Rain skirt

One of the many finishing-off jobs is putting on a rain skirt at the base of the building. This has two functions: to protect the base plate and throw any rain that runs down the building away from the foundations and also to protect the car tyres from direct sunlight, which can cause them to degrade.

Before rendering I had screwed a 2×2 timber through the woodfibre and into the timber of the base plate. This acted initially as a render stop, although the sticky nature of the Limecote mean this wasn’t really necessary. What I have now done is screw a piece of standard black stained weatherboad at an angle into this timber and nailed onto this a piece of 90 degree angled plastic beading. This beading can act as a new render stop for a a strip of render that overlaps the top of the weatherboard (see pictures).

I am pleased with the result. It finishes-off the base of the wall and gives the the whole house a slighty suspended feel.

Elsewhere, I have been finishing off the flooring and been back on the tyres and gravel again, but this time for the less critical job of providing a foundation for decking. I will use the scaffold boards so that the same material that is the interior flooring also then runs-out as ‘exterior flooring’ albeit with a small gap between the planks to allow rain to run-off as well as having set the decking on joists that are on a 1% angle to further encourage run-off.





Window detailing

It has been a long time since the last post – mostly because we have been doing ‘non-straw’ jobs, such as installing bathrooms and kitchens and painting which are not especially interesting or noteworthy tasks. I have been meaning to write a review post covering my conclusions from clay plastering, but haven’t got round to it yet. However, last week I got back onto a straw specific job, namely finishing off the exterior render around the windows.

I had left the window area without a finish coat because I needed to get the windows themselves in before doing this and I wasn’t able to do that job before the winter lime rendering window closed. As luck would have it I received a surprise visit from Arthur Phillip, the lime render man, who happened to be in the area. Arthur is probably the country’s foremost expert on rendering straw bale houses and he introduced me to PVC window frameseal beads. These are basically a strip of white PVC channel with a sticky back to it that you can put around the wooden frame edge of the windows allowing you to render up to and into the channel – thus giving you a stable and watertight edge to your render. They also have on the a tear-off sticky plastic strip which you can use to attach polythene sheeting to in order to protect the windows themselves from render spalshed – important if you would be using a gun to apply the render.

I couldn’t actually source these in sufficiently small quantities, so I made my own by buying standard PVC channeling and runing a bead of silicone up the back of them to stick these to the frame while also creating a weathertight seal. I then mixed up a batch of lime and straw to create the shaping into the window frame and finished-off with a coat of lime that then ran into / up to the window edge fo the PVC channel. This seems to work fine.



Second (and final) coat

This past week or so I have been getting on with the second coat, which I also intend to be the finish coat. The mix I have been using is firmer than that which went through the gun for the first coat, but still relatively sloppy – given the that it is serving also as a ‘float coat’ albeit I am not looking to get an especially smooth finish. The depth is probably a minimum of 10mm, although it gets much thicker in places given the uneveness of the straw and holes or depressions in the first coat.

Most important learning is the degree to which the first coat will rapidly draw the moisture out of the second coat, making it quite difficult to work – hence the need to ensure that you give the first coat a good drink before putting the second on. It is amazing how much moisture the first coat will suck-up.

I am not using the gun, just loading the hawk up with a good load of render and then pushing this on and up using a trowel. I then quickly work this to get a reasonably smooth and even coating and finish off by misting it and going over the surface with a sponge trowel. This helps cover the trowel marks and produces a relatively even but rough textured finish – which has about the right amount of ‘hand-madeness’ I am looking for.

This doesn’t require a huge amount of skill and can be done quite quickly – not quite as fast as I could go with the gun, but not far off. I reckon I can get through about 7 square metres per day. This is a relief, since I was worried that my attempt at a finish coat would either require a huge amount of time and/or end up looking like a bodge-job. This isn’t to say that plastering could be considered easy, but it is not too hard if you are happy with a what might be called a rustic finish.

I know that time is ticking on and I am now well out of what traditional lime people would consider to be the rendering season. This might be the case if I was using sand, but with the chalk and fibre mix I am confident that I have something that will be resistant to all but the hardest of frosts and that if it looks like a cold snap is on the way, putting hessian up should provide enough protection. There has certainly been no evidence of any sort of cracking developing on either the first or second coats thus far.

The only other job this week has been to order the window cills, since as you can see from the pictures, I haven’t yet finished-off the area around the windows and am waiting to get the cills in before doing this. I have set the windows quite deeply, which means the cills need to be 220mm deep – i.e. much wider than any standard widths, so I have had to order the wood (oak) from a local sawmill. I am going to use just a flat plank and set it into the frame with a downward angle, resting on top of the straw/render rather than going to the expense of having a custom-made cill, with flat base and angled top since I figured this is only necessary if you are fitting your cill onto a flat surface such a a course of bricks.



Greek yoghurt

20160914_181245I have now pretty much completed one coat along the longest wall. I have experimented with making the mix a little sloppier and reducing the air pressure. The best results seemed to have been achieved with a mix about the consistency of greek yoghurt with air pressure at 2 psi. The biggest pain has been the fact that the gun cannot get underneath the eaves sufficiently to spray right up to the top, requiring that this bit be done by hand (with a mackerel pate mix).

Other tips would be to avoid putting too much mix in the hopper – since frequent refills are preferable to aching arms. Also – as you get towards the end of a batch, it is a good idea to leave a small amount in the bottom of the mixing bin and then get a few handfuls of straw to wipe down the inside and then mix-up with this residual amount, creating a long-straw mix which you can then use to fill-up any dips that have been revealed once you smooth off the initial sprayed coat.

Note: I have given up basing the consistency of the mix purely on the amount of water added. Different bags seem to behave in different ways: one will get to greek yoghurt consistency with 11 litres whereas another may take over 13. Not sure why this would be. So I start off with 10 litres in the mixing bin, to which I add the fibres, and then progressively add powder and water.


The day of rendering is upon us

Today was the first day of rendering. I must confess I was a little nervous. Things got off to a bad start when I found that the connection on my render gun didn’t fit properly on to the hose from the compressor. However, a lead from a local builder’s merchant unearthed a relatively local company called Airpower East, who specalise in all things air powered. Here a very helpful man searched through their extensive stock to find just the thingy required. It was amazing how many types of connection there are in the air world – so I would advise you to check your connections, especially if, as I did, you end up getting a render gun off the internet and are thus not sure exactly where it comes from.

The area I was planning to start on was the gable end at the side of the house that is least visible – based on the theory that it is best to practice on the parts that people are less likely to see.

Once underway I discovered that the key to making things run smoothly is to get the right balance between liquidity of the render mix and the amount of air you put through the gun. If the mixture is too runny and the air pressure too much, it just appears to blow through the mix without catching it, or else it picks-up the wet material and leaves behind the fibres, which then clog up the gun. A mix that has 10.5 litres of water per bag and an air pressure of 3psi seemed to be the sweet spot. At one point I had a mix that was perhaps a bit too wet and this kept clogging the gun, so I decided to try putting it on by hand by simply throwing handfuls onto the straw. Ths actually worked quite well, which is good to know as a back-up, but was pretty slow.

The product itself, Limecote, is amazing stuff. No matter how wet and sloppy you make the mix it holds its shape without any slumping, because of the density of the fibres within it. It is also fantastically sticky –  especially useful when you are throwing handfuls of it at the straw. There is no way a traditional sand-based render would behave like this, or be as easy to mix.

The trickiest part was getting a good seal at the edges where the straw meets the steel underside of the roof. I did this by getting a handful of straw and rubbing render into it and then forcing this ‘long-straw mix’ into the join. This seemed to work, but exactly how this join will work long-term, I don’t know. Will have to keep a watch out for cracks / gaps.

By the end of the day I got one coat onto the whole gable end (probably about 3 square metres), which seemed like reasonable progress, given the late start.

More pictures and video to follow.


Sourcing woodfibre

The Straw Works plans I am using specify facing the foundation box beam and the roofplate ringbeam with 20mm woodfibre boards to both improve insulation and create a base for render. This always seemed like a minor detail that would be easy to to complete – but has turned out to be more of a headache.

First-off, the diagrams specify a 20mm board and Straw Works have told me that they use a Steico board. However, Steico don’t stock an exterior grade 20 mm board, only a 20mm interior one. Their smallest weather-proof board is a 40mm with a tongue and groove edge – which is significantly more expensive that their interior board. I can see why T&G is necessary for a thickish board which is going to be applied in sheets, in order to get a good join, but I am applying it in narrow strips, onto a solid timber base. There is also another issue in that the straw doesn’t have a consistent edge where it meets the beams. Just in the nature of the stuff and the way the walls were put together in some places the bales are flush with the edge and in others it can bulge 40mm or more over the top. I did therefore think that a 40mm board would help even this out but might also create too much of a bulge in places given the need to have a 20mm layer of render on top.

In the end I found a 20mm, exterior grade, square edged board called diffutherm supplied by Back to Earth. However, something else I failed to appreciate is that the board itself is very soft. I had imagined it would have the consistency of MDF and thus hold a screw or nail. There is no way fibre board could do this, so you therefore have to use special flexible plastic washers (which Back to Earth supplied) and also stainless steel screws, since the lime in the render will corrode anything else. These were surprisingly difficult to source.

As it turns out, I will not need as many of these as I first thought, because in working out how to create a render stop and also install a board which will protect the tyres and act as a skirt to throw rain as far away as possible fom the foundations, I came up with a scheme which involves using a chunky 150mm screw to secure a 50mm square timber two-thirds of the way down the woodfibre board, drilling securely through into beam underneath. I can then use this as a base to attach standard black treated weatherboard as a combined render stop / rain skirt (see first picture) but it will also serve as a way of securing the lower part of the woodfibre to the foundation beam.

Straw Works specify that this timber should be an (expensive) durable timber such as oak – but I figured that weatherboarding would be fine, in that it is clearly designed as an exterior timber and it would also be very easy to replace whenever it begins to deteriorate.

It is also worth noting that Chris Brookman at Back to Earth was insistent that I would need to apply an additional (quite expensive) coating to the fibre if it was to be able to take a lime render. However, I checked this out with Martin, my lime man, and he said that might well be the case with sand-based render, but that his mix would pretty much stick to anything and that he had already applied it to diffutherm with no problems. In Chris’s words “I can’t say you haven’t been warned!” which is fair enough, but the Limecote render I am going to be using has essentially been designed as a material that breaks most of the rules of lime rendering – and this is just something I am going to have to take a risk with.



Experimenting with render

A few days ago, Martin Browne from Warmcote, came round to try out his Limecote mix and also see how this worked using a render gun.

All-in-all it was pretty successful. My main concern was to see how easy it was to get an initial coat on using a spray and also how much a coat of render would expose the imperfections in the wall, in terms of gaps between bales and inaccurate positioning (and thus how much dubbing-out work might be required).

Getting a coat on was straightforward. The beneft of Limecote (which uses lime hydrate, chalk and a heavy dose of artificial fibres) is that it mixes with a conventional hand-held mixer and also can go through a pretty lightweight (thus cheap) gun, powered by a small (thus cheap) compressor. We were actually using a spray paint gun, not even a proper render gun, with a 2hp compressor. As the video shows, it went on pretty easily and penetrated well into the straw.

We then made up a mix with quite a lot of straw in it, as a dubbing mix. Again, we were able to do this using the hand-held mixer. This didn’t fix well to ‘naked’straw, but was fine when applied on top of a sprayed layer. By pressing this mix onto the walls it was relatively easy to flatten-out imperfections and get to a pretty good finish.

One thing that was very noticable is that the cut and folded ends of the straw take the render in different ways. The cut ends present a very firm surface, albeit with lots of vertical ridges as a result of the layering / flaking of the straw as it was packed into the baler. The folded ends didn’t have this ridging (which needs to be filled-out with render) but were much softer and thus had a tendency to form a hollow. Since we had alternate layers of cut and folded ends this had the potential to create a slightly wavy surface, which required the long-straw mix to flatten-out the hollows on the bales with folded sides. Perhaps we would have been better having all our bales presenting the same way (say cut ends on the outside and folded on the inside) – but we varied the layers following advice from someone who had found that the folded sides were slightly thicker and thus made the wall start to bow inwards or outwards after a few layers.

After Martin had gone, I prepared a mix that was even more straw-heavy – i.e. pretty much just coated straw, rather than render with straw within it. This seemed to work well for dubbing out big holes and depressions.

Martin reckoned that the best technique would be to spray an area, gently even this out with a flexible edged float and then press-on the long-straw mix to get to a desired straightness of surface (accepting that I am not looking for a perfect, flat finish). This layer would probably vary between 10 – 30mm. This should then be ready for a finish coat of about 10mm.

The test layers have now been up a couple of days and seem to have got a good fix and be setting nicely. The one slight drawback was that the straw started to leach its colour into the render, especially with the staw mixes. I guess this is less likely to happen when using a sand-based render which is already more straw-like in colour and also where the sand is much less moisture permeable than chalk. If this comes through the final coat I will have to put a straw-coloured limewash, or sufficiently pigmented to drown-out the straw leaching.


On the right: long-straw dubing mix (plus colour leaching), on the bottom left: finish after spraying and levelling , centre left:sprayed coat plus second (finish) layer



An initiation into the dark arts

5401dfdea06a6208456c949fe3ab5c70It 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). tb99

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. some-houses-in-lavenham-seem-to-be-unrealThe 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.