I thought I'd just put this out here to find out if anyone has any experience with this.

I've got a whole load of woodfibre EWI that's been sitting in my house waiting to be installed and rendered. Unfortunately I ran out of time last year before autumn and winter and I'm now planning the installation for spring.

Normally this stuff is rendered using a proprietary hydraulic lime system. However, I've been considering using non-hydraulic lime instead. In discussions elsewhere with the lime community, I've received some different opinions. Some say no way jose, and some say absolutely fine and would be great.

I don't want to discuss why I'm considering the non-hydaulic lime as I don't have any major compelling reasons other than that I would prefer the finish on my project. I'm doing it myself and although harder physical work, the non-hydraulic lime can be more forgiving of mistakes allowing time for re-work too. I also don't mind experimenting with things that are a bit different.

I'm leaning towards the fine side and at pains to think of reasons non-hydraulic lime couldn't be used on woodfibre.Those who've said not haven't been able to give me a well articulated reason why not. I'm aware that Pavatex supply/supplied a woodfibre IWI board that is fine with non-hydraulic lime plaster.

Do you have experience using non-hydraulic lime with woodfibre insulation? Any thoughts?

Thanks, that's reassuring. The stuff I have is surprisingly textured and fluffy on the outside which makes me suspect using a good amount of pressure on the base coat may be okay.

i'm a bit confused - if by non-hydraulic lime you mean hydrated lime isn't that just used as a plasticizer in conjunction with cement. On its own hydrated lime and sand won't go off properly as it needs the cement.

Maybe I have misunderstood what you mean by non hydraulic lime.

Also - what do you mean by 'leaning towards the fine side'? Do you mean fine sand?
Be aware that using an overly fine sand will lead to a weaker finish which wouldn't be something I would be happy with.

How much rendering experience do you have? A lot of the finish you get isn't so much to do with the grading of the sand but with how it is applied and when you rub it up to a finish. Why are you so against the proprietary hydraulic render? Cost I could understand. I made my own sand/hydraulic lime render for woodfibre IWI and that has worked well.

Which woodfibre board do you have?
Are you planning on a mesh as part of the rendering?

Posted By: SimonDBy non-hydraulic, I'm referring to traditional lime putty not the dry hydrated lime used as plasticizer (although I've been told this can be used and made into a putty by soaking it for a few days - but quite inferior to proper lime putty mind you).

Good, that's what I'd assumed. I believe you're right about hydrated time too and I would expect it to set in a similar time to lime putty - days or weeks, rather than hours.

Posted By: WillInAberdeenHydrated lime is any lime after the hydrating

Technically correct, but is more often used to mean the bagged stuff.

Lime putty is hydrated lime that has been left standing in water for several months so it is really well hydrated and protected from CO2

It shouldn't be. Lime putty is normally made by slaking lumps of quicklime (which have come from the kiln) with excess water. The lime breaks down into smaller and smaller pieces during the process and is left for at least three months to mature as you say.

Hydrated lime is made by slaking with exactly the right amount of water, after which it is bagged and sold.

Non-hydraulic lime is traditionally called 'air lime' or 'fat lime', especially when putty, which are less easily confused with hydraulic lime.

See https://www.buildingconservation.com/articles/limebasic/limebasic.htm

Posted By: WillInAberdeen >>> "Hydrated lime"..." used to mean the bagged stuff"

Hydraulic and non-hydraulic and partially-hydraulic lime are all sold in bags and they will (nearly) all be hydrated. If they are hydraulic then they will be marked and sold as such. If you ask for 'bagged lime' you will probably get something hydrated but not very hydraulic.

Now you're getting desperate. Of course hydraulic limes are sold in bags. Otherwise they would rapidly set. When I said bagged stuff in the context of hydrated lime, I was of course referring to bags of hydrated lime and if you try to buy that you will get exactly that. Anybody asking for 'bagged lime' deserves what they get - confusion on the part of the shopkeeper, I expect.

>>> "Lime putty is hydrated lime"..."It shouldn't be"

Yes it should. Lime putty is quicklime that has been hydrated with excess water. The resulting hydrated lime is left standing in the excess water for months to fully complete the hydration reactions.

And now eliding the context to make it look like you said something you didn't and I said something I didn't. I recommend people to read what you originally wrote and what I originally wrote in context to decide who was right.

I'm disappointed, Will. You normally talk enough sense to earn my respect.

I don't have enough experience working with lime to provide my personal input on this, but for the sake of future visitors, perhaps I can quote from the best book I've so far used to learn about this fascinating topic. It's only about 30 pages long. It's called "Lime in Building: A Practical Guide" by Jane Schofield (Revised 3rd Edition). I bought my copy from The Society for the Protection of Ancient Buildings https://www.spab.org.uk/. I highly recommend it and worth every penny of the £8 or so it cost me.

(pages 3-5)

"Non-hydraulic lime is produced by burning limestone (Calcium Carbonate CaCO3) during which process Carbon Dioxide (CO2) is driven off. The resulting 'super-dry' material is quicklime (Calcium Oxide (CaO). When mixed with water (H2O), this reacts violently and boils. This is called slaking. The resulting slurry is lime putty (Calcium Hydroxide (Ca(OH)2) which is stored in amoist state until used..

This non-hydraulic lime is so called because it requires exposure to air to carbonate, and will not set under water. This is the most permeable of all building limes, andthe most appropriate lime to use in the conservation of old buildings where maximum permeability is required.

Hydrated lime or 'bag lime', a non-hydraulic lime available from most builders merchants, is produced by slaking with a precise amount of water which is driven off during the reaction, resulting in a dry powder..

Hydrated lime has regularly been used as an extender or filler in cement mixes, but its uses as the principal binder is limited..

Hydraulic lime is produced from limestone containing clay and makes its set partly by achemical reaction with water, without exposure to air, but also by carbonisation. It can set under water,hence its name. In the ten years since this guide was first published,the availability of, and interest in, hydraulic lime hasincreased enormously in the UK. There is also a great diverity of opinion on the benefits (or otherwise) of using hydraulic lime and urgently-needed research is only now being undertaken.

Hydraulic lime is a blanket term fora range of materials with widely ranging properties which are usually referred to as follows:

Feebly hydraulic lime is defined by having less than 12% clay, which will mkae a set under water after about 20 days. It is usuallyand off-white colour,and makes a fatty (fairly sticky) mix which is easy to use. Feebly hydraulic lime is produced in the UK, frequently from blue lias or some chalks.

Moderately hydraulic limes contain 12% to 18% of active clay and sets under water in about 15-20 days. Some UK produced limes fall into this category; they are generally pale grey of (stet) buff-coloured. The mortar is less easy to use as it is not so sticky.

Eminently hydraulic limes have up to 25% active clay, make an initial set in a matter of hours, and is hard in a couple of days. It is a darker colour than other limes, and makes a mortar which is rather crumbly and difficult to handle when wet. All eminently hydraulic limes presently for sale in the UK have been imported, usually from France or Italy. Some contain cement (see page 5) or pozzolanic additives (see page 10) to keep their performance consistent.

French hydraulic limes are classified with a NHL number (HNL stands for natural hydraulic lime). Commonly available are NHL 2 which is moderatly hydraulic,NHL 3.5 which could be described as almost eminently hydraulic and NHL 5 which is eminently hydraulic.

Hydraulic lime is normally supplied in hydrated form (dry powder) although some feebly hydraulic lime is available as quicklime or short life putty."

I wanted to provide an update on this for anyone else researching this topic. I go a bit OCD on this kind of thing, wanting to make properly informed decisions so I've not pulled the trigger on ordering my render yet, instead doing as much research as I can. I'm grateful I did..

This morning I spoke to the technical department of a manufacturer of hydraulic lime mortars and renders. They told me immediately that a standard hydraulic lime and aggregate render is not suitable for woodfibre insulation. This is because the woodfibre is typically treated with a moisture repellent agent, so that the boards can be exposed to weather for a period of time (with mine it's 8-10 weeks). In addition, the relatively smooth nature of the woodfibre boards means that you're unlikely to get the necessary draw and 'stick' to the woodfibre boards. This can lead to the render unpeeling itself over a relatively short period of time (they said something like a few years).

To make the hydraulic lime render work with woodfibre, they said it needs special polymer additives within the mix.

So as a result of this, I've changed directions yet again and unfortunately it seems like it's going to result in a good hike in price!

I'm already ahead of you on that one I've just got hold of the European Technical Assessment documentation from the manufacturer of my woodfibre system giving approved render alternatives to their proprietary system - none of them are a traditional lime render but do include cement-lime options (which is typical for ETICS systems).

I've also spoken to a representative of Lime Green (whom the lime render manufacturer put me in touch with) which does provide a lime render woodfibre system but it's modified with polymers to bind with the woodfibre. It also contains modifiers to reduce its weight.

It was explained to me that internally it's not a problem applying lime plaster to woodfibre because you're not dealing with a system where the lime sits on the cold side of the insulation which can result in thermal shock.

It was also susgested during my conversations that a pure lime render can be too water permeable for the woodfibre so needs some sort of moisture repellant treatment.

I can't find any woodfibre EWI manufacturers who provide a pure lime render as part of their system. I'd love to know if anyone does?

In my particular case the recommendation to use NHL3.5 and aggregate has come from the supplier, not the manufacturer.

Perhaps it's a case of who do you believe, but one thing for sure is that the lime manufacturer has no financial interest in my situation. And I'm disinclined to ignore the manufacturer of my woodfibre.