I am in the middle of building a greenhouse. The foundation consists of 3 rows of rammed earth tires. I had set roundwood posts and beams as well as milled rafters and ceiling boards. I was planning to finish the roof and then build the North, East, and West walls up to it out of cob. Irene came through and two large trees fell on it so I'm back to just the rammed tire foundation. Now I'm planning to use the downed trees to build out of cordwood.
So, my question (finally!) is - if I use cob as the mortar, what are my options for insulation? I'm leaning toward cobwood as opposed to cordwood b/c I have a lot of clay and straw and a fair amount of sand already. I would prefer to avoid lime, mainly b/c I've never used it and don't know much about it except that I'd need to wear gloves.
Could I do something like slipstraw mixed w/ borax as an insulation layer? To be clear, I'm thinking of the Rob Roy mortar-insulation-mortar method of masonry. Do I even need an insulation layer if I go heavy on the straw in the cob? Would that sacrifice the strength of the mortar too much? What about straight cob mortar w/ no insulation and either cement or lime mixed with the cob for stability?
Thanks in advance to anyone who has experience with this!
Papercrete is insulative and cheap and DIY if you have the means to make a tow mixer. If you're interested in that, there's a papercreters group on Yahoo groups that is easily the go to place for anything papercrete.
You really don't need the lime inside the cob mix.. In fact I'd strongly urge you NOT to do that! Keep your cob just cob and use a nice lime plaster if y'er worried about the weather.
You'll want a knee high "foundation" of sorts, something that can handle roof drip splash, diagonal rain and whatnot.. Perhaps a nice lime mortared stone or urbanite (reused sidewalk etc.) stacked job. I'd build a short cob sill on top of that, say a foot of just cob, then begin the cord-wood. Finish with another foot of solid cob, they'll act like bond beams.. Top off with a roof featuring wide, protective eves and away you go.
Where you will have long term trouble is if you leave the end grain of the cob wood visible. I know it looks good in the pictures and all, but leaving the wood/mortar (cob in this case) junction open to the elements (and the critters) is ASKING for issues. Cover it WELL! This is where the plaster work comes in. I always begin with an earth based plaster of clay slip (local clay rich soil, soaked in water, mixed and screened) and chopped straw. I make it good and thick for riding over humps and filling in the lows, put it on pretty thick and even sculpt a little with it.. After that basic coat you got options, typically earthen or lime or a mix of both, depending on what you and the building/site requires.
If you DO plan to insulate, remember to keep the thermal mass INSIDE the insulation, with you... Seems obvious (and forgive me if yer already there) but you'd be surprised at how many people don't get this one. Let me repeat and rephrase; the insulation should go OUTSIDE of the thermal mass, NOT the other way around..
A good friend of mine has used lambs wool mixed with lime as insulation.. He stuck it on the outside of a cob building in Minnesota where it gets 40 below, lime plastered over it and it works like a champ!