian building are externally rendered in Gypsum and they have withstood substantial water attack!

In our house (early 1700's) we were puzzled to find pink but soft and crumbly plaster that seemed to contain Gypsum, sometimes used as what appeared to be fairly historic patch repair to lime plaster on internal walls. On one wall the pink plaster repairs were under several obviously later layers of cement render applied as an attempt to 'tank' the wall. The pink plaster (probably a lime/gypsum and possibly earth/clay mix) had, in the presence of trapped moisture, turned to a loose powdery consistency, much worse than the haired lime in the same layer which had survived reasonably well.

I think most Gypsum plaster powders you can buy are not pure gypsum but have other materials included to increase the water resistance and as such effect the materials ability to 'breathe'. Obviously modern plaster methods and additives such as water proofing chemicals or prepping with PVC would restict its breathability. How Gypsum well is crushed I would imagine would also effect its water permeability.

Whereas lime and limestone is strongly Alkaline (Calcium Hydroxide/Carbonate), Gypsum (Calcium Sulphate) is close to pH neutral and the Sulphur it contains. being in S04 form, is plant available and thus likely to encourage fungal growth in moist environments if the plaster does not have suitable chemicals added.

 

[Lime]

There's a chap on the green building forum who says that gypsum plaster has about the same vapour permeability as lime plaster: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1040 (second post)

That does not mean he's correct though ?
This is the internet after all :? :? :?

Hear! Hear!
There are many people today who consider the internet to be the font of knowledge!
What a joke.

The truth is out there but is unlikely to be on the internet particularly if it was previously published over 30 years ago.

 

[Zebra]

There's a chap on the green building forum who says that gypsum plaster has about the same vapour permeability as lime plaster: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1040 (second post)

When I was getting the spec done for my upstairs, which admittedly is different because it is timber frame, the technical expert from Thermafleece, who was very helpful and knowledgeable, said that whether you use a gypsum or lime internally won't make any real difference to the breathability. However, I'm using lime internally because of aesthetic and authenticity reasons.
Do you have a problem with water ingress Plasticpigeon? If the room is dry, then surely gypsum wouldn't be a problem? You could perhaps find out whether there is moisture evaporating from the walls by taping a plastic sheet over a section and seeing if it wets up behind it? I don't know, just guessing.
However, if your post is just evidence of your frustration with lime plastering, I would urge you to persevere with the putty mix lime, and give it time to harden. Don't give in because of frustration, you could regret later not sticking with it.....

 

[plasticpigeon]

I'm glad I haven't got completely blasted for suggesting such a thing.

I don't have any water ingress as far as I know, and you might be right Zebra, I am getting fed up with lime plastering and I certainly don't want to dogmatically perservere with something that may not be the best solution. The putty was actually very nice to use and I think I could get a decent if not billiard table smooth finish. However I can see it taking a long time to carbonate and even then I imagine it will be softer than pink plaster. I need to do my kitchen pretty soon, and I really do worry that in a working kitchen the lime might get more bashed up than gypsum. I'm also looking at my lime plastered wall in the back bedroom, where the lime plaster I put on hasn't dried in 2 weeks and now is covered in condensation. The gypsum really seems to be a better absorber of water using my unscientific spray it on and watch technique. I realise however that vapour permeability and ability to absorb liquid water are not the same thing. To try and keep the effect of condensation to a minimum, it might be nice for the walls to have some ability to absorb the condensation though, especially in a room that could get damp like a kitchen. However I have a feeling that if I left the surface of the lime plaster rough for example after using a rendering float rather than giving it a troweled finish it may be a different story and the lime might also be as able to take in liquid water quickly. Perhaps it is the closed troweled finish, and if I could get a smooth but open finish, lime would be a winner!.

It is interesting that Lime talks of using polyfilla to stop holes and small patches as polyfilla is as far as I know plaster of Paris (gypsum) and glue. I can see that gypsum would be more likely to delaminate in the event of structural movement as it will probably stay as a sheet as everything else shifts about around it.
Incidentally, the skim on my walls seems to me to be harder than I would expect for lime and I wouldn't be at all surprised if the original plaster is lime gauged with gypsum. I've not made up my mind yet what to do but I look forward to assimilating more info.

 

[Gervase]

Yes, gypsum plaster is reasonably vapour permeable (breathable). But it's also prone to expand slightly when wetted and is hygroscopic - meaning that it actually retains moisture. Continually damp gypsum plaster turns first into a weak, friable mass and then a soggy sludge, whereas lime plaster (once carbonated) can withstand immersion (provide the pH of the water is neutral or alkaline).
Lime is also flexible and capable of self-repair when micro-cracks appear, whereas gypsum is not.
Yes, plasters were often gauged with gypsum to accelerate setting, to increase hardness or allow moulding or sgraffito but, unless you are seeking a specialist finish, the best surface for a traditional wall is a plain, lime plaster.
That said, if you've a modern block wall or plasterboard partition and are blessed with concrete foundations with no prospect of movement, use gypsum. And slop on a coat of non-breathable PVA first to help adhesion.

 

[Flyfisher]

There's a chap on the green building forum who says that gypsum plaster has about the same vapour permeability as lime plaster: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=1040 (second post)

That does not mean he's correct though ?
This is the internet after all :? :? :?

Hear! Hear!
There are many people today who consider the internet to be the font of knowledge!
What a joke.

The truth is out there but is unlikely to be on the internet particularly if it was previously published over 30 years ago.

Well I'm as critical as anyone about stuff on the internet but I had formed the impression that 'biffvernon' was a pretty reliable source of information when it comes to traditional materials. Perhaps he'll drop by on here and explain the post I linked to above.

 

[Flyfisher]

That said, if you've a modern block wall or plasterboard partition and are blessed with concrete foundations with no prospect of movement, use gypsum. And slop on a coat of non-breathable PVA first to help adhesion.

When did PVA start to be used with gypsum plasters?

Part of our house is built of solid chalk block walls and is mostly plastered with gypsum. As far as I can determine, the plaster is at least 40 years old.

 

[skier-hughes]

I need to do my kitchen pretty soon, and I really do worry that in a working kitchen the lime might get more bashed up than gypsum.

I'm also looking at my lime plastered wall in the back bedroom, where the lime plaster I put on hasn't dried in 2 weeks and now is covered in condensation. The gypsum really seems to be a better absorber of water using my unscientific spray it on and watch technique.

We have very few working mishaps with the lime plaster in our kitchen which is now about 4 years old, or 3, it took an age to do!!

We have a fair few more in the bathroom, only finished January this year, but these were caused mainly by the finishing work and fitting of sanitary ware.

If your lime plaster hasn't dried, how do you expect it to absorb water? You need to dry the room, thus the plaster and then decide on whether the plaster can absorb water. A dry sponge can absorb tons of water a saturated one cannot absorb a drop!

 

[plasticpigeon]

Only 1 out of 3 areas hasn't dried, I was spraying water on the other 2 areas and the gypsum area to see what happened. I have a heater in there now and it is at last starting to dry out.

 

[skier-hughes]

Still think if one wall hasn't dried, unless it was done much later than the others, that the water from it will be in the air, thus making the other walls damper. Have you used a damp meter on them all to see what they are before spraying, after spraying (ensuring all are as equally damp) and at set times during drying time to see what happens and at what intervals?

 

[plasticpigeon]

I strayed into looking at another forum today. It was rumoured on there that some members of this forum indulge in ritualistic naked dancing around tubs of lime mortar while praying to the god of carbonation. Obviously too wet for that this year.

 

[tobydog]

You should talk to Zebra about those kind of goings on although I have no idea about the naked bit :shock: .

 

[johnobirches]

It was rumoured on there that some members of this forum indulge in ritualistic naked dancing around tubs of lime mortar while praying to the god of carbonation.

My sole purpose in joining... but yeah, too wet this year obviously.