Aaah a fellow 'limey', personally I can't get enough of the stuff.I've rendered many square metres on my own and with various builders. I Always flinch when I see a house built of stone & lime mortar being rendered in crepe (cement based) as this will not breath. It traps the damp and eventually falls off. Where the crepe or cement render is thick enough the lime mortar basically turns to into soil (not nice) Our house is an old masse, has very thick walls and has been rendered inside and out in lime render.
I've gone for a partially pointed feel on the inside leaving the stone semi revealed using very fine white sand which drys to a soft, off white. This product is great in a 4to1mix and loves sticking to existing lime mortar. The fine sand means it has to be THICK before it cracks on drying. Outside we used a coarser sand (same as James' I think) for the first coat and a fairly coarse yellow sand for the finish. This cracks more easily and can only really go to 1cm thickness. The beauty of the fine white sand is that you don't get lots of little black bits. Its more expensive (around 5€ per 30 kilo bag) but you use less if pointing and if its your first time (I'm NOT a builder) it is VERY forgiving. As James said the key is to wet the wall. If its existing lime mortar wet it with a hose and garden spray the night before then keep soaking it as you go. This doesn't just slow the cracking but more importantly it forms a vacuum so the lime render 'wants' to stick.Brian is right too - top down and work on a slight diagonal. The other key is to stay ontop of the task. If pointing really use the trowel to force the render (once its had an hour to go off) in between the stones. If you're going for a flat finish again it pays to keep 'forcing' it onto the wall to get all the air out of it. When finishing a flat wall James is right a sponge works wonders and makes an amazing difference. If you are revealing some stone and want a less flat finish a long haired, fairly stiff paint brush works well. Be warned though lime render and mortar will not go off below a certain temperature, equally high summer may test you physically. It doesn't liken being rained on 'til its dry either. On an historic note one of the things they used to colour lime render was blood, I settled for sweat and tears.
Yes Val, the finish is great. Our friend has done the entire inside of his house that way and just added slightly different sands for colour variance and effect is superb.
I have other internal walls in the house that need tidying up where they've been left to their own devices for too long so I will definitely use James's technique somewhere. I especially like the thought of cleaning off the surface with a damp sponge to expose the small pieces of aggregate.
make sure you have an air gap between out side wall and insulation - use non hygroscopic insulation or it will be a sponge!!! the p/board should have a vapour barrier (foil or similar) and the battens, if timber, should be treated (tanalised or similar) galv stud may be more suitable applied with dabs. the key is to avoid contact with the damp/wet wall. Try to vent the cavity as this will help.
Just saw James' answer about plasterboard insulated wall. Looks like a bit of a waste trying from what we were told. We have been recommended to use sheets of rockwool, then the thinnest available porous bricks and a very carefully applied mortar that breathes and also has plenty of holes left in it. The lime render over that. It costs a few centimetres off the room but splitting the difference between plasterboard and that is perhaps 1.5 to 2 cm in the end for something that will last longer.
We have been helping a friend do exterior crepie on his house, but in principle it is all very similar using enduit en sable, sable à bâtir and chaux blanche - the exterior and interior mixes are just a bit different. We have a downstairs rooms with the back wall built against bedrock and the cellars next to it. The old wall coating is impervious cement which we are having to hack off bit by bit. That should be a good, warming winter job, then the other three walls it virtually falls off in sheets. Two walls need insulation, but then we can do a finish like this on that anyway.
The application was slightly different. There was a pro involved much of the time and it is always done from the top down and across so that there is eventually always a slightly diagonal line of coating going on. Four of us did a considerable sized external wall with only one small window in a working day, so our friend who says he will help thinks each wall should be no time. It is far nicer than conventional plaster, on insulated walls it looks like real stone walls and if people want colours they can get variations in the sable à bâtir for slight difference is shade of tone colours or powdered additives for other colours like warm pinks through to burgundy, etc. Nice stuff, cheap and very effective.
Shame, because it looks great. I will go the plasterboard route - it's a corner room so has 2 external walls and would benefit from insulation - then I might arm Twerp with some old paint and let him run riot. Would add a bit of character...
Valerie, I never do this to external walls (on the inside). I would always insulate, so unless it's dripping wet, in which case you need to remedy that problem first, I would plasterboard it and insulate leaving a gap between the insulation and the wall. You can still render the plasterboard after although lime render will have a tough time sticking the plasterboard. You can staple chicken wire to the wall first to help with that.
I'm going to use this in my wine cellar, but leaving the surface finish unsealed so that it breathes. This can help with humidity and fungal problems and was a technique employed by Bouygues when they constructed the Hassan II mosque ablution rooms in Casablanca.
Brilliant James - I know I'm in the building game, but your step by step instructions are excellent.
I'd love to try this on a store room which where the pointing is crumbling away but my handyman says the wall is damp from the outside and needs battening/boarding, leaving an air gap. Do you think the rendering would stick, especially with the addition of some PVA (I used to use that trick in Malta to prevent dusty walls) or would I be wasting my time?
Just a note to draw the attention of Breton members to Tiez Breiz, www.tiez-breiz.org, who run courses on lime plastering and lots of other useful skills. I've done enduit en terre - plaster your walls with a mixture of clay from the garden and hemp and insulate it at the same time, and laying quarry tiles the ancient way - close together, with lime. Cheaper than equivalent English courses, and while I've struggled with the theory it's made sense when we got on to the practical.
Good point Ian, it is still porous despite the addition of the glue, there is just enough to keep it from being dusty. I have another wall that was done by someone else and it is constantly dusty.
as lime is a breathable material and therefore stops mould developing within the structure, it is important not to apply a paint or other products to negate the "breathability" of the wall and end up with rot etc. Great looking job.