How I used lime and hemp to plaster inside the tiny house

In this blog post I describe how I plastered the tiny house walls with lime and hemp and then limewashed them.

Mistakes were made, as they say, and I’ll own up to mine during this phase of the build!

*Note: I use the term ‘plastering’ in this article to mean the application of a finish coat of lime and hemp. I didn’t use any gypsum plaster.*

The basic idea

The basic idea was … to coat the inside of my cast hempcrete walls with some more lime and hemp. It needed be fairly smooth as it would become the finished interior surface. When the lime and hemp had dried, it would be limewashed i.e. painted with watered down lime. The resulting wall should have a soft, matt white finish.

I didn’t want this lime/hemp layer to be too thick. In fact, the thinner the better. The reason being was that I didn’t want to surface of the wall to protrude too much from the electrical socket and light switch boxes. Luckily, I wasn’t too bothered about the flatness of the walls, so I could taper the finish coat in towards the electrical boxes if need be. I was aiming for a thickness of 1 or 2 cm.

You might wonder “why plaster with lime and hemp, instead of normal plaster?” There are various reasons for this. Firstly, I had lots of leftover lime and hemp that could be used. Secondly, I quite liked the texture of lime and hemp when smoothed out. Thirdly, normal gypsum plaster is not as breathable as lime and might compromise the performance of the wall. Lastly, limewash is not recommended for use on gypsum plaster.

I could have plastered with lime and sand but that wouldn’t have given the look that I wanted and besides, I still had a big pile of hemp that I wanted to use up! The fibrous nature of lime and hemp plaster is also said to make it more resistant to cracking. In this instance, it didn’t work out quite like that, as we will see later.

Previous experience

I had, some years previously, used lime/hemp to apply a thick insulating layer over a stone wall. I had also used it over brick both during a training course and on a real building site.

At that time, I used a lime called Tradical combined with course grade hemp. While there was certainly a knack to applying it, it was possible to get quite a nice, smooth finish (with the odd hemp fibre sticking out). It couldn’t be trowelled flat like gypsum plaster but that didn’t really matter. The image above (from the Tradical website) gives some idea what it looked like after application.

Getting it on the walls

OK, getting back up to the tiny house, I had just over a week to do all of the plastering. It was during the October school holidays and the completed hempcrete walls were completely dry (they had been done in August).

One complication, as I mentioned in a previous post, was that I hadn’t had time to cast hempcrete around the gable areas of the walls. So these ’triangular bits’ had to be filled in somehow before the lime/hemp plaster could be applied.

I solved this problem by using a lath and plaster method. Strips of scrap wood were fixed horizontally to the studs and hempcrete was forced in between. The finished hempcrete didn’t have the normal thickness but it was the best I could do, given the limited time I had.

With the gables filled in, it was nearly time to start applying the lime and hemp plaster. Before doing so, I dampened the walls with water from a garden sprayer. This was to avoid the dry walls sucking moisture from the new plaster too quickly.

I used the mix ratio suggested by St. Astier. This worked out as 1/2 sack of lime : 25 Litres of hemp : 15 litres of water.

The resulting mix was fairly sticky i.e. it adhered well enough to the wall with no problems of slumping or falling off.

It was, however, not a great deal of fun to apply. It just didn’t flow very well and it needed a fair bit of muscle to get the walls covered. The fact that I was using course hemp for a thin coat probably didn’t help.

Not being an avid fan of cement grey, I can’t say that the walls looked great at this stage. But at least the patchwork of cast hempcrete began to disappear under something more homogenous. I’m fairly sure I only applied one coat, but it was nearly 2 years ago, so my memory may be playing tricks on me.

In terms of tools, I used a normal plasterer’s finishing trowel (stainless steel) to apply the lime/hemp plaster. It’s also good to have a variety of plastic floats in order to help smooth the plaster as it dries. I suggest one large and one small rectangular float as well as one with a pointed shape These can be used in a circular motion on the wall with progressively more force as the plaster becomes firmer. Small amounts of lime/hemp, placed of a float, can be rubbed onto the wall to fill in holes or low spots. The pointed float is handy for getting into corners and for ‘slicing off’ high spots of material.

The photo below shows the trowel and floats that I used. They are resting on my home made ‘hop up’ which I used to reach the tops of the walls. Making one of these is highly recommended as it can be exactly the height required and is much more convenient than a stepladder. I made mine from leftover timber and OSB shuttering. A smaller hop up can be seen to the left.

I got the plastering finished within the week and had to head home to Scotland. Ideally, I would have been able to leave the windows open to make sure the house was properly ventilated. Instead, I had to shut all the windows and shutters and leave the house until the following summer.

When I returned, the plaster had dried but it was obvious that the house had been quite damp inside due to the water released from the drying plaster. Some clothes that I had hung up were mouldy. I had hoped that the breathing construction of the building might have allowed enough water vapour to pass through. Clearly this had not been the case.

Limewash

By this stage it had been 12 months since the walls were built and 8 months since they were plastered. My first job was to limewash the walls before I could get on with anything else.

To make the limewash I mixed lime putty with water, using a paint whisk with a cordless drill. Lime putty is made from very pure lime that sets in contact with the air. As such, it comes in a tub and is covered with water. I got my lime putty in France from a supplier of eco building materials. You can get it from specialists in the UK too, such as Mike Wye.

Limewash is mixed to a thin consistency. One French artisan on-line suggested it should be like “waffle batter”. More helpfully, he said the ratio is 1 part lime putty to 2 parts water. I just mixed it using my judgement as to what seemed ok.

Then I applied it to the hemp plaster using a wide brush. Being so thin, the limewash was quite ‘splashy’. When it goes on at first it is transparent but gradually becomes white as it dries. Several thin coats are required and it’s recommended to leave 24 hours between coats. I found an excellent guide to using limewash on this UK site.

In fact it took 4 coats to get the walls looking white (or white enough). During this time, I ran out of lime putty. The supplier was a long drive away and I didn’t want to waste precious days waiting on a delivery, so I switched to using powdered lime instead, which I could get locally.

The lime I used was called in French “chaux aérienne” (air lime). In the UK, it’s called non-hydraulic lime. The sacks are commonly marked CL90. It’s the same stuff as the putty, but a bit less convenient to use. The mix ratio is similar. One online source suggests a mix of 1 part powdered lime to 2.5 parts water.

Results

I was happy with the finished walls. I wasn’t looking for perfection (and certainly didn’t get it!) but the result was in keeping with the rustic vibe of the place. The walls were not so wonky that I couldn’t hang a shelf or a cabinet. There were smooth bits and less smooth bits and the whitewashed finish came out just as I had hoped.

The only problem was the appearance of several cracks in the days following the application of the whitewash. Some were tiny and some were larger, perhaps 5mm wide. They went deeper than just the whitewash.

I was puzzled as to what had caused them to appear. Perhaps it was because the house was now exposed to the hot summer air flowing through it and it had caused a final drying (and shrinkage?) of the lime/hemp plaster.

The cracks were more annoying than worrying. The timber frame was doing the structural work and I’m sure the cracks did not go all the way through the cast hempcrete walls. In the end, I filled most of the cracks in with decorator’s filler, which seemed to do the trick.

Lessons learned

If had to do it again I would use different types of lime and hemp for the plaster coat.

Regarding the lime, I would tend towards a weakly hydraulic lime (NHL 2). St Astier do one called Téréchaux, which they say can be used with hemp for finish coats. This should be relatively pleasant to apply and possibly easier to smooth out. It would also look nicer and, being lighter in colour, cover better with limewash.

The lime which I did use (Batichanvre) was really more suited to cast hempcrete and thicker insulating layers. A plaster layer is more decorative and doesn’t need the same mechanical strength.

As for the hemp, I would use the fine grade for a hemp plaster top coat . I’m sure this would make it much easier to get a nice finish based on a thickness of 1-2cm.

As it was, I had loads of ‘less than ideal’ lime and hemp left over, so I used that and made it work. The final result was ok but getting there was harder than it needed to be.