Advice re: rigidity of bale walls before plastering (time-sensitive)

Hi, everyone.

I'm in the middle of a straw bale building project and I'm looking for some guidance.

Some background:
I live in an intentional community where a fellow member and I are putting up a bale wall in the earthship we live in. Previously, three exterior walls of the earthship duplex were built with rammed-earth tires and were set into the hillside. The fourth wall is south-facing and is comprised mostly of windows set into timber framing. The roof is wood with insulation and a rubber vapor barrier on top. Hopefully, it will be a green roof soon, but right now it's half-covered with dirt. Six years after it was built, the time has come to replace one of the outer walls because of some seriously concerning leaning shapes that the tires were making. Essentially, it was no longer a straight up and down wall. The main theory is that this problem arose because that wall was not actually in contact with the hill and instead, dirt and rocks had been piled up against it. Considering that there were plenty of holes in the wall allowing air to get in throughout the winter, it seems clear that the dirt that was there was not enough. We think that may have something to do with why it did not hold its shape. The tire wall was load-bearing, so before we took it down, we put in temporary and then permanent posts to support the roof.

And now to my point (I'll get there eventually):
We have 35 feet of wall to build and we are stacking two-string bales against the outside of four or five load-bearing posts. We are sticking two 4' rebars through the bales joining each one to the ones above and below it. Each rebar goes completely through one bale and sticks a little over 1' into the bales below and above. So if you can picture it, some bales have two rebars going all the way through them and others have four rebar ends sticking into them. We are fastening the bales to the load-bearing posts with parachute cord because baling twine was only available in huge quantities. As each course of bales goes up, we are attaching the bale adjacent to the post by wrapping three sides of the bale in the cord, pinning it below and above the bale with bent nails in the post, pulling it tight against the post and tying it.

We are now on the fourth course of bales on the side that is closest to the front wall and it gets shorter as it progresses toward the back wall. We haven't completely finished laying the first course of bales. The question that has come up is about the rigidity of the wall pre-plastering. It seems to be able to hold itself up fine and we're not at all worried about it falling down. However, it does seem to have quite a lot of give when pushed on and it is prone to sway if any pressure is put on it. I imagine that is to be expected at this point, since it is still under construction and it is not anchored at the top yet. Still, I am wondering what we should be looking for when we're done and ready to plaster. When pushing on a bale in the center (not next to a post or other attachment point), is it okay for the bales surrounding it to move slightly, or should it feel like pushing on a concrete wall that happens to be covered in a layer of straw bales? I haven't been able to locate anything in the books we have about assessing the rigidity of the bales before plastering so I assume it's not a common pitfall. Will plastering add much rigidity?

Any pointers would be helpful. Thanks!

Plaster doesn't add rigidity. It's a surface treatment, and if it is put on a backing with a lot of give, it will end up cracking. You want it to

feel like pushing on a concrete wall that happens to be covered in a layer of straw bales?

It sounds to me like what is missing in your wall is some type of sheathing -- something planar and rigid that will tie all the vertical members together so that they cannot move independently. You may want to add a network of laths or "wattles" before you put your "daub" (the plaster) on.

The bales can be made more rigid by loading them before plastering. There are a few methods for doing this. The only one I have actually used was placing a box beam on top and loading it using all-thread along either side of the wall and anchored to the foundation. I've seen diagrams using cables run through the foundation and over the top of timbers on the wall (but have never seen this in practice).

Basically, if you can compress the wall, it will get a lot tighter.

I've seen the beam box and all thread used with a dead man placed on top of lower course of bales if the all thread wasn't planned for when the foundaiton was laid.
I've seen metal banding used as well I'm sure poly banding would work as well but might not get as tight.
Is there a mechnical connection at the foundation?
Has the backfill, hill movement been addressed?

Thanks! In our research, we didn't come across much about compressing the bales in non-load-bearing structures.
I think we're going to build it up a bit more and if there's still a lot of give, we'll think about compressing. I'll report back soon.

Julia Rose wrote:Thanks! In our research, we didn't come across much about compressing the bales in non-load-bearing structures.
I think we're going to build it up a bit more and if there's still a lot of give, we'll think about compressing. I'll report back soon.

If you could include a few pictures of your earthship and new strawbale work, I bet many on here would enjoy them. I know I would.

Julia,

Not sure exactly what your framing (post layout) looks like. If you have 5 posts then you have pretty big stretches. Most designs for non-load baring seem to avoid your problem be having smaller spaces between the posts.

That said, add ing what basically amounts to a pressure plate can help considerably. This is just a piece of dimensional lumber nailed horizontally across the top of a full course of bales (often at the 4th course). Then apply some down pressure and toenail it in from the top. You can add more pressure by hammering the ends down a bit lower once you have purchase in your posts.

You might run into some bowing because of you long stretches, but would be rectified by placing the lumber on edge. Just make sure it is placed in a place where it is easy to notch the next course.

If you are sewing 1x2 or some sort of metal netting (stucco or welded wire mesh) then you really shouldn't have anything to worry about it while you stack as long as the last course is firmly squeezed between your top plate/box beam.

Hope that helps,

jc

Julia,

I am getting into this post late and I hope you have found a reasonable solution. If not consider the following.

Even if you are not doing load bearing SB the pre-compression of the bales is important. Creep is a long term movement that occurs in the wall as it settles. This process is due to the high weight of the plaster that is applied to the light weight and flexible bale stack. Over time the plaster will settle down and crack, if the wall is out of plumb then that condition of creep and cracking will be exacerbated. I am only indicating this as it would solve your first problem of a wall that is not stable/flexing under lateral pressure. As you have begun stacking already (as of your post) the foundation assembly is complete so the possibility of all thread is not possible. Robert Ray's suggestion of poly for the compression is spot on. We used packing strap with 1100 pound tension ratings spaced between 20 and 24 inches wrapped from sill to box beam at top of wall (alternate cinch side inside to outside every other strap to prevent wall from buckling during tensioning). In your sloped condition you might us rough sawn laid on flat (3x12) as your top compression plate as you already have a structure above and would like to minimize gap at top of wall/underside of roof structure.

My next concern is the bale wall sloping to the back. This indicates that the bale wall moves further back into the hill side where the wall is lower(typical earthship design). Have you got air circulation capacity around the wall on both the inside and out? The bales and adobe plasters work wonderfully together based on their individual material properties. Problems occur when you change the moisture transport mechanisms. If you shut off the evaporation potential from one side of the wall vapor transport has to move through the entire wall instead of just simple surface absorption and evaporation. This forces substantially more moisture into the wall, specifically the bales. Worse than this condition is if you have earth (the hillside) in contact with the exterior of wall. You will conduct liquid water in the soils into you bale walls and have fairly rapid decomp of the bales due to high moisture content. The tire walls work based upon mass (nearly 300# per tire) to resist lateral forces and support load (roof) above. If the hill is not stable then further erosion of the hill will create the same 'bulge' you described before only it will occur much faster this time as the bales resist lateral pressure even less than the tires.

I hope you have a successful project and we would love to see some pics.

Thanks for all the advice, everyone! Construction has slowed to a halt for the moment, since our lives have gotten much busier and it's a challenge finding time to finish the wall. We're thinking we may put up some house wrap and keep the tarps in place in the meantime. I think we are indeed going to use poly strapping. In fact, we were discussing poly strapping from the beginning, but we thought it would add unnecessary costs to the project. Now it looks like we may have access to the tools we would need, free of charge.

Christopher - thanks for your input! I think I understand what you are saying about air circulation and vapor permeability, but I don't think that will be a problem in this case because there is a wooden door leading to a wood shed between the bale wall and the hill. The wall won't be in contact with the earth at all. The pictures will probably make this clearer.

I've attached some pictures and I'll hopefully have more to share when the wall is done!

Julia,

Thanks for the pics. Good to hear that you have no earth contact. I understand the slow down on projects b/c of life. Keep up the natural work and Keep us posted.