EBC: Infilling walls with cob and light clay straw

The walls of the Earth Berm Cottage are infilled! We used a combination of cob and light clay straw. I wrote a blog post about the thermal and structural properties of cob. In addition I like both infill materials for a variety of reasons including:

1) The aesthetic and feel of earthen plaster which adheres really well to both cob and light clay straw (LCS). There’s something about earthen walls I find soothing.

2) Cob and LCS are non-toxic and easy to work with. No special safety equipment apart from possibly gloves and they don’t off gas.

3) Straw and dirt have a low embodied energy cost. Straw is a waste product and apart from transportation and creation at the dawn of the universe dirt has a negligible processing cost.

4) Cob and LCS are vapor permeable meaning if moisture gets in the wall it can escape mitigating mold issues.

5) Both cob and LCS are easily moldable. We made put in big rounded corners around the windows of the cottage. Here’s a post about making curved walls with light clay straw.


I used a concrete mixer to make cob. For each batch, I would add some water in the chamber and then 7-10 shovelfuls of dirt. Then I would add a few handfuls of straw followed by another 7-10 shovelfuls of dirt. I aim for about 90% dirt and 10% straw. For dirt I’m used sandy loam from a landscaping yard. Cob only needs a little clay (10-20%) and a lot of sand (80-90%). Since our subsoil is predominantly clay I had to outsource.

A good cob mix should roll like along the side of the chamber and have a sheen kind of like worn leather.

I used strips of 1/2” plywood fastened to the studs as temporary forms to contain the cob. Two feet tall pieces seemed to work pretty well. With the first course it’s a bit of a reach to get to the bottom of the wall cavity but you can lap subsequent sources over the previous course. Here are some other notes regarding forms:

1) 1/2” plywood seems like the minimum thickness for the forms. 3/8” plywood will bow out in the middle and you’ll have a kind of undulating wall. If you’re studs are spaced wider than 24 inches I would go with 3/4” plywood

2) Attach the forms to each stud with screws spaced 12 inches apart. If you only screw in the top and the bottom of a 2 foot tall form it will bulge in the middle.

3) Coating the forms with vegetable oil did seem to help make form removal cleaner but I wouldn’t say it’s essential.

To pack the wall I would drop 3 or 4 big handfuls of cob into the wall cavity and then smash them down with my hands working the cob around the 1×2 key and any fixtures in the wall. The keys are critical to keeping the cob from falling out of the wall. I attach them towards the middle of the stud bay.

Typically, after packing a section of wall I would leave the forms on for a day before removing them but occasionally I would remove a form an hour or so after packing and I didn’t have any issues with slumping. It was hot and dry here so the cob seemed to dry really fast. However, I don’t think I would ever try to go up more than 2 feet every couple of days.

Light clay Straw

Light clay straw is straw coated in clay slurry or slip. We harvested the clay for our slip from one of the many piles created by the natural swimming pool excavation.

Slip should be roughly 1 part clay and 4 parts water. I typically look for the consistency of slightly thicker than chocolate milk or a melted milkshake. If you end up with too much clay you can cut it with water after you’ve used some.

Light clay straw is significantly more insulative than cob. We packed it in the front wall of the cottage and in the upper sections of the walls on the bermed sides as the berm will only come up about six feet on outside of the wall. The install is similar to cob using plywood as temporary forms. However, it is much faster than cob as you can remove the forms almost immediately after packing a section.

Packing the Wall

We held three work parties to knock out the bulk of the work. Cobbing and packing light clay straw both lend themselves to communal work since the techniques are easy to teach, don’t require any protective equipment or loud machinery and really kind of boring to do by yourself. They’re also labor intensive. I estimate infilling the walls took around 80 hours. It took me about two days to build the tumbler and then I probably had a day of various prep work and picking materials.

Ultimately, we used 5 yards of sandy loam soil and 16 bales of straw (~2 for cob and 14 for light clay straw). I estimate that it takes about 8 person hours to mix and pack 1 yard of dirt and about 2.5 person hours to mix and pack 1 bale of straw. A lot is dependent on much packing has to be done on ladders. Packing the little gable ends probably took me as many hours as it did to pack the first three feet of the lower wall.