It would be tempting to view the subject of lime rendering as a secret world full of strange magicians with cloaks and pointy hats, secret recipes, potions and ancient techniques handed down from generation to generation. However, it’s not like that. They don’t wear hats.
They do have secret recipes and special techniques though – as well as beliefs in ancient practices handed down (or perhaps forgotten and rediscovered).
If I had to summarise what my investigations into lime rendering have revealed so far, you could split it into three areas – application techniques, mixes and equipment. Looking first at application, there seems to be broad agreement that spraying is the best way of getting at least the first coat onto straw. It is good to have this level of consensus, but has implications for both equipment and mixes (i.e. you need to have a mix which is sufficiently fluid to go easily through a basic, compressor-driven, render gun).
It gets harder when you enter the territory of mixes. Clearly there is the basic demarcation between the two types of flexible non-hydraulic lime: lime putty-based and lime hydrate-based and then the ‘inflexible’ hydraulic limes (NHL 2, NHL3.5 and NHL5). Hydraulic lime has, to date, been seen as a no-no for a straw bale building, especially one which has car tyre foundations which just sit on the sub-soil. Then there is the decision about whether you buy a ready mix or make your own (which has some relevance to equipment). But on top of all of this, it appears as though there is a now a divergence between traditional sand-based mixes and (perhaps even more traditional, in terms of being medieval) mixes using chalk and lots more fibre. The emergence of chalk and fibre mixes may be unique to East Anglia, or it may be just that I have come across it because we live there. Either way, East Anglia is perhaps unusual in the sheer number of medieval or Tudor timber frame, lathe and plaster / wattle and daub houses. In fact there are more listed houses in Suffolk, than any other county. This means that there is a flourishing business in both repairing these (which often involves correcting damage caused by previous repairs or refurbishments carried out using cement based plasters) and also constructing extensions or outhouses using traditional green oak, and thus flexible, framing.
As I understand it, investigations into the render on many of these old houses has revealed that sand was rarely used, but chalk was, in combination with large amounts of fibre (horse hair) and that this produced a material that was obviously breathable but also astonishingly flexible. Check out a picture of Lavenham to get an idea of why flexible render is a good idea if you want a timber framed house to last any length of time. The fact that we have all become more energy conscious has also encouraged ‘lime magicians’ to look for ways to make render more insulating, given that there is very little else you can do to improve the insulation of a listed timber frame, wattle and daub house. We live in one and you can take our word for it that the only way to keep all of it warm during the winter is to burn ten pound notes. As a result, there has been an emergence in recent years of modern / ancient hybrid chalk-based limes, incorporating insulating materials and also lots of artificial fibre – given that horse hair is out of favour because its modern cleaning treatment means it loses its resistance to the alkalinity of lime, causing it to break-down too easily.
While I had seen some of these new limes in a search through the websites of local lime suppliers and practitioners, it was Rory Sumerling who first gave me a proper introduction to this. Rory is a building restorer and one of those who started the Anglian chalk reformation (or should that be restoration) through his work investigating old renders and also having come across many failed sand-based renders on old timber houses. Rory came round recently to look at the build and also brought with him a sample of a chalk-based fibre lime mix produced by Singleton Birch. I have experimented with this mix on a bale and a couple of weeks on from its application, there are no cracks (despite paying little attention to keeping it moist etc. during its curing / carbonation period) in fact it almost has the pliability of roofing felt.
I have also been across to Shrewsbury to meet-up with Arthur Phillip. I was given his details by Dave Howorth, who has recently completed a straw bale house in the village of One House just a couple of miles away (his wasn’t the second house by the way – there are actually quite a few houses in One House). Dave used Arthur for the rendering. Arthur is probably the most experienced professional renderer of straw bale houses in the UK, if not the World – you can see him in action in this video. He hadn’t got the time to do my rendering and in any case I don’t have the budget to get a professional in, but he has such a passion for straw bale building and a desire to ensure that no straw bale rendering job goes amiss that he was happy to give up several hours of his time to give me, in effect, a one-to-one consulting session to talk about all things related to straw bale rendering. Arthur is very much a lime putty and sand man and also has a secret mix and technique designed to be used with his render gun and mixer, but notwithstanding not revealing the exact details, he was able to give me a huge amount of information on techniques, mixes, suppliers and equipment. Following our conversation, he has also been in contact with the Anglian chalk practitioners, so perhaps he may also come over to the chalk side.
Which brings me on to equipment. This is quite a big issue for me, because any equipment I have to hire will be expensive because of the length of time I am likely to need it for. Lime putty mixes require a forced action mixer to either mix them or to ‘knock them up’ if they come ready mixed. They also require quite a meaty compressor if they are going to be forced through a render gun. If you add all this to a rental charge (or purchase) of a gun, this could come to a weekly hire cost in excess of £250 per week – and I estimate it will take me some weeks to do all the rendering inside and out.
Which leads me, finally, to my currently favoured option. Rory had put me onto a chap called Martin Brown who runs a company called Warmcote. Martin is one of the East Anglian chalk and fibre men and he produces two mixes: Warmcote and Limecote. Both are bagged products like ‘normal’ plaster and can be mixed using a conventional plasterers’ mixer. Warmcote (shock horror) actually uses an NHL2 hydraulic lime, but Martin is convinced that the flexibility provided by the magic of chalk still allows the product to be sufficiently flexible. Because it is a hydraulic lime it doesn’t require the conditions required for carbonation and can therefore be applied to almost any thickness and can also go on in a single coat. Limecote uses lime hydrate – and thus still requires carbonation – but has none of the inconvenience of dealing with lime putty. Both mixes go easily through a gun and only require a small, 3.5 horse power compressor.
I know that traditionalists have been somewhat snooty about the use of lime hydrate – largely on account of the fact that it can be of poor or inconsistent quality either because of the presence of impurities or because it can be old and thus has partly carbonated before you get it. Martin’s opinion, however, is that this view stems from studies done some years back and that ‘modern’ lime hydrate, with date-stamped bags, has none of these problems and is thus fine to use.
I am favouring Limecote, because while getting a smooth finish using one coat has many attractions, I think the amount that will be required to fill-in the gaps on a straw bale house will be considerable and thus considerably expensive – and I will be better to use a long-straw mix to fill-in the gaps rather than pure render. One tip that Arthur gave me is to put a coat on first, before (or at the same time as) dubbing out with long-straw, because it is only once you have a coat on that you can really see where the gaps are. He also advised a technique where you spray-on and then scrape-back, for the first coat. Once I have a wall compressed and ready, Martin is going to come round with a sample bag and also his own gun and compressor, so that I can experiment.
So that it is. Everything I have discovered thus far about lime render.