Insulating internal walls

While I was browsing the forum, I noticed a posting some time ago asking for information on hemp-lime insulating render.

This might come a little late to respond to the original enquiry, but with the general interest in building and renovation, I wanted to answer some of the questions and tackle some of the misconceptions regarding the use of hemp-lime insulating render in renovations compared with glass wool, stud and plasterboard.

I have a lot of personal experience of both of these solutions, the first being with glass wool and plasterboard, when I started to renovate our own house ten years ago. At that time I had very little experience in renovating cob and stone buildings and since everyone I knew seemed to be using glass wool and plasterboard, I followed suit.

I had my doubts from the beginning as to whether I was doing the right thing and began to research other possibilities.

I’ve been fascinated by bio- sourced materials in building for years and when I came across hemp and lime being used as insulation I knew I’d found what I was looking for.

At first we tried applying the render by hand which certainly taught us a lot about the potential of the material but the drying time was just too long due to the excess of lime and water needed to get it sticky enough to stay on the wall.

Three years ago we invested in a system for spraying the hemp-lime mix directly onto the wall, resulting in a much reduced drying time (can be applied during the winter period) and a superior insulation value thanks to a much lower density (less lime and water).

So why use vapor permeable materials such as hemp and lime in old buildings? The answer is quite simple.Some of the oldest buildings that survive to this day are built from natural materials, wood, stone, clay, lime for example. They have survived because their builders understood the properties of the materials they used and the need to keep them ventilated and allow humidity to pass unimpeded through those materials to avoid rot and decay.

The evidence for what happens when “modern materials” such as cement and non permeable paints are employed in these old buildings is only too apparent. In extreme cases, a failed cement render on cob can allow the wall to become saturated causing it to collapse.

So to get back to the glass wool versus hemp argument here are the pros and cons in a nut shell:

Glasswool (Laine de verre)

Easy and relatively cheap to install. Although more expensive semi-rigid panels need to be used, to prevent the whole lot collapsing and ending up in a pile between the wall and the stud.

A vapor break should also be installed to prevent water vapor entering the insulation.

It’s worth noting here that the theoretical thermal resistance of a given thickness of glasswool (R value) will diminish significantly as the level of humidity in the insulation increases.

Another disadvantage of stud and glasswool is the attractive environment it presents for rodents. We have personal experience of rats coming through from an adjoining building, nesting in the insulation and gnawing through electric cables.

Aside from the purely technical aspects of the material, there are also the considerations of health and the environment.

Formaldehyde has been used as a component of the binders in glasswool for years although the major manufacturers are now trying to play down this rather unsavory aspect of their product.

The formaldehyde will gas off for a period of time in your house after being installed, so it’s not surprising we hear a lot about “sick house syndrome” these days.

If we then look at the environmental impact of the production and disposal of glasswool (high embodied energy and co2 production), it becomes apparent that it may not be the best choice.

Hemp-lime Insulation

More expensive than glasswool and plasterboard

Provides a solid wall, with no voids.

Rounded forms can be created for arches and window openings to retain the character of a building.

The cellular structure of the hemp gives it the ability to stock excess humidity in the building that will then be released through evaporation in dryer periods. This creates a natural warming and cooling cycle.

By reducing the ambient humidity in a room it will require less energy to be heated.

Unlike plasterboard, the hemp-lime allows the walls to retain their thermal mass which has the effect of buffering temperature changes.

Both the hemp and the lime binder help to maintain high air quality in the building.

The hemp also locks up significant amounts of Co2 which justifiably gives it it’s “eco”label.

Thanks to everyone who showed interest in the hemp render blog. I’m pleased to hear that Suzanne has opted for hemp render and has found a team to carry out the work. The idea of using Chanvrebloc for the internal walls is also very good as they provide excellent sound insulation.

I want to answer some of the questions that were raised concerning hemp-lime internal render or “doublage” , so I’ll state with the all important question “how much does it cost”.

It is feasible to apply the stuff by hand, which brings the labour cost to next to nothing providing you have the time and perhaps a bunch of friends that could be bribed with food and drink to help you out. The main disadvantage as I mentioned with manual application is the extra lime and water in the mix to get it to stick, which leads to a longer drying time and a higher density.

The price of a 200L bale of hemp is around 16€ TTC and about the same for 22kg bag of lime. You will need 5 bales of hemp and 15 sacks of lime to make just over 1m3. This gives a figure of 320€ to cover around 12m² of wall at 6cm.

The prices for the materials can differ wildly between suppliers and with careful negotiation you could reduce your materials bill substantially.

Suzanne Fitzgerald has found a company that hires the Euromair system which works very well. This is the machine that we use.

There are also adapted render pumps that can be hired but these machines pump the product “wet” and therefore the mix needs to be much wetter to get it through the delivery hose.

The following prices that we charge for spray application given here are approximate as the location and the size of the job will have an effect on the price.

The materials are usually supplied by the client and the price per m3 is lower, as less lime is used in the mix, (2 bags of lime to 1 bale). This gives a price of around 240€ per m3

The price for the application is 35€ per m² giving a total of 55€ per m² at 6cm.

The sprayed hemp- lime will need a finish coat of lime or earth render which can be carried out by those who would like to put their mark on their own renovation. The whole subject of finishes would fill another blog, so I’ll have to come back to that another time.

Just a last word on permeable paint, as I noticed there was some interest in the subject.

Lime wash or “Badigeon” is cheap and easy to apply, it can be bought in a range of shades, bagged and ready to mix with water. I experimented with my own lime wash with good results.

A 10€ bag of lime, some natural pigments and water will cover a room. You will also need a stabilizer to stop the surface from rubbing off. In my case I used a small amount of casein or milk protein which worked very well.

There are a load of recipes for lime wash on the internet which are worth checking out.

L'isolation thermique écologique - Terre vivante ( An essential book for those planning a renovation)

Thanks Suzanne for replying to my question.

Thanks Suzanne. Will look into it ...

I am never sure where paint Co's get their coverage figures from I usually work on 9-10mtrs/Ltr. Dulux white contains titanium dioxide same as white lines at Wimbledon etc so glows sightly under uv from electric lamps and I found that atracted mosi's etc whereas others use chalk and had noticably less flying insect attraction.

Lime paints are not really meant to cover the same way as an emulsion and that's something you have to get used to or you'll pull your hair out, it's more like french paint

If any of you are within reach of Cadouin, there is a place there where they make the stuff (paint that is) and will do the decorating for you, sell you the paint and mix colours if you are looking for something they don't have. The man who runs the place can also mix and do the plastering work with hemp and lime. The 'shop' has a few samples, many pictures and if anybody wants to take a bite, the café opposite the abbey has a ceiling and beams done in white by them. Several other houses have had work done, a few themselves, one of them has the plaster too. That is why we are impressed by the paint and will use it in a guest room I shall do up over the winter plus one wall needs re-rendering and finishing, so being a tiny room I may have a go at it myself. The finish is superb. I never actually realised that white paint could look so different as well!

I agree John, the price of the paint is eye wateringly scary. Especially as they don't seem to give as much coverage as Dulux which I'm used to. Auro looks like a 10l pot does 10m2 vs 17m2 for Dulux trade and the price per 10l pot seems to be about 2.5-3 x the price depending on where you look. If you come across any cheaper lime paint options (which don't come off on your hand like chalk) then please do let me know

I am quite used to asking for discounts now and will need discounts on the paint for sure! Thank goodness the ceilings are suspended and I can use normal paint on those.

Best price I've found so far for Auro is 44.10 HT per 10l.

Suzanne they should be just as easy to apply as normal paint but usually more coats are required. I am still surprised by the cost of these products as the raw materials are not expensive. It seem anything green has to be re assuringly expensive which is my gripe and one which probably prevents a bigger take up.

I first saw these products used when I did my course at the long barn in the new forest, they had a washroom which was natural stone with an earth bank on the outside. the inside wall was dry to the touch and did not come off on hands or clothes and had a look which was translucent and had a fine tint without losing the look of the wall behind.

Mike - I guess it depends on where the money is spent...besides the time saving it also more efficient in terms of material usage and whiilst complex for the first time it's used (which is why the supplier suggests the training and sending one of their guys for a day) it has these advantages:

• rendement entre 500-750 L/heure (en fonction des particularités du chantier).
• application d'épaisseurs importantes en une passe.
• utilisation de deux fois moins de liant (chaux) par rapport à une application manuelle, il en résulte un pouvoir isolant supérieur de l'enduit et une diminution importante du coût des matériaux.
• nécessite moins d'eau de gâchage, moins d'apport d'humidité dans la bâtiment et séchage plus rapide qu'un enduit appliqué manuellement.
I cannot remember from memory how much the machine hire was (I'm not at home at mo) but it was clear when talking to the supplier that the difference just in terms of cost of labour days by using the machine was highly significant enough to warrant paying for the machine hire.
If you're retired or are able to work from home and can attack it on a manual basis then I am sure the manual way is much more affordable to most, however as our project is large scale and we're working on an economies of scale basis - the machine won.
It's impossible for us to do it ourselves as my hubby works overseas and I have 3 minions to look after so we rely on our trades people to guide us...with lots of help and research from all you lovely SFNers too. I can safely say our project has taken shape in many ways through the help and support of SFN members.

Demo Projet chaux chanvre

above with a machine

below by hand

chaux chanvre a la main

The reason for 3 people is that one operates the machine, one spreads and one finishes. There's quite a lot about it online including videos on youtube in French

Pose Enduit Chaux Chanvre

It's beginning to look as if "isn't expensive" has a different meaning for you than it does for me!
What does that actually mean in Euros?

I believe anyone can do it Mike, its just that for the size of our surface they have recommended hiring the machine. Our builder is a skilled plasterer and can do it by hand but it would take a long time (and as we're paying rent whilst we can't move in - time is impt to us) The training isn't expensive, neither is the machine hire. To give you an idea of scale our room to insulate is 14m long x 9m wide x approx 5m high.

John - I was looking at lime paint. Just wondering how easy they are to apply?

Suzanne, That doesn't make any sense to me.
Traditional building techniques were suited to the needs of peasants who built with semi-skilled labour, using hand tools. Turning it into an industrial process means that it is only accessible to people with the financial resources to pay for it. If ecological building techniques are to have any future, we need simple processes that anyone can do.

Hi Maureen you can hire them. Ecolodeve hire them out but only if you've done the training course. For our renovation the builder is doing training and they are sending one of their guys to help for a couple of days. I believe it needs 3 people to use the machine. Ours is a large surface area and requires two passes of 4cm hemp each - 8 cm in total

Where would one get the machine for putting the chaux chanvre onto the wall please? Does one hire it or purchase it? Complete novice here.

Suzanne, how have you got on with the Stucco or Casein paint, both of those should produce a breathable but non transferable finish, sometimes a bit of olive oil or tallow is used to prevent powdery finishes and better than PVA. You can get a fantastic translucent finish not obtainable with solid paints.

more useful reading


jack in the green lime

That is a very convincing write-up.
I have seen hemp/lime render in a local gîte and know how good it looks. But what I would like to know is how much it costs relative to alternative finishes and how it is applied.
Is this something that can be undertaken as a DIY project?
Can the equipment be hired, or purchased relatively cheaply?
How do you get the stuff to stick in the right place and not where you don't want it?
Can you point me to a site where I can find some practical information, please?