Hempcrete

According to The Hempcrete Book by William Stanwix and Alex Sparrow, hempcrete is a “hemp-lime composite construction material … comprising the chopped stalk of the industrial hemp plant mixed with a lime-based binder”.

The authors go on to say that “hempcrete provides a natural, healthy, sustainable, local, low-embodied-energy building material that can truly claim to be better than zero carbon”.

In practical terms, the chopped hemp stalk is something like a cross between straw and wood chips. It is a dry, fibrous material and is typically packed into plastic bales. In the image to the left, some hemp has been put into buckets ready for mixing.

The “lime based binder” comes in a bag, just like a bag of cement and indeed the lime powder looks much like cement.

In my case, I used a lime binder that was specially formulated for hempcrete. The trade name is Batichanvre and the half opened bag can be seen at the top right of the photo.

The hemp I used is sold under the name Isocanna and is again meant specifically for use in hempcrete.

When mixed together with water, the lime and hemp make a kind of lightweight concrete. By varying the composition of the mix a variety of densities can be achieved, depending on the intended purpose. For instance, roof insulation requires a less dense mix than that used in wall construction.

The wet hempcrete can be used in a variety of ways within the building process. When used in walls, for example, it is often hand placed into shuttering that has been fixed over a timber frame. When it has set, the shuttering is removed, leaving the rigid hempcrete in place, as shown below.

Once dry, the hempcrete is surprisingly tough, rigid and fire resistant.

The advantages and disadvantages of hempcrete will be discussed in detail elsewhere on this site but I chose it mainly because of its thermal performance, simplicity, versatility and environmental credentials.

In my view, the advantages of hempcrete far outweigh the disadvantages. If your priority is to build a house as quickly, cheaply and easily as possible, then hempcrete will may not be your material of choice. If you value things like comfort, a healthy home and environmental sustainability, then hempcrete is worthy of serious consideration.

It’s also worth pointing out that a prospective house builder and owner will naturally consider the pros and cons of hempcrete with regard to their own building project. There is however a ‘bigger picture’ which concerns the impact of hempcrete on the wider environment and indeed at a global level.

In compiling this list, I consulted the following books as well as drawing on my personal experience:

1. The Hempcrete Book, by William Stanwix and Alex Sparrow,
2. The Whole House Book, by Pat Borer and Cindy Harris, and
3. The Green Self-Build Book, by Jon Broome.

Advantages of hempcrete as a building material

Feel good factor

I have found that hempcrete buildings naturally exude a ‘feel good factor’ which is difficult to put into words and really needs to be experienced. I would say that they promote feelings of well being, comfort, health and even calmness. The first time I felt this was at French Ecocentre where I visited a small octagonal building made from cast hempcrete. There was nothing inside the building, just bare walls, but I had an instant feeling of “wow, this is amazing”. The reaction of the other visitors suggested they felt something similar.

(I am 100% sure that, if the building been made from concrete blocks, my reaction would have been completely different.)

Years later, my step-daughter’s reaction, upon entering my hemp tiny house after the walls had been built, was the same. Surprise, a big smile and words (in French) to the effect of “oh, this is very pleasant!”

Whether this ‘hempcrete effect’ is due to the hempcrete’s thermal, acoustic or vapour regulating effects, or because it’s a free from synthetic chemicals, or something else – I don’t know. Perhaps one could feel similarly in a house made from straw bales, cob, raw earth bricks or timber.

Thermal performance

Hempcrete was described to me (by a French Eco Builder) as a good compromise between thermal insulation and thermal mass. By varying the amount of lime, relative to the hemp, the insulation value can be adjusted. The greater the proportion of lime, the denser the mix becomes and the poorer it becomes as an insulator. Unlike lightweight insulation, the thermal mass of hempcrete helps smooth out daily temperature fluctuations.

In theory, hempcrete does not insulate as well as some other materials of an equivalent thickness. For example, I found a UK hempcrete block supplier who stated that their 15cm blocks had a U-value (thermal transmittance) of 0.43 w/m2k. By way of comparison I read that a 14cm exterior timber stud wall with mineral wool insulation has a U-value of just 0.29 w/m2k.

In practice however, hempcrete tends to perform better than it should. This is partly due to hempcrete’s combination of thermal mass and insulation and partly due to the effect of water vapour as it moves into, through and out of the hempcrete.

As long ago as 2001, as part of a Suffolk Housing Society development, 2 hempcrete houses were built alongside 2 conventional brick houses and the constructions were monitored and compared. They found that the hempcrete houses maintained an inside air temperature 1-2 deg C higher than the brick built houses.

My own experience of living in the hempcrete tiny house last summer was also positive. After days of hot weather (30-35deg C) the house did get warm inside (but never to the point of being uncomfortable). I remember one evening it was 29 deg C inside the house and in the morning it had descended all the way to … 25 deg C! (the outside temperature would probably have been about 15 deg C). The thermal mass of the hempcrete acted a bit like a storage heater – absorbing the heat during the day and releasing it slowly during the night.

Acoustic performance

The previously mentioned Suffolk Housing study found that hempcrete wasn’t quite as good as a cavity block wall at sound attenuation (I.e. at blocking sound) . Hempcrete was, however, a very good sound absorber. The helped create a comfortable acoustic environment inside the house.

In the Hempcrete Book the acoustic quality of hempcrete is described as “unusual and distinct from that of conventional building materials” and depends on hempcrete’s porosity. It is suggested that the acoustic performance can be varied by the amount and type of binder (lime or cement) in the mix and how tightly the hempcrete is compacted.

It seems to me that hempcrete buildings do have good acoustics but this hasn’t been properly quantified or explained yet.

Vapour permeability

Thanks to it’s porosity, hemp is permeable to water vapour. It is also hygroscopic I.e. it can store and release moisture in the surrounding atmosphere. This is due to the microscopic structure of the cell walls.

The lime binder used in hempcrete is also permeable and hygroscopic (to varying degrees, depending on the lime used).

These characteristics are beneficial to the indoor air quality by helping to maintain a steady humidity and inhibiting the formation of condensation and mould on the surface of walls.

Fire resistance

According to The Hempcrete Book, the French manufacturer Isochanvre has produced test results that indicate hempcrete is a “non-flammable material”.

I can confirm that this is true. As an experiment, I cast hempcrete around some hollow concrete blocks to make a basic wood stove. Despite being in contact with burning wood, the hempcrete didn’t catch fire. Instead it just became progressively more charred and eventually crumbled away.

If you still need to be convinced, there are several videos on youtube of fire tests being conducted on hempcrete, such as this one:

Mechanical strength

Considered as an insulation material, hempcrete is tough and rigid. Compared to regular concrete, hempcrete is significantly less strong (about 1/20 the compressive strength).

However, if a low density hempcrete mix is cast around a load bearing timber frame, the hempcrete itself doesn’t need to have a great compressive strength. However, when fully set, the hempcrete does have a structural role in that it provides racking strength. I.e. it helps the building resist lateral movements. This has the advantage that the number of diagonal braces and horizontal noggins can be reduced or even eliminated completely.

I wish I had known this when I built the tiny house. My carpenter, who helped me frame the building, did a great job. This included diagonal bracing to make the structure nice and rigid. For good measure, I added horizontal noggins right around the building. However, these braces were a real pain when it came to casting the hempcrete around the frame. When I saw how rigid the fully hardened hempcrete was I realised that the braces were unnecessary. If I were to do it again I would suggest some kind of temporary bracing that could be removed in stages as the hempcrete was cast.

Resistance to moisture damage

I already mentioned that hempcrete is vapour permeable and can also store and release moisture. This is significant, as water does not get ‘trapped’ in a building’s walls (as it could do in, for example, a timber framed house with a faulty vapour barrier and glass fibre insulation). So, with no trapped water, the timber frame will not rot and the hempcrete can go on absorbing and releasing water vapour indefinitely.

As regards exposure to rain on exterior surfaces, hempcrete is normally protected by a lime render or timber cladding. However, in The Hempcrete Book the authors describe how hempcrete test panels were left outside, unprotected, for several years without deterioration. My own experience backs this up. At a French Ecocentre, while doing a training course, I saw some hempcrete that had been applied externally as an experiment. A few years later, it was still in good condition.

Non-toxic

Hempcrete produces no toxic emissions during the use of the building and does not pollute the indoor environment.

Furthermore, as it regulates humidity and reduces condensation, hempcrete promotes a healthy indoor air quality and restricts the growth of potentially harmful moulds.

Finally, the thermal mass of hempcrete (heat energy stored in the walls) means that building can be well ventilated without cooling down to any great degree. This again contributes to a good indoor air quality.

Environmental and societal advantages of hempcrete

Sustainability

The use of hempcrete, rather than concrete, is preferable in terms of environmental sustainability. To quote Jon Broome from the Green Self Build Book:

”An extremely low amount of energy is required to produce the hemp material, and the emissions of CO2 from the manufacture and curing of lime are very much less than Portland cement, which is now one of the principle sources of greenhouse gases in the earth’s atmosphere.”

The authors of The Hempcrete Book go further and point out that hempcrete is a natural material that is actually “Carbon Negative”. This means using hempcrete in construction will lock up more CO2 in the building than is emitted during the hempcrete’s production, transportation and disposal.

Contribution to rural economy

Greater use of hempcrete in the construction industry would require more hemp to be grown. Although I am not an expert on agriculture or rural economics, from what I have read and watched, this looks like a positive development. The video below goes into more detail on wider benefits of planting hemp on a larger scale for various uses.

Better health of individuals and benefits to society

There is no doubt in my mind that a house made with hempcrete, using natural finishes, is healthier and more pleasant than a conventional new build using highly processed, synthetic materials.

It stands to reason that living in a hempcrete house would have a beneficial effect on physical and mental health. If hempcrete were more widely used in construction, it would mean less pressure on health services and a positive effect on people’s wellbeing.

If this sounds too much like a hippy dream, consider the negative effect of poor housing. It’s well known that damp and mould in houses can be very bad for the respiratory systems of their occupants. Now imagine all of these people lived in hempcrete buildings with no mould and excellent air quality – I’m sure their health and happiness would be much improved!

Disadvantages of Hempcrete

Cost?

Does a hempcrete building cost more than a conventional building?

Referring back to the Suffolk Housing Society test build, they estimated that the additional cost of their hempcrete buildings was 10% more than the equivalent brick built houses. This was attributed to the increased labour, cost of the materials and the learning process of the contractor. Even so, 10% premium doesn’t seem like much to pay when you consider the thermal performance and comfort obtained.

In The Hempcrete Book, the authors suggest that the construction cost of a hempcrete building should be “broadly similar” to that of a conventional construction. They also make the point about comparing like with like. I.e. the hempcrete building must be compared with a conventional building with high-spec insulation. They also say that there are non-monetary benefits of hempcrete that may be important to the owners e.g. a healthy indoor environment and a low carbon emission construction.

When designing my own timber framed tiny house, I considered other environmentally friendly insulation materials such as wood fibre batts (which I had used before). They are good (but not particularly cheap) and they are not ‘monolithic’ in the same way that hempcrete is. In other words they have to be used as part of a wall system with extra layers (plasterboard, vapour barrier), which adds to the cost and complexity. Hence the reason why I chose hempcrete – it was simpler and no more costly.

To conclude, it’s by no means certain that a hempcrete build will be more expensive than the equivalent conventional construction, especially when you include the long term benefits of living there. As with any big project, you would need to seek quotes from various contractors and be sure that they are comparable.

Availability

Hemp shiv and lime binders are not common building materials. You could probably drive into any builder’s merchants and be able to buy foam, glass fibre or mineral wool insulation off the shelf. They almost certainly wont have any hemp in their warehouse.

However, even here in the UK, a quick google search turned up several suppliers of hemp shiv suitable for use in hempcrete. In France, I was able to order both the hemp and lime quite easily from my local builders merchant. So, in Europe at least, the materials are available but you will need to plan ahead and order them from specialists.

Unfamiliarity

As a ‘novel‘ construction technique, it’s safe to say that there are relatively few contractors who have the skills and knowledge required to build with hempcrete (that’s not to say that they could not acquire them for a particular project).

I guess that, for a hempcrete building, the options are:

• Find a hempcrete specialist
• Persuade a ‘normal’ building company to do the job (perhaps via a trusted architect)
• Build it yourself (or at least the hempcrete part).

Drying time

Cast hempcrete walls must be allowed to dry sufficiently before a finish, such as lime plaster can be applied. This takes at least 6-8 weeks, possibly longer if the weather is cold and wet.

Obviously this factor will have to be incorporated into the building schedule.

Clearly, using hempcrete is much less convenient in this respect than using lightweight insulation and plasterboard (drywall).

Safety

The main drawback of hempcrete with regard to safety is the alkaline and irritant nature of the hemp binder (lime). Care should be taken to avoid inhaling the dust and getting it onto the skin and into the eyes. The mixed hempcrete is also an irritant and gloves should be worn when working with it. So, as common sense would suggest, make sure to use the correct protective equipment (goggles, gloves, masks) and have eye wash or clean water to hand.

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

Mixing Hempcrete

The process

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.

The equipment

1. Plasterer’s mixer/whisk..

For small quantities you could get away with using a plasterer’s mixer like the one shown below.

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.

Pan Mixer

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.

Safety considerations

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!

Using Hempcrete

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.

You can read how I insulated the roof here and how I insulated the floor here.

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.

Sprayed Insulation

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.

Precast Hempcrete

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.