In this post I will go into a bit more detail about the types of hemp shiv and binders that can be used to make hempcrete.
There are 2 types of hemp shiv: course and fine.
Course Hemp Shiv
When building my tiny house I used course hemp shiv of the brand ”Isocanna” which is sold by St. Astier (who also make the lime which I used.)
According to The Hempcrete Book the pieces of hemp shiv of this type should be 10-25mm long and should be as free as possible from dust. The course hemp shiv produces a hempcrete matrix structure that is strong but also fairly open and breathable.
Fine Hemp Shiv
I only recently learned that hemp shiv was available in a finer grade. It’s now about 15 years since I first got some training in the use of lime and hemp and nobody mentioned it then!
According to the French supplier Isol Naturel, the size of their fine hemp shiv is 5-10mm.
St Astier likewise have a fine version of their Isocanna hemp and in the UK I found that Lincolnshire Lime also stock fine grade hemp shiv.
Interestingly, the fine hemp can be used to make an insulating ‘lime plaster’. In other words, a finish coat of lime/hemp applied to the rather more coarse hempcrete walls. I wish I had known this before building the tiny house as I would have used the fine grade shiv for this purpose. Instead, I used the coarse grade for everything, which wasn’t ideal.
Another use for fine hemp shiv, according to The Hempcrete Book is in spray applied hempcrete.
Types of lime and cement
I’ll discuss briefly some of the different types of lime/cement that are available (please note, I don’t claim to be an expert or want to write a book on the subject!). My knowledge was picked up in France, so the terminology may not be the same in your country.
It seems to me that the various types of lime are based on the amount of ‘impurities’ contained within them, which has the effect of altering their properties.
At the very pure end of the scale, we have the very white ‘air lime’ (Chaux Arienne in French). This only sets in contact with the air and can be bought as a powder in sacks or in tubs as a putty. It’s not mechanically ‘strong’ and is often used for more decorative purposes. I have used it for making a brilliant white lime wash (think Greek houses) and, when combined with marble dust for making Venetian Plaster (imitation marble). Because it is so creamy and sticky, it could added to other lime mixes to make them easier to work with. Sacks of this kind of lime are normally labelled CL90.
Hydraulic limes (Chaux Hydraulique) contain impurities (such as silica and alumina) and require water and air in order to set. They come in different grades from weakly hydraulic to strongly hydraulic, depending on the levels of impurities. The weakly hydraulic grades are softer, more flexible and lighter in colour. The strongly hydraulic limes tend are more ‘cement like’ – less sticky, stronger and grey. These types of lime are labelled NHL (Natural Hydraulic Lime). They are available as NHL2, NHL3.5 or NHL5 (least to most hydraulic).
Lime with more than 20% of cement added is labelled NHLZ.
Natural cement is extremely hydraulic and sets very quickly and very hard. Critically, it is also vapour permeable.
Portland cement is made from a mixture of limestone, shale, gypsum and other additives. It is strong and hard but not vapour permeable.
Binders for use with hemp
As far as I can see, the self builder has a choice between using a binder that has been specifically formulated for making hempcrete, or another type of binder that is normally used for other purposes.
In The Hempcrete Book, it is advised that the binder should have a strong initial set so that (when building a wall) the shuttering can be removed fairly quickly (within 24 hours) without risk of the lower parts of the wall failing.
The formulated hempcrete binders do provide the reassurance of this initial set. The brands Tradical and Batichanvre, for example, are thought to achieve this by a mixture of lime , Portland cement and possibly other additives. The volume of added cement (20-30%) is said to not inhibit overall vapour permeability. Another formulated option example is the Prompt brand of natural cement made by Vicat.
The non-formulated lime binders are perhaps more of a risk but may be cheaper and more easily available. They also have the advantage of not including portland cement. This may be important to you from an environmental impact point of view. I don’t know to what extent the avoidance of cement improves the vapour permeability of the wall.
I found online some guidance from an association of French hemp producers on the use of natural hydraulic limes as binders for hempcrete.
For external walls, they suggest the following mixture:
200L of hemp: 70kg of Lime (NHL 3.5 or 5) : 80L of water : 20-30L of clean sand (optional).
For my tiny house I used the formulated binder Batichanvre. It was indeed grey and cement like. I never had any problems regarding the strength of the hempcrete – it came out pretty solid.
The authors of the Hempcrete book strongly advise against the use of non-formulated binders.
Despite this, were I to build with hempcrete again, I would experiment with the NHL mixture above. Assuming that I had enough time, I would use the mixture on a small ‘test wall’ and see how it fared. If it stayed solid after the removal of shuttering, was not crumbly, and seemed to be drying out properly after a few weeks, then I would be confident enough to use it on other walls.
Ultimately it’s up to each builder to choose what level of risk they are prepared to accept and what their priorities are.