Cob: cob vs strawbale

On Tue, 29 Aug 2000 08:37:38   Russel Johnsen wrote:
>Yes, I am going to have to go with strawbale.  Everything
> I have seen points to this as best for our climate here in >Winnipeg.  
[snip]
>This is what I will have to do -- with the addition of
>cob inside.

Do you mean cob just for the interior walls? Or cob exterior walls with strawbale insulation wrapped around the outside? 

If you want the latter (for the extra creativity/thermal mass/work) there could be a legitimate concern during the extended drying-in phase. (there are a LOT of well-meaning strawbale people who would disagree- and could present reasonable historical-fact-based arguments to support their contention.) The cob wall, which can continue "curing" for a year and more, could potentially contribute enough moisture to the bales to begin rotting them if the entire assembly - that is, cob/bales/plaster - is made all at once (assuming that the cob is of typical cob proportions and methods, rather than being an extra-thick earthen plaster).

Straw-clay (a.k.a. light clay, a traditional German timber-frame infill material called Leichtlehm, with centuries-old examples extant) practitioners typically limit the thickness of their walls to not more than 12" because the straw in the center will rot before the wall can dry^E and straw-clay presumably dries faster and has less initial moisture than cob.

On the other hand, if the bales do not have some kind of plaster (such as mud) applied directly to them with good adhesion, an air gap can develop (or be inherent in the wall system) which will reduce the wall's performance in both insulation and fire resistance. (Not to say that it would be any more of a fire hazard than the trailer you're planning on living in while you build, or the RV we're planning to live in while we build.)

To avoid these problems, you could build the cob part of your exterior walls and let them dry for a year while you finish the roof and other things, and then add straw bales to the exterior. Dip one side of each bale in clay slip to seal (somewhat) the edge that will face the cob, and let them dry before putting them in place. (Tom Rijven has been doing this in France for some time with good results, and Paul Lacinski in Massachusetts is a fan of the technique as well.) Then, as you're stacking the bales, stuff straw-clay in the gap between the cob and the bales. Let that assembly dry (for some weeks or months of low-humidity warm weather) before plastering the exterior, preferably with an earthen plaster. It might be worth considering a layer of lime to finish off the outside, especially if you don't build the wonderful wrap-around porch already suggested by a couple people.

Remember that a straw out-sulated cob wall built this way will result in deep exterior window reveals (unless you wait and put your permanent windows in the bale part of the wall, flush or preferably proud of the outside surface). Those "ledges" are a potential source of moisture problems for cob and straw alike. Then again, the aforementioned wraparound porch is a longstanding favorite for wall protection.

You might want to imbed anchors in the cob so you can tie through the bale wall to an exterior pin system of bamboo or saplings to help draw the bale part of the wall to the cob part of the wall.

While learning all the strawbale related details you'll need to know to design the house, be sure to play with some bales to see how creative you can get with your wall shapes. Bales can be bent into a shallow curve, cut into wedges and other shapes, etc. 

Another option, which would yield lower thermal resistance - and probably be more work - would be to make straw-clay blocks in forms (say 12" x 6" by 4"-6" deep), let them dry, and then mortar them in overlapping layers with mud onto the exterior of the cured cob wall. The emphasis here is on the straw; use as little clay as possible in the straw-clay. A clay-rich and highly-compacted Leichtlehm may not have significantly better thermal resistance than cob.

Mark Piepkorn 
(former editor of The Last Straw   http://www.strawhomes.com )
 & Sarah
mudhome at lycos.com
(formerly at sarahm at erols.com)

Ps: There is a lovely straw-clay house featured in a recent issue of Natural Home Magazine, but the magazine calls it cob!




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