Light clay straw infill, a.k.a. slip straw

The plan for our interior walls is to pack them with straw coated with clay slip which will then be plastered. This method is referred to as light clay straw infill or as slip straw.

The process involves mixing clay slip which is a clay-water solution about the consistency of heavy cream. The clay slip is then used to coat loose straw, kind of like putting salad dressing on salad. The straw doesn’t need to be soaked, just coated with the clay slip.

Packing the clay-straw mix into the walls involves the use of moveable forms. I have some 1/2 inch plywood cut into 24” by 48” pieces. Two of these are screwed onto opposite sides of the wall, creating a cavity into which the clay-straw can be packed. The clay-straw is put between the forms and then tamped firm. We use some 1”x1” boards for tamping.

Once the cavity has been packed to the top of the plywood forms, two more forms are added above the first ones. The new cavity is then packed full of the clay-straw mix in the same way. When it is packed full, the first two forms are removed and placed on the wall above the second set of forms. The process of packing and piggy-backing forms continues until you reach the top of the wall.

The boys and I tried out this method late last summer on one wall in the house, and I was impressed with how well it worked. I learned about the technique by reading about it and watching a couple of videos on YouTube. Using slip straw will allow me to plaster the interior walls like the exterior straw bale walls. It will provide some sound insulation between rooms, and it means I don’t have to use drywall. That, and it is inexpensive, using straw left over from building the exterior walls and clay I dug out of the ground.

Today, the boys and I decided to pack a wall, if we could. We were able to infill two cavities between studs on one wall which is 9.5 feet high.

The photo on the right shows our progress at lunch time. We had finished filling nearly to the top of the wall and had removed the lower forms. Because there wasn’t enough space above, we couldn’t just piggy-back the lower forms above the others. I actually removed all the forms, secured ones near the top of the wall, and then put the others directly below these. Adding the forms below helped to assure that we didn’t push any of the material out of the wall when tamping it firm.

You might notice that there are boards attached to the plywood. I bought 1/2” plywood for the forms, but it is really not thick and rigid enough – it will bow out some when the clay-straw is tamped in firmly. I screwed some 1”x2” boards to them to provide more rigidity. Thicker forms would also work, of course.

One of the challenges is what to do when you reach the top of the wall. The top plate of the wall and the ceiling are in the way for feeding in more clay-straw mix and are in the way for tamping it into place between the forms. The solution that we tried out today was to put a form on one side all the way to the top of the wall while leaving the one on the other side several inches lower. Then, into the space we packed some heavy clay-straw mix – basically a cob. This mix sticks together better in this application and will stay in place better than the light clay straw mix. This solution seemed to work well, as long as there’s no problem with it drying properly.

We learned some lessons from today’s activity. One is to put the straw in to be packed in small quantities rather than a lot at a time. The smaller amount will be more evenly packed within the wall. Larger amounts tend to not get packed all the way and leave voids in the wall. The voids on the surface of the wall are the only ones that are visible, and they will be filled with plaster later on. So, it’s probably not that big of a deal, but it is nice to get the material packed in firmly, though.

Another lesson is to pay particular attention when starting to fill between a new set of forms. For some reason it is at this point that the most visible voids and loose packing seems to occur. It might be that we would start with too much straw, but I’m not sure. I just know that we need to take a little extra time and pay a bit more attention to the filling and packing at this point in the process.

A third lesson is to be careful around electrical wires in the wall. I ran the wires for the outlets in this wall about 12” from the floor. The straw has to be packed on either side of the wire, but it’s quite easy to hit it with the tamping stick. It might have been better to have routed the wire along the bottom of the wall to eliminate this minor hassle. I thought that it would be less of an issue than it was today and that it would also help to hold the clay-straw in the wall.

We are happy with what we accomplished today. It will take it a few days to dry. As long as the weather is not too cold or too humid, it ought to dry well. We’ll be keeping an eye on it.

A bit of wiring

With the interior walls framed, it was time to get started on the electrical wiring in the house. I’ve done some wiring at different times in a few different houses over the years, although I’ve not wired a whole house. Still, like most things, it’s not that complicated if you understand the concept.

A few weeks ago, I made a wiring diagram which shows the location of outlets, switches, and light fixtures. One of the considerations is how to route the wires. In many places, they have to go through or around the timber frame. In other places, like for many light fixtures, there is no space within the ceiling to run the wires. I haven’t tackled the light fixtures yet, but I have a plan for concealing those wires by running them along a floor joist and covering them with some wood trim.

The starting point for wiring was to install electrical boxes. Armed with my wiring diagram, I mounted boxes in their locations within the interior walls – this was easy since these are conventionally framed walls. The straw bale walls, on the other hand, require a different method.

I cut a 2x2 into 9 inch long lengths and tapered one end to a point. After attaching a standard electrical box to the 2x2 stake I made using a couple of drywall screws, I was able to drive it into the wall. I thought I would need to notch out a space for the box to recess it into the wall a little, but it works out that it’s not necessary – it can be driven into the wall far enough without a notch carved into the straw.

In order to run wires between the outlet boxes in the straw bale walls, I use a 1x1 to push the wire 4 to 6 inches into the wall. Between outlets, the wires are running between the first and second courses of bales. For switches, I am still able to push the wires into the straw bales even though they aren’t necessarily between courses.

Once I have the wire ends into the electrical boxes (I leave plenty of wire to work with when putting in switches and outlets later – I’ve worked on homes where the electrician left hardly any wire in the box, and it wasn’t fun), I push the box into the wall leaving about an inch exposed past the surface of the bales so that the outlets and switches should end up flush with the wall surface when the plastering is completed.

I was able to get a fair bit of the wiring completed last week. I wasn’t installing the outlets and switches yet, just putting in boxes and running wires. I have a few more wires to put in the walls. I was also running wires back to the location for the circuit breaker box for the different circuits we’ll have in the house.

Since our electrical system will be solar powered, not grid-tied, I’m not overly concerned with code stipulations about location and number of outlets (although, in most areas, we’re meeting or exceeding code). The wiring will be safe and functional. The wires are properly sized for the loads on each circuit. Actually, with our current system, we are only running an 1,100 watt inverter which is about 10 amps of power (it can surge to about 20 amps). So, other than the convenience of having different sections of the house on different circuits and possibly upgrading at some later date, there is actually little reason for multiple circuits.