How about Living in a Hemp House

Artikel auf Deutsch: www.hanf-magazin.com/wj1

One of the oldest plants, cultivated by man, was the cannabis sativa. Nowadays we call it ‘industrial hemp’.

von Wolf Jordan

Nowadays we call it ‘industrial hemp’. Virtually every part of this fast growing plant can be used. The seeds, the leaves, the fibres from the stem and even the left-over parts of the wooden stem: the hemp shivs. This is what I will tell about, since this material has an enormous potential. If we mix the hempshivs with a binder, then we can use this for insulation of existing houses and for building completely new projects. This mix, usually referred to as ‘hempcrete’, gives a light weight material that in many ways will not obey the ‘normal’ physics and calculations, used in modern building … Now, this becomes challenging!

First, there are aspects that fully correspond with normal physics. Like a cement-concrete essentially is composed of a binder and filler, the same works for this ‘hempcrete’.

So, in order to obtain an optimal result, the granulometric composition of the fillers is crucial. You will need big particles, to give your mixture strength. But the cavities between those particles, have to be filled up by smaller particles, and room between those, need still smaller particles, and so on, until you have a maximum of filler, from big to very fine. The granulometric curve of hempcrete should be similar to that of a ‘normal’ concrete.

This homogeneous mixture needs to be glued together, and therefore we need a binder. For hempcrete we take earth (loam, which is composed of clay, silt and very fine sands) or preferably lime.

Why preferably lime? Well, it is one of the best building materials. Look at our ancestors, and even further back to old Roman times: those buildings still stand after more than 2000 years. Talking about sustainability… Lime is making a strong comeback, but is still strongly misunderstood. The finest and the purest lime is the best, but then we need a pure limestone.

If a limestone, or calcium carbonate, is heated (temperature needed is between 800 and 900°), then the chemical structure breaks up. Water and carbon dioxide (CO2) escape and what is left, is calcium oxide, or CaO. This is very reactive, we call it quicklime, in French ‘chaux vive ‘ (Branntkalk auf Deutsch). If you mix this with water, you will change this quicklime into hydrated lime (chaux hydratée) or slaked lime or calcium hydroxide, the formula is Ca(OH)2. In the past our ancestors did this with a large amount of water, so they obtained a ‘fat lime putty’. Modern technology and perfection allowed to slake the lime into a powder: easier to handle and to work with.

If you use this hydrated lime to make mortars, renders, paints, etc., then the lime will react with the CO2 from the air, it will start to petrify and will become calcium carbonate, virtually the same material as it had been before the treatment. The circle is closed, because the CO2 that had escaped during the heating process, will again be absorbed.

Typical for this type of lime, once it has carbonated, is the breathability combined with its microporosity. Although a good lime plaster will perfectly be able to regulate the humidity inside your house, the micropores are so fine that in fact there is not enough room for damp. So, when too wet, it will pump out this damp as fast as possible.

To give you an idea: gypsum also is breathable. But the micropores are larger, there is more space inside, so gypsum can take up more moisture. It will even suck up humidity, will easily become wet, grow moulds and start to rot. You don’t want that in your house. Now, if you mix this super fine lime system with hemp shivs, you get a combination that defies normal physics… You see, the microporestructure of hemp happens to be exceptional too. Which is quite normal, considering hemp grows to a height of 3, up to 3,5 meter, even 4 metres, and are ready to be harvested within 3 months of ca. 90 days. Trees would need 15 to 16 years to do this! This is a fabulous thing, only bamboo does better… So, hemp has the natural capacity to pump up water and nutrition at a phenomenal speed. Inside the stem there is an ingenious capillary system, capable of transporting humidity and nutrients in an extremely fast and efficient way. Don’t forget that at night plants reverse this system: they capture CO2 from the air and pump it down through the roots, into the earth, where they feed the microorganisms and the mycelium, preparing the nutritional ingredients for the plants. A wonderful world that is. You can imagine something special to happen in the hemplime mixture, indeed. Especially in wintertime, we have a high damp pressure inside our houses. It is a warm air, we breathe, wash, cook, etc., so we create an inside air containing lots of humidity resulting in a high damp pressure. Outside, when it’s freezing cold, the air is much dryer, so the outside damp pressure is very low. Thus the damp will be pressed into the walls from the inside, creating lots of potential problems like condensation, saturation points, wet walls, etc. That’s why in modern building, damp screens are used, avoiding the condensation problems, but causing moisture problems inside and moulds. So, a forced ventilation system is installed, which is quiet expensive and will continuously consume energy. Compared with clothing: if you wear plastic, this cannot breath, you will sweat and get a cold.

Good clothing should breathe, let the damp pass, you stay dry and you keep warm. Exactly this will happen with a hemp-lime, but with an extra surprise: So, the lime will take up the damp pressure, it will pump the damp into the hemp shivs (with those very special capillary pore systems) but by doing so, this damp undergoes a phase change in order to enter the hemp structure. This generates heat. So, the energy within the damp pressure is transformed into caloric energy. This not only means that the problem of this difference in pressure is solved, but it also means that, the colder it becomes, the better the thermal insulation performance of this hemp-lime will be. Or, the colder, the better the ‘lambda’. It becomes a dynamic value. This is no fantasy, it has recently been measured at the University of Bath, were a team of specialised scientists is doing impressive research on this material. Add this to the high thermal inertia (or the capacity to store heat over a long time) and this material will perform much better than any physical formula would suggest.

Does hydrated lime and hemp only have advantages? To be honest, no. Hydrated lime takes a long time to cure. This is not accepted anymore in the building sector, spoiled by the fast curing material cement … Some builders will add a substantial part of cement to the hemp-lime, hence the name hempcrete, a name I personally prefer to avoid. Because cement does not breath and it will change the properties of the mixture. Cement will cause radon gas to evaporate, it demands double quantity of energy for production, it reacts with water (forming complicated gels) so will not recuperate CO2, on the contrary, it has a very high carbon footprint. Many builders use hydraulic lime. Now, what’s that? Is it better than cement?

Yes, definitely. Is it as good as hydrated lime? Definitely no. So why then? Because the curing process goes a bit faster.

But what is the difference between the two? As I said at the beginning, the best and purest hydrated lime can only be made from pure limestone. But most limestones are not pure. They contain more or less some clay. Geologically, the most important components of the earth are calcium carbonate and clay. So, most quarries have limestones, naturally contaminated with clay (which is silicium, aluminium and ferrous oxides). For lime, burning temperatures of 800 to 900° are enough, for burning clay you need higher temperatures. The more clay, the higher the temperature needed, the higher the carbon footprint. And there is more: the clay will change at least part of the lime’s reaction, by reacting with water (forming gels, like what happens with cement, hence the name ‘hydraulic’) and we will not anymore have this closed circle we had with the pure hydrated lime (which cures not with water, but with CO2 from the air, remember).

Hydraulic lime is also better water resistant than hydrated lime. Sometimes this is useful. But inside your house? Like if you invite your friends to dinner, would you keep wearing your raincoat at the table?

Hydraulic lime will have larger micropores after curing. That is because they consume water in order to form their gels and this leaves cavities. This will change the absorption of damp pressure, loosing partly or totally, the heat-generating change of phase. The lambda of hemp-lime with hydraulic binder scores poorer than hemp-lime with hydrated lime. This has been measured. This is a fact.

Since the contamination with clay was a natural, geological process, this type of lime nowadays is called ‘natural’ hydraulic lime, because the word ‘natural’ sells well on the marked. As a matter of fact, there are far better ways of giving lime hydraulic properties, ways that allow to perfectly control the dosage and the degree of hydraulicity, but that’s another story …

To conclude.

Like all plants, hemp absorbs carbon gas from the air to transform this into matter, into carbon compounds. Huge amounts of CO2 can be sequestered this way. A hemp building will be carbon-negative. The far-most best binder in this case is hydrated lime. Although building blocks from hemplime are available on the marked, the in-situ application is best. You can put the mixture on existing walls, from inside or outside, or both, as well as for building new houses (mainly post and beam structures). Up to 8 cm you can put on as a render, if thicker you can use a formwork. In France people often use mixtures of hydrated lime with hy-

Hemphouse in Kontich, Belgium, before the finishing render

House in Meerdonk, Belgium, a bamboo structure with hemplime fill in.

Chalet in France, at the foot of the Mont Blanc

Grocery Store in Belgium, with an experimental finish of oil instead of render

Fence at the office

draulic lime, but that makes the working site unnecessarily complicated. We have here natural additives, allowing to move up your formwork after one (yes, 1) minute. This way you can work extremely fast (even much faster than a spraying machine) getting perfectly straight walls.

What’s more, the hemp will be one piece, so no cold-bridges (using blocks, every joint is a potential cold-bridge). And if you love to work with your hands, this material invites to mold, to sculpt your house, creating round corners and structures, transforming your dwelling into a hobbit-style house. Either way, your environment will become alkaline, neutralizing the acidity of air pollution. It also largely protects against electromagnetic fields. It will create a healthy environment, a constant humidity and a constant temperature. You will easily save 50 % up to 75 % on heating cost.

Local farmers will be stimulated in growing hemp. Grow your own house.

Grow hemp.