Advantages and disadvantages of hempcrete

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.