I’ve been re-reading older posts to try to get my head around the various types of lime plaster out there and I’m still confused! I’ve only just realised that the N in NHL doesn’t stand for Non! Unfortunately we’ve had to strip our walls more or less back to brick as a lot of the old lime plaster was blown. Also, some had paint on it which I didn’t want to chance leaving. I’ve found a plasterer and we’re planning woodfibre boards directly onto the brick, then plastered. He is suggesting using Baumit RK 39 I think. Also, Baumit Thermo in a couple of rooms where we’re not going to use boards as we don’t want to rip out original features. It’s a compromise but one we’re okay with. Is hydraulic lime suitable? I find the array of limes super confusing. We’re going to the effort and expense to ensure the walls are still vapour open so I don’t want to mess up with the wrong type of lime. And do we need to use similar on internal walls? Many thanks. I feel like I take one step forward and two back when trying to figure this stuff out.
- Thread starter Maria D
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One other question, we haven’t removed all the old plaster quite yet, but it does sound hollow in places which is why we are planning bringing it back to brick. Is this necessary? It’s quite crumbly but is that just because it’s lime rather than it being defective?
Lime is created by burning limestone in a kiln at high temperature.
If you start with a very pure limestone, then what comes out of the kiln at the end of the burn is a non-hydraulic lime. Non-hydraulic lime is very breathable and is relatively soft. It takes longer to harden than hydraulic lime (see below).
If you start with an impure limestone (containing some clay or other impurities), then what comes out of the kiln at the end of the burn is a naturally hydraulic lime. This is typically what is referred to as NHL. Naturally hydraulic lime is still pretty breathable, but less so than non-hydraulic lime and it is relatively hard. It hardens more quickly than non-hydraulic lime. Naturally hydraulic lime is categorized by hardness numbers, namely NHL 2 (softest and most breathable of the NHL bunch), NHL 3.5, and NHL 5 (hardest and least breathable of the NHL bunch).
So you can roughly line up all the limes discussed above across a common spectrum. And you can even throw Portland cement and clay onto this spectrum as well. Moving from most breathable, softest and slowest hardening to least breathable, hardest and fastest hardening; it goes
Clay
Non-hydraulic lime
NHL 2
NHL 3.5
NHL 5
Portland cement
The Naturally hydraulic limes (NHL 2/3.5/5) come in bags of dry powders only. Non-hydraulic lime comes in bags of dry powder as well but also in a moist putty form. So if you hear somebody talking about a lime putty, you know that’s a non-hydraulic lime.
Once you’re clear on the above spectrum then it’s just a question of deciding where on that spectrum you want to land for a given project / to solve a given problem.
If you start with a very pure limestone, then what comes out of the kiln at the end of the burn is a non-hydraulic lime. Non-hydraulic lime is very breathable and is relatively soft. It takes longer to harden than hydraulic lime (see below).
If you start with an impure limestone (containing some clay or other impurities), then what comes out of the kiln at the end of the burn is a naturally hydraulic lime. This is typically what is referred to as NHL. Naturally hydraulic lime is still pretty breathable, but less so than non-hydraulic lime and it is relatively hard. It hardens more quickly than non-hydraulic lime. Naturally hydraulic lime is categorized by hardness numbers, namely NHL 2 (softest and most breathable of the NHL bunch), NHL 3.5, and NHL 5 (hardest and least breathable of the NHL bunch).
So you can roughly line up all the limes discussed above across a common spectrum. And you can even throw Portland cement and clay onto this spectrum as well. Moving from most breathable, softest and slowest hardening to least breathable, hardest and fastest hardening; it goes
Clay
Non-hydraulic lime
NHL 2
NHL 3.5
NHL 5
Portland cement
The Naturally hydraulic limes (NHL 2/3.5/5) come in bags of dry powders only. Non-hydraulic lime comes in bags of dry powder as well but also in a moist putty form. So if you hear somebody talking about a lime putty, you know that’s a non-hydraulic lime.
Once you’re clear on the above spectrum then it’s just a question of deciding where on that spectrum you want to land for a given project / to solve a given problem.
I find part of the issue is proprietary products where the manufacturer is reluctant to say what their products are based on. Looking at Baumit RK 39 it is almost certainly NHL 2 based, but they don't advertise that, presumably as it is way more expensive per Kg than buying a bag of NHL 2 and some sand. That said there is definitely something to be said for consistancy, particularly if you are paying a plasterer who is happy to use that product. The plasterers time will be worth more than the saving of mixing your own.
I know some people don't like NHL, but many (myself included) have had good results and are happy with the compromise.
I know some people don't like NHL, but many (myself included) have had good results and are happy with the compromise.
While @aockenden ’s info is a really great overview, there is a bit of context and one issue with NHL which is lack of consistency. A 2 is not a 2, but minimum of 2… various studies have found many of the NHLs to be double their stated hardness once gone off.
Properties vary massively across brands, if you just use it, then St Astier is the best and softest.
It depends on usage - NHL has breathability and ability to manage moisture as well as vapour. However, it doesn’t like movement and sets hard.
So, for timber panel infills for example, it’s not great - frames shrink and swell, new oak shrinks for years, NHL just stays where it was put leaving gaps/cracks.
Whereas some of the original chalk lime I have will literally fold over double quite happily where it has detached from the daub beneath.
Properties vary massively across brands, if you just use it, then St Astier is the best and softest.
It depends on usage - NHL has breathability and ability to manage moisture as well as vapour. However, it doesn’t like movement and sets hard.
So, for timber panel infills for example, it’s not great - frames shrink and swell, new oak shrinks for years, NHL just stays where it was put leaving gaps/cracks.
Whereas some of the original chalk lime I have will literally fold over double quite happily where it has detached from the daub beneath.
Lacan07
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If you're planning on insulting with wood fibre then no there's no point going back to brick. Mechanical fixing are used to secure the wood fibre to the wall. If anything is obviously loose then pull it off but keep anything that's solid. Old dry lime can be quite crumbly in texture, non hydrated lime is soft by nature as its required to allow for movement in the building.
Thanks for the information. I’m conscious that the term “breathability” can often be used erroneously by some companies (paint for example). I’m trying to manage moisture on a redbrick Edwardian house with no cavity. I don’t want to make the wrong move with the IWI and plaster and regret it down the line. Interesting that the number next to NHL only refers to a minimum rather than an exact level.
Even if it sounds a bit hollow in places when you tap on it?
Lacan07
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Will be insulated with wood fibre board?
Lacan07
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If existing plaster is Lime (with no modern paint) and its flat / plumb then affix the wood fibre board to that. Anything from 40 - 80mm is fine, however much room space you feel you can afford to lose. 80mm is the limit however as depending on the thickness of the plaster there's a slight risk of moving the dew point into the fabric of the wall. The mechanical fixings will pull any 'hollow' or dislocated patches of plaster into the wall again as its tightens the wood fibre against the wall.
If the hollow patches are very large then yeah you might want to pull them off and patch with some base coat first but I wouldn't pull the whole lot off. That's a waste of time and also you don't know how flat and plum it will be underneath. The interior skin of 9" solid brick walls are often ramshackle, they saved the neat work for the exterior, and you may end up having to re-plaster it again.
When you apply wood fibre board the wall has to be relatively plumb, you don't want to leave any gaps behind if you can avoid it. Some people use and adhesive between the wall and the board but I think its unnecessary. Bear in mind the 40mm and 60mm boards have a relative amount of 'give' in them which will help with any minor undulations in the wall.
Flyfisher
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Good points about consistency. I wonder, would it be reasonable to assume that modern lime production is more consistent than historic lime production?
I'm guessing that when the builders of old collected their lime from the local lime kiln it wasn't in bags conveniently labelled NHL2, 3.5 or 5. I imagine it was just 'lime', which they mixed with a local sand to make a suitable mortar. All a bit 'artisan' and possibly the reason why some period properties have survived better than others, with some having probably failed completely and fallen down - ie the ones we don't know about or see!
As for Lime/chalk mixes, I've read that these were widely used throughour East Anglia for plastering, which makes sense because chalk is widespread throughout the region and its proven flexibility is a good match for timber frame properties that are particularly prevalent in the region, especially Suffolk and Essex.
I don't know how a chalk/lime mix would perform as a mortar for brick or stone but it would certainly be softer than an NHL2/sand mortar. What I do know is that when we re-rendered the outside of our timber frame section of our house it was with a chalk/lime mix and it has performed very well over the past 10+ years with none of the cracking that was evident in the previous cement-based render. I've also used chalk/lime plaster extensively internally, on both lath-and-plaster and brick/rubble walls and it has also perfromed well.
It’s not a regional thing in my experience, well it probably is, but more widespread. Turn of 17c Cottage in Berkshire, and 15/16c wool barn in Wiltshire, both same 2-3mm chalk flexi coat directly to daub. My ceilings weren’t added to the barn until 1600, yet they have the same top coat, on top of a sandier base. Berkshire was flint heavy, but here we’re on the edge of the Cotswolds limestone belt.
The Georgian outbuilding attached to our barn, built in brick early 18c is bedded in a chalk heavy lime mix, penny struck for good measure. It’s still sound now, never been repointed as it never was to start - bedding mix is the pointing, one solid mix.
Back in the day, they were slaking quick lime though, often on site, so the base material wasn’t as tough like an NHL. You’ll often find the tell tale big chunks of white lime in the mortar where it wasn’t slaked fully…
they certainly did some experimenting too though, as we have panels re-rendered in an early poziment on one elevation, somewhere late 17c/ early 18c - yet to analyse what the pozi was, but it’s harder than cement, bloody thick and straight onto the wattle. It does however act as a lime would, isn’t freezing and manages water. It’s clumsily haired in chunks, and full of varying aggregate too… oddly it’s on the side most sheltered from the weather.
There’s a really interesting paper about the use of metalwork into timber buildings to deal with failures, it means that poor framing methods and joints are preserved whereas normally they’re lost to history. A great insight into early trial and error and joints that got phased out… we’ve got some oddities in our and some creative joints from when it became a dwelling in 1600.
It wasn’t built to last, intentionally - they had the timbers and skills, as evidenced in some areas, but elsewhere they choose not to build to the same quality and spec. But the fact that it was converted to a dwelling with money and craftsmanship has saved it - there are very few examples elsewhere.
dowsingarchaeology.org.uk
The Georgian outbuilding attached to our barn, built in brick early 18c is bedded in a chalk heavy lime mix, penny struck for good measure. It’s still sound now, never been repointed as it never was to start - bedding mix is the pointing, one solid mix.
Back in the day, they were slaking quick lime though, often on site, so the base material wasn’t as tough like an NHL. You’ll often find the tell tale big chunks of white lime in the mortar where it wasn’t slaked fully…
they certainly did some experimenting too though, as we have panels re-rendered in an early poziment on one elevation, somewhere late 17c/ early 18c - yet to analyse what the pozi was, but it’s harder than cement, bloody thick and straight onto the wattle. It does however act as a lime would, isn’t freezing and manages water. It’s clumsily haired in chunks, and full of varying aggregate too… oddly it’s on the side most sheltered from the weather.
There’s a really interesting paper about the use of metalwork into timber buildings to deal with failures, it means that poor framing methods and joints are preserved whereas normally they’re lost to history. A great insight into early trial and error and joints that got phased out… we’ve got some oddities in our and some creative joints from when it became a dwelling in 1600.
It wasn’t built to last, intentionally - they had the timbers and skills, as evidenced in some areas, but elsewhere they choose not to build to the same quality and spec. But the fact that it was converted to a dwelling with money and craftsmanship has saved it - there are very few examples elsewhere.
Historic Ironwork Repairs in Timber-framed buildings. Index page.
Lacan07
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This is true, studies have shown that NHL's can set way harder than their grade. Having said that NHL's can be a very useful addition about the house, for example, I often employ NHL mortar for repairs and patching holes around services. It's useful for its structural properties, or external work where weathering is an issue such as plinths. Its less fussy to use and colour consistency is very good if that's important to you. When collecting some plaster at the Lime Centre the other day I was told professionals are turning to it a lot more these days.
Sadly, the cynic in me thinks there is a growing number of ‘professionals’ jumping on the demand for heritage work and claiming to be lime experts, who default to NHL for ease of access and not having to understand much to use it…
It’s got its place, don’t get me wrong - we’ve rebuilt our stone plinth in it, and some brickwork where appropriate. I’ve also rendered the most weather gable elevation … time will tell if this is an error….
stuart45
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I used to work for a firm about 25 years ago that was the only one in the area that I knew about using lime. Nowadays there's loads of them. Most builders merchants stock NHL3.5.
Going back further, a 1/2/9 cement/lime/sand mix was often specified for older properties.
The problem with renders and mortars is that it can take 20 years or more to find out of it really was the correct mix.
NHL3.5 is popular because it's a relatively easy and reliable binder to use.
It's a bit like eating foods that have ' heathly and fill of vitamins ' on the packet. Makes people think they are doing the right thing without too much effort.
Going back further, a 1/2/9 cement/lime/sand mix was often specified for older properties.
The problem with renders and mortars is that it can take 20 years or more to find out of it really was the correct mix.
NHL3.5 is popular because it's a relatively easy and reliable binder to use.
It's a bit like eating foods that have ' heathly and fill of vitamins ' on the packet. Makes people think they are doing the right thing without too much effort.
Stevers
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My father showed me how to put a dash of washing up liquid in my cement mortars to make them workable - no doubt they would have been 3:1 mixes - thus proving that you shouldn't always listen to your parents! From the moment I began my extreme DIY phase on moving here in 1997, I have used 1:1:6 and that worked well other than it not staying put when I used it to repoint a section of garden wall. I then experimented with 1:2:9 on the basis that it would work even 'better' (or something), but in one of the garden walls that I rebuilt with it, lime has leached out in considerable quantity and the coping has detached from the wall underneath. I'm not sure exactly why I then tried NHL for the next bit of garden wall in 2010, but I did and have used it successfully ever since.
stuart45
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A dash of Fairy isn't a bad tip for making cement mortar workable, although technically plasticizer should be used. It does the same job, keeping thousands of tiny air bubbles in the mix, which act as ballbearings.
Without this, cement mortar is virtually unworkable for a bricklayer.
Without this, cement mortar is virtually unworkable for a bricklayer.