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Integrated Straw Insulation for Cob House  RSS feed

 
Matthew Jackson
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So this is my first post here at Permies. I'm building a cob house in Northern VT and I'm looking to boost my insulation (and reduce the amount of cob I have to build!) by adding some straw. The foundation and first lift is already finished, and I'm considering either staggering flakes of straw straight from the bale in between lifts or using sand-bag bags stuffed full with straw in a similar fashion. I was wondering if anybody has tried either of these, knew of any problems with either, or had any other suggestions.
Thanks!
 
Jami McBride
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Location: PNW Oregon
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I haven't tried it, but if I was going to I would run something (sticks maybe) through it, tying it into the surrounding cob.

As for 'filler' remember in the old Tudor houses (cob + cow manure, with lots of rocks, sticks and other land debris thrown in as filler). They were thinking just like you...

So maybe don't work so hard, filling bags with straw and such, instead fill the cob with straw, sticks and other stuff willy nilly not clumped up together, which will help keep it uniformed.
The Maure made their cob stickier which helps hold all that stuff in place and keeps the cob from pulling away. So consider adding some manure or poo-straw to help with your fillers.

Edit: Oh yea, and WELCOME To Permies, we'd love to see pictures of your build ♥
 
Matt River
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If you google supercob some pictures of straw lifts incorporated into walls will come up. I have not used this technique myself, but straw layers, if pinned vertically, might have substantial benefits in decreasing the total weight of the wall both for loading purposes and ease of construction. The cob will also be more permeable and will dry faster in such a system. What I am unaware of is if the walls are finished with a stress skin of some type, or if the system was being used with post and beam or columns etc.

Another system that has run through my mind is simply to stomp extra straw in the center of the walls, letting the regular cob mixture encapsulate around the straw core. the straw would function strongly as a cross-tie between the two more dense outer walls, but each lift of cob would overlay and reconnect the structure to maintain its action as a monolithic unit.

generally, cob walls are so thick and insulating that they will perform great using standard mixes. another option is to do light-straw clay packed with temp forms on the inside of your cob walls. this is a great way to square up the inside of a home for, say, cabinets and dressers etc. the disadvantage? you are losing your thermal mass and creating vapor/water permeation issues that do not apply to a more conventional type of contiguous wall. experimentation can yield great results for many people, but can also mean a lot of headache for the originators. a moveable slipform could also be used to create freestanding light-straw clay shapes that are then covered with traditional cob outside and in, or by a thinner portland type stress skin.

something that I am building right now is a thermal mass masonry fireplace that will vent through a serpentine path inside my cob wall - the wall will become a radiator. with proper burn chamber design and firebuilding, masonry stoves have been built that approach 95% efficiency, with many of them in the 70% range. another way to capture heat is to use a passive (not connected to the burn chamber) ducting system that flows through natural convection - with a small fan or some planning, you could use drain tiles or mason blocks under your floor to store heat in a very large mass. a good steel or cast stove with encapsulated or surrounded by mass with ducts inside the mass will provide passive or active heat depending on the design parameters.
 
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