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Stone cottage - what mortar and plaster?

 
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Location: Ireland
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So this summer my partner and I will be restoring an old farm building with traditional drystone walls and a corrugated roof.

We're in Ireland, so it's damp all year round, but it's bone dry inside the building.
Keeping the sheer amount of rainfall we get in mind, we're seeking the wisdom of others in regards to what mortar and plaster to use for the walls.

I was considering cob for the interior walls finished with a limewash and putting in a limecrete floor. But I'm wondering about the exterior walls. A hot-lime mix for the pointing both inside and out is the first port of call. Any suggestions on what to use on top of that? We were advised to put in plasterboard inside but I'm not so sure as breathability is key to prevent mould.

We want the space to be comfortable and warm, it'll have a wood burner and we might put in underfloor heating if we can get it airtight enough.
 
master pollinator
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Wow, sounds like an adventure!  I hope it all goes well!
I don't know the answer and will love seeing the replies to your question.
 
Billie Bushe-Murphy
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Thanks Jane! I'm really excited to get started šŸ˜Š
 
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Great questions. Iā€™m looking for similar answers to repairing an internal basement wall, so look forward to seeing what answers crop up. Sounds like an interesting project.
 
master gardener
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I like the idea of cob, do you have a source of clay locally?
I can't really help with the stonework, but share your dislike of plasterboard. We've been lining our house (1920s stone built 'whitehouse') with planking. Actually thin floorboards, since i like to be able to put up shelves or pictures anywhere with proper fixings. Behind that we put not-very-permie PU insulation sheets with silver coating and taped joints to keep the moist warm air from the cold stonework. The PU insulation was sandwiched with battens to the wall using very long stainless rawlbolts, so as to get continuous insulation. The one room we have actually finished is much warmer as a result.
Have you looked at the rocket mass heaters? If you're starting with a bare shell you might want to look at those instead of a metal woodburner.
 
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There is the Traditional Lime Company in Carlow ---they have all the stuff you would need and have demonstration days, and in the north Prof. Tom Woolley who runs courses in lime and lime based insulation , dont do dryline boards on the inside ---its the incorrect advise, causes a major headache years later when it will have to be torn down and replaced ---or if you stick to the advise ---replace it and repeat the cycle. Just like my old cottage was done ---yes it lasted  15 years --but eventually the mold grew  from the back out to the front of the boards.
 
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I love old stone cottages. Pembrokeshire, where I live, is full of them. I only wish that I could afford to buy one.

It's interesting to read that the walls are drystone, something I've not encountered over here in buildings. Perhaps it was once common but that everything has since been pointed in lime or cement.

On that point, please don't use cement. I'm sure you know this but stone (and cob) houses like to breath - it allows the moisture in and out, preventing it from building up and creating damp. Further, cement would likely be harder than the stone used in construction and could compromise it as the building thermally expands.

I would seek an expert opinion on this. Find someone with experience restoring old properties and see what they suggest. My suspicion is that you'll be recommended a soft lime plaster. I do think a cob internal skin could work, however.
 
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Billie,  Dia Dhiut,
from the statement" the sheer amount of rain  we get"  can I take it you are on the west coast?, where you can get 4 seasons in one day and in winter rain coming at you horizontally.
In the past I worked a lot on old  farmhouse cottages in Kerry, Limerick and Clare rewiring them. Those that had lime plaster ( some with it there for over 150 years) on the interior walls were excellent and bone dry, those with plaster board  nearly always had mould and damp some where in the space behind them with masses of wood louse and ear wigs and rotting stud frames that supprted the boards.  
I would seek out advice  given here on using a lime mix for pointing the exterior or use a lime plaster/putty to cover the entire walls.  Using the lime plaster softens the look as you can curve it and take away those sharp angular corners and give a cosier feeling and keep a lot of the character of the building,  my own opinion is they don't have to perfectly flat as the little imperfections also add some character.  
I now live in Africa and  when I meet people for the first time they often ask do they have rainy seasons in Ireland? I go" sure of course they do, they have 365 of them".  
 
tony uljee
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just re reading the post , cob applied to the inside walls wont help much as its very dense and not insulation but more of a thermal mass heat storage ,it also needs to be able dry out  on both sides ,then limewashed or clay plastered to water proof it  ,your stone walls are going to be damp inside for awhile so the cob mix might fail as it can not breathe out moisture all the while ,maybe once the house is warmed up by putting  a fire down over several months the building will start to dry -- old cottages had chimney / open fireplace -to help draw out moisture  as well as cook and warm up people---not always very efficiently though. Hotmix lime is not lime mortar ,its the start of lime putty which has to be matured under a water layer for quite sometime then used as a basis of making up a lime mortar with clean sharp sand and applied as a pointing  , usually hydrated lime powder is used and  an add into the mix of some hydraulic lime or very fine crushed fired clay ---from old brick and clay tile is used as an accelerant to set up the mortar . A limehemp mix on the inside of about 4 inches thick would provide insulation value and will breathe/vent off moisture , it can also be lime plastered and limewashed over to a smoother but still rustic finished surface.
 
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