The YouTube video below shows hempcrete being mixed in a small drum mixer, similar to the one I used.
I’ll assume that, as you read this, the video is still on YouTube and I’ll also assume that you have watched it.
The question for me then is “is it worth writing the rest of this article?’
Well, yes, probably … because I will give my own experience of mixing and using hempcrete to build an actual tiny house
As demonstrated, the mixing process consists of combining the water and binder to make a slurry, to which is added the dry hemp shiv.
I would suggest using the lime binder manufacturer’s instructions as a starting point. I think that every sack of lime or cement that I have ever used has had the mixing information printed on the sack. Failing that, there should be some information on the manufacturer’s website. In the case of St Astier, they provide lots of useful information on how to use their product for different applications.
Depending on your mixing equipment you’ll probably have to convert the mixing quantities provided by the manufacturer into something that makes sense for your situation. For example, St Astier recommends using 1 sack of binder with 100 litres of hemp and 30 litres of water for a hempcrete wall insulation mix. If you have a normal, small, drum type concrete mixer then you probably won’t be able to mix this amount of material in one batch. Instead you might try 1/2 sack of binder to 50 litres of hemp and so on, until you get a batch size that suits the mixer.
You will then want to convert these quantities into ‘buckets’ for convenience of measuring. I.e. your mix may work out as 2 buckets of water + 1 bucket of binder + 3 buckets of hemp (note: this isn’t a real recipe, I’m just illustrating the point!).
To recap: add the lime binder to the water and mix until you have a consistent slurry. St Astier recommend a mixing time of 3-5 minutes. The hemp is then added and the whole lot mixed together until the hemp shiv is uniformly coated with the slurry.
It’s simple enough, but there are a few things too watch out for. If you mix for too long, part of the hempcrete batch can form little balls. It’s best to stop the mixer as soon as you notice this happening. Another potential problem is lime forming a clump on wall of the mixer drum after it has been added to the water. If this happens, the slurry effectively becomes less rich in lime and so does the resulting hempcrete. To combat this it’s best to keep an eye on the drum as it spins around mixing the slurry. If you see a clump of lime on the back of the drum, stop the mixer and scrape the lime off with a trowel or a shovel. Start the mixer and let it run for long enough to incorporate the dislodged lime into the slurry.
I should point out that this is an abbreviated explanation of how to correctly mix hempcrete. Glancing through The Hempcrete Book, I realise that there is much more to the subject than I have written here. I wish I had read the book before beginning my tiny house! So if you are reading this blog post, and are seriously thinking about building with hempcrete, I would say that The Hempcrete Book is indispensable.
1. Plasterer’s mixer/whisk..
I have used this method to mix up some hempcrete ‘plaster’ to insulate a stone wall in a loft conversion. It was quite handy to be able to mix the material right next to the job rather than go up and down stairs to a cement mixer.
For a bucket I used a large polypropylene type from an agricultural co-operative (I think it was intended to be used for animal feed). Similar buckets are available online.
I would strongly recommend this type of rigid bucket as opposed the the flexible ‘gorilla’ type buckets that are used for mixing plaster. I have found them to be much more durable.
While using this method for a relatively thin coat of hempcrete plaster (5cm) is OK, you really need a bigger mixing system if you want to make enough hempcrete to be able to construct walls and floors.
2. Drum or bell type cement mixer
I used a ‘DIY’ type electrically powered concrete mixer to build the tiny house. It has a capacity of 160 litres and cost 270 Euros.
I was lucky that, in France, every DIY store seems to have similar types of reasonably priced mixers. In the UK they are less readily available and a bit more expensive. For instance, this 160L model from Machine Mart costs £359 (about €416).
Larger, industrial quality, drum mixers have much greater mixing capacities but also cost a great deal more. E.g. with a much greater mixing volume (280L), the model below costs more than £2600.
Of course, you could hire a larger machine rather than buy one. This might be worth it if you were confident of completing all the hempcrete work in a relatively short space of time. Other options are to buy a machine (new or second hand) and recoup most of the money by selling it at the end of the job.
If you are on a tight budget, but have a team of people to help with your build, I would suggest buying 2 small electric mixers. By running both mixers at the same time, you get a decent output. I would think that 4 people would be ideal: 2 to operate the mixers and 2 to use the hempcrete.
This is the type of mixer that is best suited to making the large volumes of hempcrete necessary for a ‘normal’ sized house.
As I was building a tiny house (on a tiny budget) on my own, it was too big and expensive for my needs. Having never used one, I can’t say I know much about them.
The picture to the left shows the type of 800L hydraulically powered pan mixer mentioned in The Hempcrete Book. It currently costs £320 per week to hire it and presumably you also have to hire the telehandler (and driver?) needed to make it function
The same company who hire out the above mixer (Kilworth Machinery) recently sold a used stand alone electrically powered pan mixer (shown below) for £7850. Not exactly a budget option!
I also found brand new hydraulically driven Pan Mixers for a relatively affordable £2,350 (Agitrend UK). See below.
These machines are obviously specialised pieces of kit that are beyond my level of experience. Clearly though, they are the largest, fastest but also most expensive option.
Given the alkali and highly irritant nature of lime, some safety precautions should be followed.
Firstly, avoid inhaling the dust by wearing a mask when handling the dry lime. This is especially necessary when tipping the lime into the mixer. It’s at this point that a puff of powdered lime tends to be released upwards (into your face!).
Secondly, safety goggles or glasses should be worn as it’s quite common for small amounts of hempcrete to splash out of the mixer as it turns. You don’t want this to get into your eye. If it does, then wash the eye out with eye wash or clean water and seek medical assistance if necessary.
Thirdly, wear thick chemical resistant gloves to protect your skin from wet hempcrete. I found that flock lined gloves were the most comfortable. Another thing I recommend is having a cup of olive oil nearby to use as a cheap and effective moisturiser/barrier. Long sleeved tops and trousers are advisable too. I found that, when wearing shorts, the little ‘chips’ of dry hemp tended to get into the tops of my shoes and get stuck in my socks. This soon became incredibly scratchy and annoying. You have been warned!
Casting around timber frame
In this case, hempcrete is formed (cast) around a load bearing timber frame using shuttering.
For my tiny house I chose to have the exterior of the frame exposed to allow the weatherboarding to be easily fixed in place. The exterior shuttering was therefore screwed directly to the timber studs.
On the inside face of the wall, I wanted the hempcrete to completely cover the timber studs (giving a continuous surface for the finish coat of lime-hemp plaster). In order to do this, I spaced the shuttering back by about 5 cm. This was easily achieved driving long wood screws part way into the studs before attaching the shuttering. I left 5cm sticking out, thereby spacing the shuttering 5cm from the studs. When the shuttering was removed, so were the long woodscrews (they were reused for the next shuttering ‘lift’). The sketch below illustrates this system.
The photo below shows one of my walls during this process, with the outer shuttering removed and the inner shuttering still in place. Note: you can also see the pieces of scrap timber that were fixed to the sides of the studs to provide a mechanical key for the cast hempcrete. With these in place, it’s impossible for the set hempcrete to move relative to the studs.
You can choose to have the frame exposed, or not, on either or both sides, depending on how you will finish the wall (cladding, plaster, render, etc). Obviously, if the frame was completely covered with hempcrete, that would give the best results in terms of insulation and timber protection.
After some experiments, I settled on 12mm thick OSB (Oriented Strand Board) for the shuttering material, cut to a height of about 60cm. OSB is fairly strong and cheap and the tongue and grooved boards can be fitted together end to end for long runs of shuttering.
I tended to leave the shuttering on overnight and the hempcrete was always solid enough when it was removed the following day.
When it came to placing the hempcrete in the shuttering I’m afraid that I did compact it quite firmly using tamping sticks. I probably made it fairly wet too. It was only later that I learned that the hempcrete should be fairly dry and only pressed down by hand.
Consequently, I have reduced the insulation value of my tiny house walls but have gained a little in thermal mass. That’s perhaps not such a problem in warm and sunny France. Another point in my favour is that the timber weatherboarding is bound to add a little to the overall insulation of the walls.
Roof and floor insulation
Insulating the roof and floor or the tiny house was more straightforward than the walls. The hempcrete mix was less dense (50% less lime) and the shuttering was less complicated.
Hempcrete render (lime and hemp plaster)
My plan was to cover up the rough inside face of the cast hempcrete with a relatively smooth lime and hemp plaster. I had some experience of doing this from my eco-building days many years ago. From memory, we used the Tradical binder, to which we added some weakly hydraulic binder (possibly NHL2) in order to improve the creaminess and stickiness of the mix. Using this method it was possible to get a reasonably smooth (if undulating) wall that was a kind of oatmeal colour.
Fast forward to my tiny house. The house had been built in the summer and I came back in October to plaster the walls. I had about a week to complete the task.
Regarding the lime, I had lots of Batichanvre left so I decided to use that. The manufacturer’s website advised on the correct mixture to use and stated that it should be done in two coats, 2 or 3 cm per coat, with 20-90 minutes between coats.
I thought about adding some NHL2 or CL90 lime to improve the plaster’s workability but I didn’t want to take the risk. Had I had more time, I could have experimented a little.
Anyway, I applied the plaster as directed. It was hard work as the cement-like mix didn’t flow very well and I had to use a lot of muscle to get it on the wall! The grey colour wasn’t very attractive but I knew I would be coating it with white lime wash later, so it didn’t matter.
I didn’t try very hard to make the walls perfectly flat. Having worked as a professional plasterer in ‘conventional’ building, I took some pleasure in making my own walls a bit wibbly-wobbly 😉
Unfortunately, I had to lock up the tiny house and head back to Scotland after a week. This meant the wet plaster would not be very well ventilated. I hoped that the released water vapour would be able to leave the building to some extent via the ‘breathable’ floor and roof, but it was not as good as leaving the windows open for a few weeks.
When I came back the next summer, the plaster was dry but the cabin had obviously been very damp inside. Some clothing had become mouldy, for example. The pitfalls of building abroad and having to leave in a hurry!
I was surprised to see that there were no cracks in the plaster (I had expected some in the corners of the walls or at the edges). Strangely though, numerous cracks did develop over the next few weeks. I don’t know if this was due to some more drying as the building was aired and the temperature inside increased. It may also be due to the movement of the building as it was being occupied and work was carried out inside. Perhaps if I had added some ‘softer’ lime to the mix and let it dry properly, the cracking would have been reduced.
Annoying as it was, I knew that the cracks were only superficial and the the timber frame meant the house was still structurally sound.
We’re I to plaster with lime and hemp again I would (1) use a finer grade of hemp (2) use a less hydraulic lime (3) put on a thinner coat and (4) make sure it can dry properly.
I know that hempcrete can be sprayed on but I have no experience or knowledge of the process. It’s not a technique that I was ever going to consider for a tiny house in rural France. However, if you were building a larger house and had access to a contractor who specialised in this method, then I guess it would be worth investigating. Although it wouldn’t be cheap, it would be quick and these time savings could be important to you.
Hempcrete is available as precast blocks, which might be convenient in some situations. I did not look into the possibility of using these for my tiny house. However, I have permission to build a larger house on my piece of land and I will certainly consider using them for that build (if and when it ever happens!). When I have dome some more research on these products I’ll make them the subject of a separate blogpost.