Clay, Sand, Straw and Water...

These are the basic ingredients to the base coat of render that we are using to protect the straw bales.

All the straw bale walls are all up (with only a few minor parts to fix) and we are rendering. We have started with the outside first to help protect the bales from the elements. The roof overhang does do a pretty good job of keeping the water off the walls but no amount of overhang will stop the almost horizontal rain we sometimes get throughout the year.

First coat of render on the bathroom wall, the top part is cob which fills the gap between the top of the wall and ceiling

The amount of work to render the house:

• we have about 50m of straw wall that is on average 2.4m high
• which is 120m² (subtract 20m² for windows and doors, leaves us with 100m²)
• 100m² x 2 sides (inside and out) is 200m² of 50mm render
• or 10m³ of mud to mix and apply

This is not an insignificant amount. We are mixing and applying the render manually.

We have decided to go with a natural mud render rather than a cement render for a variety of reasons. The recipe we have chosen at least for the base coat is as above. So far we have been mixing up the render in a wheel barrow using a shovel, the wheel barrow makes it easy to get the render to the part of the wall we are working on. It is a lot of work mixing the render in the barrow though. The steps to mixing are:

1. Put four shovels of sand in the barrow
2. Add four shovels of clay (generally we get more clay in each shovel over the sand, I would estimate the ratio to be 3 part clay to 2 parts sand)
3. Mix the dry clay and sand so they are basically combined, chopping up the clumps of clay
4. Add a few hand fulls of straw (so far we have been using up the stuff that has fluffed out of the bails in the wall whilst putting them up)
5. Mix straw in
6. Add water and mix in
7. Keep mixing until the render changes colour slightly and is very sticky.

Preparing a load of mud render

The recipe does not have to be overly precise and during the render process we have tweaked it slightly, for instance we have lowered the sand content and added the straw. We did get hail on one wall which has made us realise we will need to add lime or some other weather proofing agent to the final coat. I have been using a short spade to "chop" the straw into the mud mix. The water content also plays a part in the ease of which the mixing is, a wetter mix for instance is much easier to mix and apply but it may also make it more susceptible to cracking during drying.

Wetting down the bales about to be rendered

Applying the render is pretty simple. First the bales that are going to be rendered are wet down slightly, this stops the straw from drying out the render too quickly and is supposed to help with adhesion. We used a garden spray pack and also wet the surrounding timer that the render was going to attach to. We then make balls about 15-20cm in diameter and squish them onto the straw. The mud is then worked in so that the mud penetrates the bail. We have noticed that the fluffy side of the bail it the hardest to work in but we get a much better mud penetration. We are leaving the first coat rough to hopefully help the second coat stick better

Working in the render

We have also been using the mud render mix to make cobs to fill in the gap between the top of the wall and the roof. We have been really happy with the result. The mud mix does have to be on the dryer side otherwise we have found that we get some slump which leaves a gap at the top (it is pretty easy to fix with a little bit of extra mud however). We also added a strip of wood (1/3 of a bit of 4x2) to the ceiling to key the mud to. A key was also added to above the doors and windows.

It is really exciting now seeing how the walls are going to look. We have been really happy with the colour and how it is all coming out. The rendered curved walls look fantastic. Before the final render coat is applied we need to finish off around the window and doors jams (we are thinking of using several planks of cypress glued and biscuit joined to fill the 550mm width).

Left side un-rendered straw wall, the right wall has the first coat applied

The rendering process is pretty slow and involves a lot of manual labour (mixing is a complete upper body workout) and there is soooo much to do, so a word of warning to any friends and family coming to visit over the next few months, wear old clothes ;)