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Community and Q&A

Exterior siding on cob walls

Hap Mullenneaux | Posted in Green Building Techniques on

My wife and I live in a hand built cob house in Iowa.
While we have no desire to change our home, I would like to have an exterior insulation layer in the next cob house I build. I envision a 4-6″ cob wall just inside a standard stud wall. I would like to do the stud wall with diagonal bracing and no plywood or osb. Cellulose insulation would be inside the studs and housewrap on the outside. Fiber cement lap siding would be the exterior.
The fiber cement siding would resist the mechanical weathering from rain which is a real issue for natural builders here in the Mid-west. The housewrap resists air and water infiltration. The insulation keeps the heat in. The cob is thermal mass and helps moderate the interior humidity. There would be a clay based plaster inside the cob.

First, are there any flaws in this system?

Second, which housewrap? Tyvek has a perm rating of 50 while Typar is about 11. That seems like a major differences and yet they are considered interchangeable. I would be inclined toward more permeability.

Thanks, Hap

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  1. GBA Editor
    Martin Holladay | | #1

    You've begun to see some of the disadvantages of traditional building materials. (A 15-year-old log house that I drive by several times a week is now getting exterior retrofit work -- new siding over vertical strapping.)

    Some cynics will note that your proposed solution to the disadvantages of cob construction amount to building a house around your house. Sure, it will work -- but you could probably safely eliminate the cob and end up with a simpler house that is much easier to build.

    I don't think it matters whether you choose Typar or Tyvek. You could also use #30 asphalt felt, which is an excellent WRB.

    My one suggestion: include a ventilated rainscreen gap between your siding and your sheathing. After all, you don't want your cellulose to get wet -- and if it ever does, you want it to dry quickly.

  2. Riversong | | #2


    You're goals are admirable but constrained by your attachment to cob, which is designed to be the structural wall, and 4"-6" won't do that, and it's very difficult to build such a thin wall out of cob (though you could use adobe bricks). If you will also have an earthen floor, then you may end up with too much interior mass. Once the mass is contained within the thermal envelope, it has to be properly balanced with solar glazing to achieve optimum performance. Either too little or too much will undermine the ability of the house to maintain homeostasis.

    You can certainly achieve your goals with a thicker structural cob wall and an exoskeleton for insulation and siding, but that is wasteful of materials. Have you considered structural or infill straw bale? That will give you the cellulosic insulation and you can apply an inch or more of interior clay plaster for both mass and hygric buffering (as well as those wonderful negative ions). A strawbale wall can be either plastered or sided on the outside.

    But I'm wondering why someone who prefers natural building would use plastic housewrap and fiber cement siding, which seems oxymoronic. Lap or shiplap wood siding has stood the test of time in rainy climates, as long as it is back-sealed, sealed at end grain and maintained on the exterior with opaque latex stain. A thin drainage plane behind it can be beneficial if over earthen materials but is not necessary over wooden sheathing, and the sheathing may not be necessary unless on a structural frame.

    #15 felt (not #30, which is far less permeable) makes a fine WRB (better in some ways than plastic), and while Tyvek hypes their high permeability, it's far more than is either necessary or wise. An exterior perm of 5-10 is perfect, otherwise it's too open to inward vapor drives.

    Another option would be a double wall system, with dense-pack cellulose in the outer and middle cavities and clay slip-straw in the inner stud cavities, which is an excellent base for earthen plaster, would provide some thermal mass as well as hygric buffering.

    The key to any successful natural building strategy in a cold climate is air sealing at junctions between dissimilar materials, but this can be accomplished with felt gaskets and some kind of lathe or burlap to ground the plaster.

  3. Hap Mullenneaux | | #3

    Yes, Robert, I am very attached to cob. We have been in our house for two years and we love it. I'm looking for ways to make it faster to get a roof up and have an exterior insulation/ rain barrier while still enjoying those negative ions around me.

    I have used wood siding in past conventional construction, but the best builders in my area have gone to fiber cement lap. Wood is not abundant in Iowa and doesn't seem to be the ideal exterior although most builders don't do everything you recommend.

    From the two responses, it sounds like my proposed system will work if I make sure that the lap siding gets air behind it. The Typar sounds like the better housewrap.

    Many thanks, Hap

  4. Riversong | | #4


    You seem to be missing my point. You don't need cob to get all the benefits you love. Cob is simply one of several ways to build with clay, sand and straw - and they all offer the same values. Cob is designed for thick, structural walls and works best in the desert southwest. In colder climates, particularly those with easily available straw, earth plasters on straw bales makes much more sense.

    I would strongly encourage you to investigate other ways to use cellulose and earth that are more suitable to your climate and more sensible to use.

  5. James Morgan | | #5

    Huh? A cob wall 4" thick? Cob is not just a material (basically mud, sand, horse-shit and straw), it is a way of building. A true cob wall is laid down without formwork in lifts of up to 12" in height and shaved to a final thickness of 24" or more after it dries. Yes it takes time, this is part of what cob is. To quote the Cob Cottage website: "racing to build fast is missing the point". So is wrapping its soft, fluid, sculptural exterior with a boxy linear siding. Call me a purist, but we'll surely have to come up with another name for mud blocks used in this way. How about Gringo Adobe?

    I have a great affection for cob, having grown up amongst many ancient cottages built in this way. I have also spent a few sleepless nights in an old cob mill house listening to rats racing through the tunnels they had carved in a material they regarded as just another earthen bank.

  6. J Chesnut | | #6

    I helped build a small 4 season hybrid cob/strawbale one room building with a rocket stove in Wisconsin. A structural stray bale wall was coated on the interior and exterior with a 2-3" layer of cob. It was a pleasure to build and the cob application to a straw bale wall worked well to lock everything together and fill in the gaps. I think the walls ended up with decent insulation value and are likely sufficiently air tight with good hygrothermal properties. ~2' overhangs are meant to keep the rainwater off the exterior walls so the exterior wall finish can remain cob.
    The thermal weakness of the overall design was the roof and wall to roof transition. The designer/builder had collected 3" ironwood trunks from the site and assembled a roof structure in place from this material. The organic form made it difficult to insulate, air seal and the rainwater management is questionable.

    If you are going to a frame wall to add insulation why not do a natural clay plaster on lathe for the interior finish. I've seen successful and beautiful applications of this and am in process of doing this in my own home. With a 3/4" thick layer of clay plaster I believe you the benefits of the properties of clay that you get with cob.

    I'd be interested in learning a little about your lifestyle. Natural buildings are viable but the inhabitants lifestyle is the key factor. I aspire towards living in this type of structure myself (more challenging in the cold climate) but my wife has other expectations (despite having grown up in a cob/thatch roof structure she has fond memories of).

    I have to say your proposed wall assembly doesn't strike me as the best solution to a well insulated structure built of natural materials. Please post again however I'd like to hear more discussion around these building techniques.

  7. Hap Mullenneaux | | #7

    "Cob is not just a material (basically mud, sand, horse-shit and straw), it is a way of building." Actually, you can cut the shit, so to speak. We use cow dung in our exterior plaster but not in cob itself.

    Very few cob structures in the US follow the English tradition. Cobbers on this side of the pond play with the unique structural and sculptural properties of cob to produce very artsy dwellings. Our house is tame by US standards, but we do have some interior walls that are about 5" thick. Such walls can be done safely with cob if you know the material intimately.
    1. Cob bonds best with itself. Avoid material like posts that run through the cob and break up its monolithic strength.
    2. Curves can be stronger than straight walls.
    3. The orientation of the straw in the cob can be critical especially where it is not so thick.
    4. Lateral support can buttress a narrow wall.

    "racing to build fast is missing the point" In England the rain usually falls down. In Iowa the rain often resembles a car wash. Storms begin with gusts perfectly designed to remove tarps, temporary roofs and small animals before the rain starts. A building systems that starts with quick structural walls and a permanent roof will allow you to then take your time and enjoy the cobbing and plaster work.

    The purpose of my original post was to see what other experienced builders thought of a possible hybrid wall system. Sounds like it would work, but I'm the only person who would consider doing it. I will continue to consider other options.

    After building our cob house, we did a small bale/cob hybrid similar to the one described in J Chestnut's post. Our final render was a cement stucco called Litewall which was said to have high permeability. The cabin was closed in last December and the rocket stove fired up to finish drying all the interior cob. The water vapor that was pushed into the bale/cob walls got as far as the Litewall where it was trapped. We noticed that the plaster behind the Litewall would get saturated and large chucks would fall off. We put a moisture probe into the bales and got readings from 24 to off the meter throughout the structure. In the spring we put up a temporary support for the living roof and took out the external cob and bales many of which were well on their way toward compost. What we were left with was an internal cob wall under a massive roof. I did a stud wall to hold up the roof and now you know why I started pondering cob inside a conventional exterior.
    Thanks again, Hap

  8. Riversong | | #8


    Your experience with the hybrid building should have taught you what all cold-country straw bale builders already know: that you can't use cementitious exterior renders on straw - they are simply not permeable enough - rather than turn you off to an excellent natural building system that would perfectly meet your goals.

    Almost all north-country straw bale buildings are infill, precisely so that the roof can be built before the straw arrives.

    The reason I keep encouraging you to consider other materials and methods is that is how the design process ideally works. With every design project I've done, the client comes to the table with specific ideas or mental pictures (or photographs) of "things they like" and then they want a super-insulated, healthy, passive solar home which is either incompatible with the "things they like" or can be designed in a much more efficient, effective and less expensive or easier way.

    So the design program always begins with "what are the elements of shelter that are important to you?", "what life-style functions do you want to contain in it and what are they're relative priorities and relationships?", and "what are your design goals?".

    From this basic ingredient list, a number of options will emerge that will satisfy those functions, goals and priorities - and they often look very different from what the client had initially imagined.

    What I'm hearing from you is that you want a natural, healthy home, with earthen free-form wall surfaces, some curvilinear partitions, a structure that will allow the roof to go up before the earthen elements, and a weather-resistant low-maintenance exterior.

    There are a number of paths to that endpoint, and you're severely limiting your options by starting the process with a fixed idea in mind (and one that, quite obviously, doesn't make much sense to experienced builders).

    My experience and knowledge of natural building (as well as structural and hygro-thermal engineering) leads me to suggest a stick-framed, straw-bale infill (properly configured for a wet climate), with thick interior earthen plaster and interior cob walls and structures, an exterior render of earthen plaster and exterior wooden horizontal shiplap siding with no WRB but spaced an inch away from the exterior straw/clay surface and with open (screened) bottom drainage.

    Other possible options I've already discussed.

  9. James Morgan | | #9

    Hey Hap, I didn't mean to disparage the American way of cob, just think it might have useful to use a differentiating name. The quote on slow building came from the Cob Cottage Company website, based in Oregon - they call their system, developed by Ianto Evans and others in the 1980's, Oregon Cob to distinguish it from the British tradition. And by the way, it rains - a lot - in the traditional cob country Southwest Britain, Wales and Ireland. Often for days at a time, often in combination with sustained gale force winds in excess of 40 mph.

    And Robert's comments on adjusting building methods to fit final goals are spot on. Allowing means to determine ends is not atypical of our culture, but it often results in a costly lesson from the non-negotiable laws of nature.

  10. Hap Mullenneaux | | #10

    "you can't use cementitious exterior renders on straw - they are simply not permeable enough"
    The autoclaved, aerated Litewall was introduced to us by a strawbaler who thought it was an exception to the rule based on its perm rating. Based on experience we now know that you are 100% correct.

    "stick-framed, straw-bale infill (properly configured for a wet climate)"
    Is there anything you would do to configure this for a wet climate in addition to the rainscreen type siding and plaster on the exterior of the bales?


  11. Riversong | | #11


    The essentials for a wet-climate straw-bale building include:
    - straw bales kept at 14% moisture content before and during construction
    - good ground clearance (typically 18" or more)
    - large roof overhangs (typically 2' or more per floor)
    - capillary breaks between straw and foundation or slab
    - no metal pins, rods or rebar within bales
    - integral air barrier at discontinuities
    - exterior lime render for breatheable water and mold resistance

    With those precautions, most New England straw bale homes have plaster exterior finishes that can be maintained with limewash, though some use a wooden rain screen cladding, more for aesthetics than protection.

  12. Daniel Ernst | | #12


    I doubt this is new information for you, but it might be good information for other readers.

    The Litewall stucco that you used is manufactured by Elite Cement down in Georgia.

    The product was specifically developed for Aerated Autoclaved Concrete substrates. It is also approved over concrete block or poured concrete surfaces.. There are only a handful of "approved" coatings for AAC. The manufacturers (like Aercon in Florida) approve products based on their compatability with the AAC: mainly they are interested in keeping the coefficient of expansion in close line with the substrate. They are also interested in permeability (35 perms), capillarity, elasticity, and ease of application.

    As you said, it's a cement based product (primarily gypsum), and it's polymer modified to boot. I've used it on AAC with good success. It does resist wetting - initially - but wind driven rain will soon break the surface tension and allow the product to absorb moisture. It does provide exterior drying, but as you found, not at the same rate as you would expect from a clay / lime plaster.

    With any reservior wall you are creating a balancing act between wetting and drying. Masonry substrates - structural brick, AAC, concrete block, etc. - are more resilient. Straw bale walls are more delicate.

    Only you can determine your real goals for construction AND how much risk you are willing to assume. You can either follow a presciptive path (using those materials and methods that have stood the test of time), or you can venture out into new territory. But without significant testing and risk analysis, you are taking a gamble on any hybrid wall.

    I would recommend researching what other builders have done in your climate (not that you haven't already). Is there an assembly of straw bale or cob buildings local to you? What about buildings with double-stud or larsen truss walls? If your goal is cellulose and earth, then you have a lot of different options. But, personally, I wouldn't try to mix any of these techniques.

    Good luck!

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