Hello all! I've been reading and learning from the experience of folks on this forum for a long time, and I'm finally ready to jump in with my first building project. We're going to build a 10x15 chicken coop for my folks, not because this will be the most efficient/inexpensive method of doing so, but as a test structure to learn for future projects on a larger scale. I've been kicking around the different alternative building techniques, and I've come down to one I think I may want to try that would involve poles harvested from the land, cob, and straw, and utilizing a rubble trench/gravelbag foundation. Here's the basics of how it would work:
1 - First I would erect a pole frame for the structure using poles harvested from the property.
2 - Outside of the pole framed wall, I would build a cob shell that would "cup" the outside of he poles. External surface of the cob would likely be covered in lime plaster.
3 - I would then line the inside of the walls (between the pole and the full depth of the poles) with straw, which would then be covered with earthen/lime plaster to seal and prevent fire/insect issues.
4 - Inside walls may be covered entirely with a plaster (including face of poles) or covered with wood slats attached to the exposed face of the poles (once again, slats harvested from land).
5 - Finally, I would plan to shingle the roof with hand split shakes (you guessed it, once again harvested from the land).
I thought this might be a good combination of building techniques to take advantage (in varying degrees) of the thermal mass of cob, the insulation properties of straw, and the beauty of wood all in one structure. What I would like to hear from you fine folks is if you see any inherent issues in this sort of design. Any feedback, input, criticism, etc., is welcome!
I'm neither a strawbale, nor a cob specialist, but if you want to avoid to have your dew point in the middle of your wall, or even on the inside of it, you should reverse your layers. This means you put the insulation (straw) on the outside (don't forget some weather protection) and the cob inside. I found suggestions for straw bale/cob hybrid buildings, to use cob for inner walls and straw bale for the outer walls. But a chicken coop probably does not have inner walls. On the other hand, if you use wood for the structure, you probably don't need any cob at all, except perhaps for plaster.
I'm not an expert either, (although I do live in a passive solar house so have researched the subject), but my first thought on reading was also that the insulation should be on the outside in order to get the benefit of the thermal mass, ie stable temperature, inside the structure.
I agree with the above posters about putting your insulating layer on the outside of the structure. If you look at the REMOTE Manual (the concept isn't exactly permaculture-friendly, but I think it's a far sight better than current building practices), it explains the "why" of insulating the exterior.
Thanks to all for the input. This has been helpful! This is one of those projects that is continually evolving. I'm not certain how it will turn out, but your input is helping me shape how/what I'll be doing. In any case, I expect it will be a lot of fun, and likely more work than I expect!
I think your best bet would be to first lay down a rock foundation, both cob and straw bale will need to be raised from the ground in order to work properly. As others have mentioned, insulation on the outside is best. Since this structure is a chicken coop there are certain tings that will need to be addressed just because of the occupants of the structure such as 1. They love to scratch and peck, the interior wall will need to be able to withstand these two habits. 2. The floor will need to be able to keep out predators, the rock foundation would be a start there. 3. you will need nest boxes of some sort, unless you aren't planning to go that route. 4. The roost area either will be attached to the framework or free standing, that also would need to be decided before construction I think.
When planting the poles are you going to simply ram earth to stabilize them or are you going to use some other material such as concrete? What method of moisture barrier are you going to use to protect the in ground portion of the pole for longevity is also something to think about. Do you plan to have an above grade (raised) floor?
There is also the option of building a double wall with insulation between the two.
I live in Arkansas, and the coop we built is located in a wooded area, so it is mostly shaded in the summer. We have it raised floor on rock foundation with foam insulation and the ground side is decked as well to keep the local predators from getting in, should they infiltrate the fence line ( buried 1.5 feet). It has walls built using pallets and a wattle and daub exterior, wood slat interior with foam insulation between. The nest boxes are fastened to these walls and two high. The Roost is Fastened to the walls at the far end of the coop with a slide tray for cleaning, there is also an automatic water set up using an old toilet tank and schedule 40 pvc pipe and nipples ( this set up also has a line to the outside so it provides all their water needs. I did this setup so we don't have to change water, this system is hooked up to a gravity feed from a rainwater collection set up just for the coop and the tank still has the float in it ( took this idea from MEN). Their permanent pen is aprox. 1/8 acre and they can get out to free range anytime they want to. Hawks and other flying predators are discouraged because of the location of their coop and this permanent pen. The windows are high and double screened and provide air circulation. We designed this "simple" structure over a two month period, drawing and redrawing until my wife was satisfied, then we started the build. The Galvalume roof is also insulated with foam and covered with thin ply wood on the inside just incase. We also have an area fenced off inside for storage of feed stuffs and spares.