I have the bare bones of an idea for an insulated wall system that I'd like to propose here. Feedback is most welcome. I'm not at the point where I'm likely to be trying this anytime soon, but if I'm lucky, others may like the idea enough to try it out in the mean time.
The seed of this idea is in the style of insulated cord-wood masonry wall depicted in this video.
Here the cord-wood pieces are cut long enough to allow mortaring at both ends of the wood, while leaving a gap in the middle to take some sort of insulation. You can think of this sort of wall as a double wall system: two simple cord-wood walls with an insulated gap between them, where every piece of cord-wood is also a tie between the inner and outer walls. I like this design a lot, but it requires a lot of wood. There's also a lot of thermal bridging in the wall. A variation on this design has only a portion of the cord-wood going straight through from outer to inner wall to tie them together, thus reducing the thermal bridging, and to some extent, the quantity of wood required.
At some point, it occurred to me that the construction on either side of the insulated gap does not have to be the same. In fact, though one side must be load-bearing, the other can be much lighter-duty. It really just has to be strong enough to support itself, keep the insulation in place and cope with whatever activities are going on inside the structure. It also occurred to me that a lighter inner wall would not require as much support, so the ties could be smaller, further reducing the thermal bridging.
So here's the essential idea. You build an outer load-bearing wall using some well established method such as cord-wood, cob or earth bag construction, but embedding suitable ties, e.g. cleft hazel as used in hurdle-making, at regular intervals -- say every 2-3 feet. The ties point inward, and at the distance where the inner wall is to be built, they are bent/twisted up or down at a 90 degree angle. The inner wall uses wattle and daub construction and the vertical ends of the hazel sticks coming from the outer wall are woven into the wattle as it is being built. Once the daub has been applied, insulation is stuffed or poured into the space between the walls. The inner wall could be finished with a natural plaster as you might a cob or straw bale wall.
It's a simple idea, and I'm surprised at not being able to find it out there anywhere. Does anybody see any problems making this work? There are refinements needed to address things like plumbing and wiring, hanging kitchen cupboards and accommodating doors and windows. More thinking is needed about how to anchor the hazel ties into the various types of load bearing walls one might use. And the points where this wall meets interior walls, the roof and the foundation probably deserve a close look too.
Does anybody see themselves trying this out? If so, I'd like to hear about your attempts.
Well, I am thinking about experimenting with this idea. I had nearly the same idea while coming up with a method of using earth to finish my pole barn interior walls. I stumbled upon this youtube video. I think I will experiment with this on one section of wall to see how it performs. Basically, my load bearing structure is the posts and stringers. Exterior steel panels give the building a monolithic envelope of rigidity. I can simply take some 1" x 2" slats and nail them across the posts. And build the walls up like this guy did. I will put a moisture barrier at the base, and weep holes to allow moisture to escape. Also I will line the air gap between the earth wall and the steel with straw for insulation.
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Permaculture Technology Jamboree: June 29th-July 10th, 2020, Wheaton Labs