Insulating plaster comparison and damp walls

[Sam W]

I started 'redecorating' our dining room at the start of the week. Initially this was going to be strip wallpaper, tidy up walls and repaint. On stripping the wallpaper I found that all walls had been cement rendered to about 1.5m height. I'm fully on board with the importance of breathability in controlling damp in our (stone) walls, so cement render is now off and I'm wondering what to replace it with. A few questions

• Top of the list of render options are Lime Green Ultra and Cornerstone Insulating Render. Interested in hearing people's experience, particularly if they've tried both. The Cornerstone is quite a bit cheaper, is there a reason for this?
• EcoCork is also a possibility, but one of the walls is always going to be a bit damp as outside ground level is about 25cm above internal ground level. There is zero chance of getting this changed in the short term, and not much more chance in the long term. Does anyone have experience of EcoCork on a damp wall? I know cork is good in wine bottles, but ultimately it is still wood and presumably susceptible to holding moisture/rotting.
• Plasterer is telling me bottom of the wall with earth on the outside will need tanking. I can see where he's coming from, and don't want the new plaster to fall off. I guess the tanker may not be necessary if the plaster can breathe quick enough to get rid of moisture. Interested to hear from anyone else in a similar situation

All comments appreciated.

 

[malcolm]

Modern trades people are going to suggest tanking. It's a bad idea. The concrete stuff you removed was tanking. A small amount of water will make it's way up from the bottom of a wall and it needs to be able to evaporate away in order for the wall to stay dry. If you tank the lower 1.5m of the wall then the wall will slowly get wet and cold, and after a few years it'll make it's way above the tanking and cause problems.

Before the widespread use of portland cement in 1920 and worse the widespread use of plastic paints in the 1970s everything was breathable and everything worked fine. Rising damp only started happening in the 1970s because waterproof paint no longer allowed water to escape from walls. So I'm with you on the breathability thing. Lime plaster direct on the wall is good.

I've not tried an insulating plaster. I was compelled by building regs to insulate and used wood fibre board. I did consider cork and don't remember why I went for the fibreboard. The insulating plasters didn't have the U value to satisfy building regs.

I've had some horrid arguments with building regs because I will not agree to insulate a wall if it is damp. There will be a water gradient in the wall and if you are relying on drying to the inside then you'll end up with wet insulation which is pointless. I stripped plastic paint off external bricks and dug a trench around the house to lower ground levels to dry a wall before insulating.

 

[a twig]

Quickest / easiest / cheapest way to insulate with lime is to use wood wool boards with a lime skim - lots of examples/methods in this thread here:

Plastering question

Hello all, After nearly a year of serious unanticipated structural work we've now finished and can get on with fitting windows, plastering and everything else that we thought we'd be finished by now. I have a question regarding use of wood wool boards on an external facing brick wall. This...

www.periodproperty.co.uk

 

[paulc]

I've not tried an insulating plaster. I was compelled by building regs to insulate and used wood fibre board. I did consider cork and don't remember why I went for the fibreboard. The insulating plasters didn't have the U value to satisfy building regs.

A work colleague used insulating hemp plaster in her little cottage. She is far from impressed with it. And judging by the values given for thermal resistance, you need a very, very thick layer to get anywhere close to building regs requirements.

 

[Lacan07]

I started 'redecorating' our dining room at the start of the week. Initially this was going to be strip wallpaper, tidy up walls and repaint. On stripping the wallpaper I found that all walls had been cement rendered to about 1.5m height. I'm fully on board with the importance of breathability in controlling damp in our (stone) walls, so cement render is now off and I'm wondering what to replace it with. A few questions

• Top of the list of render options are Lime Green Ultra and Cornerstone Insulating Render. Interested in hearing people's experience, particularly if they've tried both. The Cornerstone is quite a bit cheaper, is there a reason for this?
• EcoCork is also a possibility, but one of the walls is always going to be a bit damp as outside ground level is about 25cm above internal ground level. There is zero chance of getting this changed in the short term, and not much more chance in the long term. Does anyone have experience of EcoCork on a damp wall? I know cork is good in wine bottles, but ultimately it is still wood and presumably susceptible to holding moisture/rotting.
• Plasterer is telling me bottom of the wall with earth on the outside will need tanking. I can see where he's coming from, and don't want the new plaster to fall off. I guess the tanker may not be necessary if the plaster can breathe quick enough to get rid of moisture. Interested to hear from anyone else in a similar situation

All comments appreciated.

Tanking is the antithesis of breathability, if you plaster the walls with lime they may get wet but they'll also dry out. The benefit of lime is it has this capability without blowing off the wall. If your going to tank the wall you needn't have bothered removing the cement render, whats the difference?

Wood fibre board has a better UV value than insulating plaster but you may be reluctant to go down that road if you think your wall will always retain a bit of moisture due to the high external ground. I'm pretty sure the wood fibre manufacturers will recommend you avoid using their products on a permanently wet wall. As others here have stated insulating plaster can't really compete in the UV stakes but its better than nothing, particularly in awkward areas. And if your walls are relatively dry because they're breathable then thats a bonus in keeping the house warmer anyway.

I've used Lime green Ultra before but the Lime Centre (near me in Winchester) started producing their own version called Limco. Although its meant to be exactly the same (don't quote me) its cheaper and I found it much easier to use than Ultra which goes off like a pig. The substrate needs to be well watered and I would considered battening the wall if you can to get a flatter result. These products are incredibly light weight because they contain fibres rather than aggregate so its not exactly a chore to get it on the wall. If you use Ultra you'll hardly get any time at all to go back in and flatten with a hard edge before it goes off, at least that was my experience. If I need to use an insulating plaster I'll opt for Limco every time now. Unfortunately I don't know anything about Cornerstone.

 

[a twig]

Wood fibre board has a better UV value than insulating plaster but you may be reluctant to go down that road if you think your wall will always retain a bit of moisture due to the high external ground. I'm pretty sure the wood fibre manufacturers will recommend you avoid using their products on a permanently wet wall.

Don’t know about any manufacturer recommendations but I used wood wool boards skimmed with NHL in exactly this same (moist, below ground level) circumstance with still no issue 3 years later…

 

[Sam W]

Thanks for all comments.

I would be keen to add wood fibre boards, but the doorway into the room is really tight to the wall, even when they originally installed it they had to cut down the architrave. As a result I've got very little scope for building the wall out into the room. Adding to the problem, the underlying stone work is very lumpy, average depth of plaster to get back to the original surface would be about 40mm, but in places it's probably only going to be 10 or 15mm. Hence the appeal of an insulating plaster which can easily be worked round the bumps.

Plasterer came at lunchtime, I've discussed comments above with him and I'm now waiting to see his quote. Bracing myself for a bit of pain on this front, when I first started stripping the wallpaper I was only planning minor plaster repairs and then a bit of painting.

 

[Lacan07]

Don’t know about any manufacturer recommendations but I used wood wool boards skimmed with NHL in exactly this same (moist, below ground level) circumstance with still no issue 3 years later…

Twig, may I ask what did you skim the boards with, was it a home-brew mix? I've been told it has to be Lime Green Solo but I'm not so sure its that restrictive.

 

[a twig]

Twig, may I ask what did you skim the boards with, was it a home-brew mix? I've been told it has to be Lime Green Solo but I'm not so sure its that restrictive.

Sure thing, when I was doing it myself I just did a 1:3 mix of NHL (whatever my local Travis Perkins had in stock, usually 3.5) and builders sand. One time I had some kiln dried sand left over from a patio so threw some of that in in place of the builders sand.

When I had trades in doing it, the plasterer had never used lime before so I used this stuff as it was easier for them to basically just add water to that then get on with it

SecilTek Reabilita Cal AC

SecilTek Reabilita Cal AC is a natural hydraulic lime top coat used for the external renovation of old and new masonry and insulation boards

www.mikewye.co.uk

Worth either taping the joints (mesh tape
not paper) or putting mesh into the whole thing - as per my previous post I linked to somewhere in a convo recently - losing track!

 

[Lacan07]

Sure thing, when I was doing it myself I just did a 1:3 mix of NHL (whatever my local Travis Perkins had in stock, usually 3.5) and builders sand. One time I had some kiln dried sand left over from a patio so threw some of that in in place of the builders sand.

When I had trades in doing it, the plasterer had never used lime before so I used this stuff as it was easier for them to basically just add water to that then get on with it

SecilTek Reabilita Cal AC

SecilTek Reabilita Cal AC is a natural hydraulic lime top coat used for the external renovation of old and new masonry and insulation boards

www.mikewye.co.uk

Worth either taping the joints (mesh tape
not paper) or putting mesh into the whole thing - as per my previous post I linked to somewhere in a convo recently - losing track!

Thanks.

Im guessing a lime putty mix isn't strong enough for wood fibre board

 

[88v8]

I would be keen to add wood fibre boards, but the doorway into the room is really tight to the wall, even when they originally installed it they had to cut down the architrave. As a result I've got very little scope for building the wall out into the room. Adding to the problem, the underlying stone work is very lumpy, average depth of plaster to get back to the original surface would be about 40mm, but in places it's probably only going to be 10 or 15mm.

Don't know how old your place is, but if you want character, the wall does not have to be flat. Let the plaster undulate according to the contour. Put it on with a float, but knock it into to the dips with a churn brush.

Won't be possible to wallpaper any more though....

Ivor

 

[a twig]

Thanks.

Im guessing a lime putty mix isn't strong enough for wood fibre board

Don’t see why lime putty mix wouldn’t work? The board will provide a good key and the increased flexibility might help more with avoiding cracks to be honest.

I just used NHL as it goes off quicker and was more similar to what the plasterers were used to.