Cork lime plaster experience?

[aockenden]

Hope you’re keeping well , Ive started on another room on Saturday, this time with quicklime only and cork & hemp , slaked the quicklime for around 2hrs and then added the hemp & cork !!
had a look earlier today & it had lots of cracks and it didn’t seem to be stuck to the wall especially around the edges, would you know the reason for this , I did wet the wall good , but I didnt scratch it , I don’t want it to fall off in a few months , probably should have stuck with the hydrate lime

Hey, assuming you’re planning to do a finishing skim layer of just traditional plastering sand and lime, then shrinkage cracking isn’t a big deal on these thicker underlying insulating coats. Ours cracks as it dries as well (see pic). In general I think the wetter your mix is the more shrinking and cracking you’ll get, but personally I prefer it pretty wet and sticky to ensure a strong “grab” onto the wall and I’m willing to tolerate some cracks as a result.

It’s possible that your total water content increased when you switched to using the quicklime. It takes a while to fine-tune the ratios of things as you move to a new product. Was your quicklime highly reactive (aka, releasing tons of heat and bubbling like crazy when you added water)? 2 hours is a bit on the short-side to wait before using it, 12 hours is my typical (with the metal bin-lid on top tightly to hold in all the steam to help the slake). That 12 hours just helps to ensure there are no unslaked pockets of lime lurking around. Stirring / mixing with your aggregates should do a good job of preventing that problem tbh though, even with only 2 hours of slaking.

As for it not sticking to the wall, what makes you think the middle sections of the plaster havn’t grabbed onto the key of the underlying wall? Is it falling off in clumps or something? How did you apply it to the wall initially? The edges tend to pull-back slightly over the first couple days as part of the shrinkage / drying process, which can make the edges look a little “loose” / on the wall. Take some pics of those edges?

 

[Mikeyboy7]

Hey, assuming you’re planning to do a finishing skim layer of just traditional plastering sand and lime, then shrinkage cracking isn’t a big deal on these thicker underlying insulating coats. Ours cracks as it dries as well (see pic). In general I think the wetter your mix is the more shrinking and cracking you’ll get, but personally I prefer it pretty wet and sticky to ensure a strong “grab” onto the wall and I’m willing to tolerate some cracks as a result.

It’s possible that your total water content increased when you switched to using the quicklime. It takes a while to fine-tune the ratios of things as you move to a new product. Was your quicklime highly reactive (aka, releasing tons of heat and bubbling like crazy when you added water)? 2 hours is a bit on the short-side to wait before using it, 12 hours is my typical (with the metal bin-lid on top tightly to hold in all the steam to help the slake). That 12 hours just helps to ensure there are no unslaked pockets of lime lurking around. Stirring / mixing with your aggregates should do a good job of preventing that problem tbh though, even with only 2 hours of slaking.

As for it not sticking to the wall, what makes you think the middle sections of the plaster havn’t grabbed onto the key of the underlying wall? Is it falling off in clumps or something? How did you apply it to the wall initially? The edges tend to pull-back slightly over the first couple days as part of the shrinkage / drying process, which can make the edges look a little “loose” / on the wall. Take some pics of those edges?

View attachment 12738

Hiya, I think I was overreacting as the first wall didn’t have any cracks at all , and this one did , again thinking out loud do you think the hydrate lime will be good enough ,
It was a lot easier to make as you didn’t have to wait around , but a lot of the home insulation hemp plaster seem to mix it in a mix with quicklime & perlite, as I said thinking out loud

 

[Mikeyboy7]

second wall , with quicklime, cork & hemp

 

[aockenden]

It’s looking good mate. I do think the hydrated lime should be good enough assuming it’s fresh. You can test that your hydrated lime is fresh enough by mixing a small amount of powder with just enough water to make a thick putty and then flatten it and let it slowly dry inside over a week or so. Assuming it hardens properly then you should be good to go. So if you’re more comfortable with that just go with it.

We’ve done plenty of batches with only hydrated lime + small amount of NHL and no quicklime and those batches perform just as well as the ones with quicklime instead of hydrated lime.

 

[Lacan07]

It’s looking good mate. I do think the hydrated lime should be good enough assuming it’s fresh. You can test that your hydrated lime is fresh enough by mixing a small amount of powder with just enough water to make a thick putty and then flatten it and let it slowly dry inside over a week or so. Assuming it hardens properly then you should be good to go. So if you’re more comfortable with that just go with it.

We’ve done plenty of batches with only hydrated lime + small amount of NHL and no quicklime and those batches perform just as well as the ones with quicklime instead of hydrated lime.

Exactly right, always put to one side a brick of fresh hydrated lime putty to make sure its good.

 

[aockenden]

Exactly right, always put to one side a brick of fresh hydrated lime putty to make sure its good.

Yes to be fair you were one of a couple people who originally suggested that to me: sound advice so passing it on. Credit due to you though!

 

[Mikeyboy7]

Thanks guys , will do

 

[Mikeyboy7]

Hi guys ,just wondering how long should it take the lime insulation mix take to dry out , as mind seems to have dried within a few days ,( 4 days approx) is this to quick

 

[aockenden]

Hi guys ,just wondering how long should it take the lime insulation mix take to dry out , as mind seems to have dried within a few days ,( 4 days approx) is this to quick

It seems to be a sacrosanct tenant of the lime world that lime shouldn’t dry out too quickly. I’m unclear if that only applies to hydraulic lime or not and I’m also unclear on what the real danger is if the lime dries too fast.

With all the lime work I’ve been doing over the last couple years, I’ve been erring on the side of caution and just following the advice with the goal to have a full dry in no-less than 1 week. I suspect 4 days will be fine, although I AM surprised that this insulating render with large amounts of hemp and / or cork is drying that quickly for you. Most of ours has been taking a couple weeks at least. A couple patches upstairs I noticed drying quicker and I did spray it down after 2 or 3 days just to be safe to slow the drying.

How thick did you apply your mix to the wall? Ours is an average of 2 cm for the first layer. I suspect thinner coats would dry significantly quicker.

 

[Mikeyboy7]

It seems to be a sacrosanct tenant of the lime world that lime shouldn’t dry out too quickly. I’m unclear if that only applies to hydraulic lime or not and I’m also unclear on what the real danger is if the lime dries too fast.

With all the lime work I’ve been doing over the last couple years, I’ve been erring on the side of caution and just following the advice with the goal to have a full dry in no-less than 1 week. I suspect 4 days will be fine, although I AM surprised that this insulating render with large amounts of hemp and / or cork is drying that quickly for you. Most of ours has been taking a couple weeks at least. A couple patches upstairs I noticed drying quicker and I did spray it down after 2 or 3 days just to be safe to slow the drying.

How thick did you apply your mix to the wall? Ours is an average of 2 cm for the first layer. I suspect thinner coats would dry significantly quicker.

 

[Mikeyboy7]

View attachment 12836

Big wall done at the weekend 6m x 3.5m

 

[Rob Duff]

Hi,

I am new to this forum & look forward to learning from you all.

We (my wife Terri & I) bought an early 17th century Somerset farmhouse two years ago that was modernised with cement in the early 1950's, split into 3 flats in the late 1960's.

We have removed almost all of the cement paths, patios, tanking, pointing & floor coverings, lowered external ground levels below internal floor levels, fitted French drains, repaired the roof adding wood fibre sarking, repaired the rotting sash windows & allowed the house to start to dry out. With walls from 25 to 33 inches thick, it is a slow process.

It is not all bad as we have found 4 flagstone floors, a 17th century kitchen with drying chamber, washing copper & bread oven, two painted Tudor Arch fireplaces, 1st floor smoking chambers & several remaining lime plastered walls with murels/walls paintings on them, plus paintings of churches & windmills on the inside of the fireplaces.

We are now at the point of putting things back into the house & re-plastering with lime.

We propose to use hot lime, which is supported by our local Conservation Officer, but are unsure about the advantages & disadvantages of addng insulation.

We have read the Heritage Science 2021 report ( https://doi.org/10.1186/s40494-021-00546-9 ) which concluded that hot lime has a different crysteline structure with smaller cyrstals that hold more moisture, so it is more hydroscopic., plus it has smaller spaces between crystals for the water droplets, so less chance of freeze & thaw if used outside: All good.

We have also read the DEIS /Leeds Sustainability Instutute research paper on the Demonstration of Energy Efficiency Potential (DEEP) which concluded that wall insulation is the most effective method of improving the thermal efficiency of a house (more effective than loft or window insulation!), especialy as part of a whole house approach.

However, some people argue that their is no real evidnece on the long term impact of insulated renders/plasters, & that they could detrimentally alter the performance of the house as a whole, especially reducing its ability to hold moisture.

Our question thus is whether anyone on this forum has any experience or knowledge of whether adding cork &/or hemp changes the hydroscopic nature of the plaster, reducing its ability to hold water & help keep the house dry?

Also, is anyone aware of research which illustrates the thermal benefits of adding cork or hemp?

Our gut feeling is that the cork & hemp is in addition to the hot lime mix & thus should not alter its hydroscopic ability, & that the thermal benefits outweigh any possible downside.

Any views on this will be greatly appreciated.

 

[Lacan07]

Big wall done at the weekend 6m x 3.5m

Nice one m8! Keep up the good work.

 

[aockenden]

Hi,

I am new to this forum & look forward to learning from you all.

We (my wife Terri & I) bought an early 17th century Somerset farmhouse two years ago that was modernised with cement in the early 1950's, split into 3 flats in the late 1960's.

We have removed almost all of the cement paths, patios, tanking, pointing & floor coverings, lowered external ground levels below internal floor levels, fitted French drains, repaired the roof adding wood fibre sarking, repaired the rotting sash windows & allowed the house to start to dry out. With walls from 25 to 33 inches thick, it is a slow process.

It is not all bad as we have found 4 flagstone floors, a 17th century kitchen with drying chamber, washing copper & bread oven, two painted Tudor Arch fireplaces, 1st floor smoking chambers & several remaining lime plastered walls with murels/walls paintings on them, plus paintings of churches & windmills on the inside of the fireplaces.

We are now at the point of putting things back into the house & re-plastering with lime.

We propose to use hot lime, which is supported by our local Conservation Officer, but are unsure about the advantages & disadvantages of addng insulation.

We have read the Heritage Science 2021 report ( https://doi.org/10.1186/s40494-021-00546-9 ) which concluded that hot lime has a different crysteline structure with smaller cyrstals that hold more moisture, so it is more hydroscopic., plus it has smaller spaces between crystals for the water droplets, so less chance of freeze & thaw if used outside: All good.

We have also read the DEIS /Leeds Sustainability Instutute research paper on the Demonstration of Energy Efficiency Potential (DEEP) which concluded that wall insulation is the most effective method of improving the thermal efficiency of a house (more effective than loft or window insulation!), especialy as part of a whole house approach.

However, some people argue that their is no real evidnece on the long term impact of insulated renders/plasters, & that they could detrimentally alter the performance of the house as a whole, especially reducing its ability to hold moisture.

Our question thus is whether anyone on this forum has any experience or knowledge of whether adding cork &/or hemp changes the hydroscopic nature of the plaster, reducing its ability to hold water & help keep the house dry?

Also, is anyone aware of research which illustrates the thermal benefits of adding cork or hemp?

Our gut feeling is that the cork & hemp is in addition to the hot lime mix & thus should not alter its hydroscopic ability, & that the thermal benefits outweigh any possible downside.

Any views on this will be greatly appreciated.

Sounds like you’ve been just as busy as us over the last couple years! I bow to your work ethic.

As for your concerns / questions about insulating internal plasters / renders: this video (plus 3 miserable cold, humid, damp winters living in a cement box) convinced us we needed a breathable internal wall insulation.

I think this team thinks very deeply about issues surrounding energy efficiency, environmental issues, and of course moisture-management. They impressed me with their thoughtful approach, explanations and their well spoken intelligence, a far cry from the grunting local bog-standard builders we discussed our property with at the start.

They chose to retrofit this house with diathonite as internal insulation which is a clay, lime and cork insulation mix applied directly to cold masonry. They went 8-10 cm thick and got the old stone house up to passivhaus standards! I figured if it was good enough for them, it was good enough for me. We would have used diathonite ourselves but it’s cost was prohibitive so that’s when we decided to simply mix up our own version using the same principles of combining a natural breathable binder with insulating aggregates. We subbed out 50% of the cork for coarse hemp to reduce costs.

 

[Rob Duff]

Sounds like you’ve been just as busy as us over the last couple years! I bow to your work ethic.

As for your concerns / questions about insulating internal plasters / renders: this video (plus 3 miserable cold, humid, damp winters living in a cement box) convinced us we needed a breathable internal wall insulation.

I think this team thinks very deeply about issues surrounding energy efficiency, environmental issues, and of course moisture-management. They impressed me with their thoughtful approach, explanations and their well spoken intelligence, a far cry from the grunting local bog-standard builders we discussed our property with at the start.

They chose to retrofit this house with diathonite as internal insulation which is a clay, lime and cork insulation mix applied directly to cold masonry. They went 8-10 cm thick and got the old stone house up to passivhaus standards! I figured if it was good enough for them, it was good enough for me. We would have used diathonite ourselves but it’s cost was prohibitive so that’s when we decided to simply mix up our own version using the same principles of combining a natural breathable binder with insulating aggregates. We subbed out 50% of the cork for coarse hemp to reduce costs.

Hi,

Thansk for your assistance, really interesting. Did you go down the route of a heat exchage unit to remove moisture & add clean air?

 

[aockenden]

Hi,

Thansk for your assistance, really interesting. Did you go down the route of a heat exchage unit to remove moisture & add clean air?

No for a few reasons: our house probably won’t end up quite as air-tight as theirs did. We’ve also got trickle vents on our windows and we might install single-room MVHR systems in the bathrooms / toilet rooms and kitchen. Lastly, the homes in that video are in Yorkshire which has one of the drier climates in the UK. We on the other hand are in mid-Wales which is one of the wettest climates in the UK. Therefore, bringing tons of very high RH external air into our house seemed less advantageous.

I’d rather have a small backstop dehumidifier monitoring humidity and kick-on occasionally if things are really too wet. But generally I’m hoping all the natural materials (lime, clay, hemp, cork, sheepswool, wood-wool, wood, etc.) will provide enough buffer to keep our humidity under control passively. We’re already seeing improvements in the way our RH is behaving as more and more cement comes off and is being replaced with breathable insulative lime render. We haven’t needed our dehumidifier in over a month. Just gentle heating and passive ventilation seems to be keeping it under control, and we’ve only removed 40-50% of the internal cement render so far.

 

[Rob Duff]

We were thinking along the same lines as you, so pleased to hear that the house is drying & the dehumidifier needed less often.

We just need to decide whether to use insulative lime & the what to use.

I went at the cement tanking with gusto, using lump hammer & chisel where possible & a small breaker on the really hard stuff (some was 100mm thick!) The result was, alas, carpel tunnel operations on both hands (I awoke one morning & could not open the lower three fingers on either hand!) & 8 months out of action or on light duties only: So take care on the further cement removal.

I read Liverpool John Morres University's research paper on cork based morters last night ( https://researchonline.ljmu.ac.uk/id/eprint/11653/1/AB_FG_PF_Energy and Buildings_2014 (1).pdf ) whih concluded that cork does reduce the breathability & hydroscopic nature of the morter & reduces running costs by 8%, but that was with a high % of cork (up to 70%) & applied to external walls. We were not thiking of adding anywhere near that much cork.

It, alas, did not assist in answering the question on whether the benefits of cork outweigh the risks associated with reduced breathability & ability to hold moisture.

 

[aockenden]

We were thinking along the same lines as you, so pleased to hear that the house is drying & the dehumidifier needed less often.

We just need to decide whether to use insulative lime & the what to use.

I went at the cement tanking with gusto, using lump hammer & chisel where possible & a small breaker on the really hard stuff (some was 100mm thick!) The result was, alas, carpel tunnel operations on both hands (I awoke one morning & could not open the lower three fingers on either hand!) & 8 months out of action or on light duties only: So take care on the further cement removal.

I read Liverpool John Morres University's research paper on cork based morters last night ( https://researchonline.ljmu.ac.uk/id/eprint/11653/1/AB_FG_PF_Energy and Buildings_2014 (1).pdf ) whih concluded that cork does reduce the breathability & hydroscopic nature of the morter & reduces running costs by 8%, but that was with a high % of cork (up to 70%) & applied to external walls. We were not thiking of adding anywhere near that much cork.

It, alas, did not assist in answering the question on whether the benefits of cork outweigh the risks associated with reduced breathability & ability to hold moisture.

Yeah our cement internally is 100 mm in places as well. Absolutely unforgivable and ridiculous the suffering it’s causing humanity right now.

Sry about your hand, I came close to that after doing the entire outside and all the internal window reveals with hammer and chisel as well. Then I bought an SDS and it’s going easier now. I’ve developed some nasty nerve impingement / vertigo issues myself from this project which is miserable. Can’t wait for it to be over.

Our final, dried insulating lime mixture is probably about 60% hemp / cork and 40% lime by volume. Everywhere We put it it’s staying bone dry (once it’s initial drying is finished which does take a long time), even in condensation-prone corners / along the floor etc.

 

[irishcottagerestoration]

Hi all. Apologies for picking up an old thread but I've just stumbled across this and very tempted to give it a go. Do you have any updates on how the plaster has been for you since you did it? Do you have any updated pictures? How did the finish coat go? Sorry for so many questions!

 

[aockenden]

Hi all. Apologies for picking up an old thread but I've just stumbled across this and very tempted to give it a go. Do you have any updates on how the plaster has been for you since you did it? Do you have any updated pictures? How did the finish coat go? Sorry for so many questions!

As I sort-of hijacked and resurrected the original thread the first time, I’ll assume this is directed at me and not the OP. The plaster has been great for us, however believe it or not we still haven’t done the finish coat anywhere in the house! The wall temp to the touch is dramatically improved and overall the rooms feel notably warmer where we’ve done this (vs the rooms which we haven’t done yet). Drying time and the hard work mixing the insulating plaster are the biggest drawbacks. Careful using it in places where there is perpetual moisture as the bioaggregates like hemp / cork can start to mold if the wall is too slow to dry.

We had a minor spot of that near the bottom of one wall where a plastic underfloor DPM retrofit laying under a poured-in cement floor keeps the bottom of this wall slightly damp from moisture migrating out from underneath the plastic. Small patches of rusty orange mold appeared after that corner still hadn’t dried out 100% a month after application. So I ran a dehumidifier and small space heater in that corner, finally got it thoroughly dried and the mold has now died and faded and the plaster in that corner is now staying bone dry without any heater or dehumidifier help.

Everywhere else it fully dried out within 2-3 weeks and had zero problem. If I were doing it again, in any areas with some persistent moisture issues I would just swap the cork for perlite.