Hi everyone. First post here.

We've taken on a renovation recently. Curiously, it was originally a single story stone (rubble filled) cottage that was later extended upwards to 2 storeys with single skin brick in 1911. The brick extension was done functionally rather than beautifully so it was always intended to be rendered.

Today we find it with several layers of render. Varying in type, with dash in places, thick cement render, Tyrolean, the works!

We think we have 3 options:

1) Hack it all off and re-render with a breathable render. Either a modern breathable system, e.g. silicone or traditional lime based render.

2) Hack it all off and install EWI and render over the top with a breathable render.

3) Don't hack it off and affix EWI on top of existing render and top with a breathable render.

Here are some images that show the layers of render and the condition of the walls - https://imgur.com/a/Wj6bhNM . We assume it's important that being a stone wall it needs to breathe, so therefore, option 3 would mean locking moisture in under the existing render. Or is that not a problem? Where we have removed render on one side of the house, we did find the mortar in the stone work was wet, loose and missing in places. In one area, so bad that the outer leaf is bowing and we think we need to rebuild in blockwork. We think the rest of the house is sound though as that was the wall that faced the elements and moisture had got behind the render there.

So, to EWI or not to EWI and to hack off or not to hack off, that is the question?

Thanks!

Hi and welcome
I have a couple of similar buildings. One I have insulated with EWI. It is a stone / rubble walled structure with walls 50cm thick and 36M long One side of the building had render in good nick, the other side had no render at all.

I put on EWI with the standard EPS EWI, adhesive, EPS adheive render with glass mesh then thin film render to finish. I have had no problems since doing it and it has made a big difference to the livability of the building.

If the render is sound I would leave it on (I did). On the side with no render I put on the EPS with a thick(er) bead of adhesive top and bottom of each 50cm x 100cm EPS slab with 4 big dollops in the middle. This was pressed on to the wall with each slab lined up as best possible (the wall was a bit uneven) and when the whole wall was done and set (3 days) any mis-matched edges were sanded to even prior to the render/glass mesh coat. (with a building that long and over 100 years oId there is no way it was ever going to be flat and straight). I decided against rendering the wall first because of cost and time. No mechanical fixings were used because the stone is basalt with sand/lime mortar on the outsides and rubble/earth infill. The drill bit breaks if it hits a stone and everything falls out of the mortar.

Stone rubble walls are notoriously draughty but putting on the EWI solved this problem. Definitely put on EWI. It was done DIY

The standard EPS EWI systems are sufficiently breathable. IMO the expensive grey EPS is not worth the extra, just use a bit thicker standard white.

So IMO if the render is sound leave it, if not hack it of. The re-render of places where the render was hacked off or fallen off, it may be cost effective to use thicker beads of EWI adhesive rather than re-render. Mechanical fixings are worth doing - if the stones allow

Thanks for the comments so far. We're not builders ourselves so we'll be paying someone else to do the work and we've had quite a few quotes, all in the region of £14-15k, so we are desperate to get it right.

We do like the sound of the first 2 responses because we would save money on the hack off (around £3k). However, this article is telling us that both cement render and EWI are doing terrible things to our walls, so maybe that would be a mistake - https://www.heritage-house.org/damp-and-condensation/types-of-damp-what-have-i-got/damp-problems-caused-by-cement-render.html

Jonti's suggestion does sound sensible, but with the cost of the hacking off and an additional base coat before the EWI it's going to be expensive. I wonder, is the additional basecoat essential? Example here where not done that way - https://cdn.mos.cms.futurecdn.net/bxbXuk7drPhhpe6zeTwMaB-650-80.jpg

It's a shame there isn't only one proper and approved way to proceed.

Tricky!

Yes it is very tricky and in the end it is down to going with what you feel is best. The problem with relying on articles is that it is usually possible to find one that will suggest the exact opposite to the last one you read.

As for the cost of removing old render and adding new it depends how hands on you want/can be. It really is not that difficult and if the render is cement based then your better removing it.

Still not an easy dilemma.

interesting thread as I'm currently pulling hair out over similar issue, only MUST do something like this on the perimeter insulation ( base of wall ) where I can't use woodfibre boards due to very seldom risk of wetting ( e.g. a few hours per year is expected, also very spordaic, has so far happened 3 times in 8 years.. near a river and the water occasionally backs up to around ground floor level, usually just for a few hours and then quickly goes back down again )

What I'm concerned about, is the issue with EPS and having to have rock solid details of closing the perimeter around each board, windows and everything correctly, not leaving open voids at the plinth level ( must be fully flat glued, no dot and dab ) and not having any cracks or gaps filled with anything but insulation ( no patching with mortar ).. I'm getting this info from watching some youtube videos of a German building inspector ripping apart customers insulations and explaining why they won't pass.. but it seems for e.g. mineral wood boards that carry render you don't need to close the edge of the boards.. then dot and dab is ok.. but I don't want to use MW at plinth level either. Having reviewed all the systems and options, even thing like Multipor / Gas concrete / Calcum silicate everyone still uses EPS/XPS at plinth level, not matter what they are putting above.. but I really don't want to have to add cm's of very expensive adhesive render over the old stone work to bring it all perfectly flat. If it is the case that the issues are only related to the adhesion of the boards.. and/or moisture effects on the render, then I am considering I could dry lay an eps board around the plinth with profiled edges ( to slot together ) and then build a small brick plinth in front to protect it and hold it in place, thereby circumventing all the issues.. so in effect a simple cavity wall but only at plinth level, render directly onto the brick/block and then continue above with a timber contruction ( this is already done on 1 side, so theplinth insulation has to be fitted in underneath ). Anyone able to blow a hole in the mini cavity wall idea or add any knowledge / detail about the issus with EPS fitting and detailing? thanks ps. I've run all these ideas through a U / Condensation calculator and EPS seems to be fine with everything, also inside the cavity with e.g. a 12 cm deep concrete block or Lime/Sand brick in front, I only get minor condensation on the outside of the EPS when the inner cavity leaf is a 36 cm deep Poroton brick, but it is accetable.. or a may stick the EPS directly on this one end of the building because it is modern and nearly flat already.