Hoping someone can offer some advice please. In the middle of a long renovation of our upstairs. Ended up back to brick. Have applied diathonite lime insulating plaster and wood fibre board to external facing walls. Internal walls and ceilings have been re-skimmed with gypsum. Our amazing plasterer is going to give baumit rk70 and then glatt a go over the woodfibre boards after the lime plasterer I found (with difficulty) had to cancel due to illness. The system seems fairly straight forward but he has very little lime experience so trying to help with research as much as I can! Our main issue is how to deal with the joins between the lime and gypsum. Our upstairs is in the eaves so we have corners, walls/cielings, and a could have verticals walls where the top half is gypsum (dormer cheek) and bottom is lime. We have discussed stop beads and mesh options but any thoughts appreciated!
Lime meets gypsum
- Thread starter RosieB
- Start date
Tricky one.
Even trickier without pics.
It's a shame you can't just lime the lot, all bases covered that way, where the pink meets white (as one will breathe and the other not), unless the juncture is well away from the cold wall - you WILL get mould!
May have to return a few inches off of each and start from there. Time consuming and a bit of agg.
Even trickier without pics.
It's a shame you can't just lime the lot, all bases covered that way, where the pink meets white (as one will breathe and the other not), unless the juncture is well away from the cold wall - you WILL get mould!
May have to return a few inches off of each and start from there. Time consuming and a bit of agg.
Woods
Active Member
gypsum is vapour open. I don't think mold on the interior walls is going to be an issue at all. Unless the entire house is damp AF maybe. Of course, don't go putting plastic paint on the gypsum. Use clay or lime or some other vapour open paint.
@RosieB are you asking about joining up the two walls? In Belgium it is generally recommended to cut corners and caulk them. Since they're two perpendicular faces, it is always possible that some movement between the two will happen. Not all inside corners crack, but I've seen it enough to see why cutting and caulking is a reasonable thing to do. But it doesn't seem to be something our ancestors did.
If there are going to be cracks, it will be due to movement though. Because if you properly scour your base and finish coat, you rarely (if ever) get shrinkage cracks.
So if I were you, I wouldn't worry about it and just have your guy plaster straight up to the gypsum but tell him to properly scour, especially the corners. I would also recommend to scour the finish coat. It seems not many people do this nowadays but scouring the finish coat will make it more durable and reduce/eliminate shrinkage cracking.
Of course, I do not know the ingredients and proportions of the materials you're using as I exclusively use my own lime composites, so I can't predict what your material will do. It's also much easier to get a real nice, smooth and even finish when you wet trowel right after the final dry scouring.
Woods
Active Member
gypsum is not a moisture barrier mate
Take plasterboard for example; if it didn't have suction, it wouldn't be possible to get a good bond when skimming it.
When you wet a gypsum wall, you can clearly see that the wall sucks up a portion of the water
Gypsum is doesn't stop water mate. But it's not as good at drying out walls as lime and clay plasters.
Take plasterboard for example; if it didn't have suction, it wouldn't be possible to get a good bond when skimming it.
When you wet a gypsum wall, you can clearly see that the wall sucks up a portion of the water
Gypsum is doesn't stop water mate. But it's not as good at drying out walls as lime and clay plasters.
Woods
Active Member
sand/cement or hydraulic lime float + gypsum set is more of a moisture barrier though, but that's due to the cement rather than the gypsum. Apparently gypsum sucks up moisture pretty well but it is much slower to release it again compared to lime or clay binders.
Our ancestors used predominantly lime in their finish plasters but would sometimes mix a little gypsum into it in order to increase the set.
The issue with damp walls is more important on the outside though: drying out the walls is all about the wind hitting the facade. If you have lime or lime wash on the outside, it has two functions: 1. it helps protect against water ingress and 2. it speeds up the drying of the entire wall assembly because of its superior convective drying capabilities.
Everyone talks about "breathability" but it's not an ideal descriptor of the process. If you're talking about a wall that stays dry then describe it with "convective drying capability" because that's what it is.
The wall is damp. But then the wind hits the facade and dries out surface. Because the surface is now dry, the moisture deep inside the wall is now being "sucked" towards the surface where the wind dried it out. Then the surface is wet again, but the wind dries it out again. Then more moisture from deep inside the wall gets sucked to the outside. This process repeats constantly. Lime has such great capillary activity (it facilitates the movement of moisture) that the convective drying capacity of the wall is increased significantly, resulting in a wall that will dry itself out perpetually.
All the old houses do this. Unless someone did repairs to it with hydraulic lime or cement or another type of moisture barrier which will trap the moisture inside the wall and lead to damp problems.
Moisture barriers should only be used in damp courses, to mitigate moisture traveling up the wall from the ground. Our ancestors often used hydraulic lime and cement (concrete, béton, ...) below the ground in order to create a moisture break so that the foundations would not suck up a bunch of water which would then travel up into the walls above ground.
Basements were always meant to be damp because it's where they stored food. To store food you need a low, stable temperature and a high humidity. That's why basements always had adequate ventilation. And that's also why they used hydraulic lime aka concrete. Concrete (unreinforced) loves water. It will literally set under water and become even stronger because of it.
Gypsum can only be used on the inside and above ground though because it doesn't tolerate water well. Using it on the interior is not problematic because it will allow moisture/vapour to pass through so it will help regulate the indoor climate.
Of course, lime plaster is much more durable especially if it was done well.
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Woods
Active Member
I know there's a lot of myths out there, a lot of them created and perpetuated by lime (plaster) manufacturers. Keep in mind, they just wanna sell product. So it's in their best interest to convince you that gypsum isn't "breathable". But it is. The only things gypsum isn't suitable for is outdoor render/finish and durability. Lime is much more durable on interior and the the de facto stuff you should use on the outside. On the outside, that's where it makes the biggest difference.
But it's perfectly fine to plaster your old house with gypsum on the inside. It won't be as durable as lime plaster, but it won't hurt your walls or create damp problems at all. This is a lie pushed by certain lime companies.
Sure, the cement industry has a lot of liars but so does the lime industry. I used to believe that gypsum wasn't breathable either but then I researched it and found it was a lie.
Having said that, I will always advocate lime on the inside because it's a more durable finish and much more beautiful if done well. If done well, it doesn't need paint. Just a few coats of black soap with water to give it some protection from dirt and people's dirty hands when they can't stop themselves from touching up the wall. f**k**g pervs...
But it's perfectly fine to plaster your old house with gypsum on the inside. It won't be as durable as lime plaster, but it won't hurt your walls or create damp problems at all. This is a lie pushed by certain lime companies.
Sure, the cement industry has a lot of liars but so does the lime industry. I used to believe that gypsum wasn't breathable either but then I researched it and found it was a lie.
Having said that, I will always advocate lime on the inside because it's a more durable finish and much more beautiful if done well. If done well, it doesn't need paint. Just a few coats of black soap with water to give it some protection from dirt and people's dirty hands when they can't stop themselves from touching up the wall. f**k**g pervs...
Woods
Active Member
ok fair, I'll listen; got a source for your claim?
Entirely up to you.
Stevieo
Royal Spin Doctor
He expects the rest of us to so why not you?
Woods
Active Member
But do you?