29 December 2010

Strawbale vs. Cob

Which material works best for which application...

I often get asked this question:  Which is better to use for my natural building, strawbale or cob?

The answer is simple: IT DEPENDS!  (of course!)
To help sort out if strawbale or cob walls are better suited for your application, I'm going to describe the basic properties of each material, and then how to use those attributes to your greatest advantage.

strawbale detail around a beam
STRAWBALES insulate.  And they are thick, so they insulate really well.  What this means is that a wall built of strawbales slows down heat energy traveling from one side of the wall to the other.  A good insulator acts like a down jacket that keeps your body heat inside the jacket instead of getting disbursed to the cold winter air.  A well-insulated house will use less energy to heat in winter than a poorly insulated house, because the insulation keeps the heat inside.  If you use an air conditioner in summer, insulation will keep the heat outside, so again you need less energy to keep cool.


sculpted cob thermal mass wall
COB is a thermal mass because of its principal ingredients: clay & sand.  Cob has limited insulating properties.  Instead, a thermal mass is like a storage battery for heat (or cool) energy.  This means cob is good at absorbing heat energy from the sun or a fire and storing that heat.  When the air temperature around the cob is lower than the temperature of the cob itself, it releases its battery storage of heat into the air.  In this way, cob can absorb a lot of heat energy and then release the heat over time, long after the heat source is gone.  Conversely, a shaded thermal mass with no heat input will stay cool in the summer and absorb heat energy out of the warmer air around it (thus having a net cooling effect).


So how to best use these characteristics to your advantage?
The answer depends on your climate and what you are building.  If you are using energy to change the inside temperature and keep it something different from the outside temperature, then you generally want a good insulator...ie, strawbales.  If you live in a mild climate where the temperature swings are day-to-night instead of seasonal, then a thermal mass exterior wall generally will help to average out those temperature swings...ie, cob.  Thermal mass can also provide a highly beneficial interior element in conjunction with passive solar design, to capture heat from the Southern sun in the winter (when the sun is low) but remain shaded when the summer sun is high in the sky.

STRAWBALES work best...
  • as exterior walls anywhere you are trying to keep the inside temperature different from the exterior temperature; the insulating strawbales will help keep the temperature exchange to a minimum, so the energy used to change the inside temperature will be minimized

COB works best...
  • as thermal mass built around a masonry heater or rocket stove (or near a wood burning stove), where the cob can absorb heat from the fire, and store the heat energy even after the fire is out
  • for trombe walls in passive solar design, with the cob thermal mass inside, where it is warmed by sun coming through South-facing glass
  • for any interior element when you are trying to keep the inside cool; this can be the same thermal mass used to keep warm in winter as long as there is no heat source warming it when you want to stay cool

I use a combination of strawbale and cob.  Because I design for a climate that requires several months of heat in the winter, I use the insulating strawbales for exterior walls.  This ensures that only a minimum amount of energy is needed to heat the spaces and the heat stays inside.  I then position some cob element to the interior.  Either it surrounds a wood-burning heat source or it is positioned so that low winter sun shines on the wall from the South.  That same cob element is shaded and cool in summer.  This way the cob helps to regulate the interior temperature in both winter & summer.

56 comments:

  1. Amazing information. Thank you so much for openly sharing all that you know. I could not possibly express my appreciation in one paragraph. Sigi, you are an amazing individual/teacher!

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  2. Thank you so much for putting all this info out here for us to read!

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  3. Hi, I live in southwestern British Columbia (think: Pacific Northwest climate) and I'm struggling with the choice of cob vs. strawbale. I think we need the insulative properties of strawbale, but we live in a very wet climate and I'm concerned about keeping the walls dry (both rain and humidity are high during the winter). Cob is better for that but I'm concerned about it's poor insulating properties. The west and north walls of our site get no sunlight at all, and even though the building will be south facing with passive solar design, we get little sunlight during the winter months.

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  4. If you live in a climate where a well-insulated home is more comfortable and cheaper to heat/cool, then absolutely you want strawbale exterior walls. You can augment with an interior cob wall that is placed in such a way that it gets ample sun in winter, but is shaded in summer. But your barrier between inside and outside wants to be a good insulator. Building with cob as your exterior walls in a cold climate can result in a condensation point and moisture build-up inside your wall...which can lead to mold problems, and even freeze-thaw cycles in your wall (which can cause the wall to break apart).

    No worries...you can absolutely build with strawbales in a wet climate! The bottom line is that you can build with straw anywhere you can build with wood. They both decompose at the same moisture content, so if wood construction is viable, so is straw. That said, you need to follow a few simple "best practices" to ensure that your bales walls last a long time. In short...

    1. keep the bales well off the ground
    2. make sure you have a good roof overhang
    3. install redundant moisture barriers at all window sills & tops of walls
    4. don't allow condensation points (ie, anything cold) inside the wall...so no rebar pinning!
    5. no plumbing lines in strawbale walls

    For more in depth info, you can read my article "5 Tips to Keep Strawbale Walls Dry in a Wet Climate":
    http://www.buildnaturally.com/EDucate/Articles/DryStrawbale.htm

    Hope this helps!
    Sigi

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  5. Hi! Someone just pointed me to your site, and I am very grateful for it! I want very much to build some kind of a green and affordable house in the next year or two, and have been reading all about Earthships, cob houses, strawbale, etc. It's looking like the strawbale exterior/cob interior combination would be the best (I live on Maryland's soggy Eastern Shore). But cost is a major factor, so I'm trying to gather all the information I can about how much I can expect the project to cost, as well as every possible grant and incentive program out there, and every possibility for getting help with the labour. Without having any building experience (but plenty of energy and a quick capacity to learn), how much can I hope to do myself? Can you advise on any possible resources? I'm on something like a $100K budget, including land, well, and septic, so I need all the help I can get!

    For instance, I'm wondering whether there might be any college programs which would send students to assist with/learn from such a project. Or whether someone like you who teaches workshops might use my house as a hands-on workshop project.

    Also, what do you think of incorporating some of the Earthship design into a straw bale/cob house? Specifically, the big greenhouse south side with the sloping windows, as well as the clever water system.

    And finally, permitting is a huge concern. They're awfully backward out here on the Shore and I'm guessing they're going to need to see very proper architectural plans before I'd have any hope of permits. Do you make such plans? Could I hire you to design this house with me?

    Thank you in advance,
    Gryphon Corpus

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    1. Hi Gryphon, and thanks for posting a comment. Lots of questions...I'll do my best to answer.
      >>for your budget, it sounds like you are going to be on a shoestring. So you will need to maximize your sweat equity and plan out your costs really really well ahead of time. You could easily spend your whole $100k on just well, septic & land... It is feasible to do a lot of work yourself, assuming that you have the time. However, I recommend to be realistic about your time and also to inform yourself superbly before you begin. It will be a steep learning curve, but no doubt rewarding as an accomplishment in the end.
      >>Not sure about college programs, but I would contact regional universities, perhaps starting with UMD. You could certainly host workshops. Personally, I do workshops only on projects that I have designed, so that I know that all of the detailing is designed to last in our climate.
      >>Earthship concepts are definitely congruent with natural building materials, including strawbale & cob.
      >>Non-loadbearing natural buildings MEET all current building codes, assuming you are using insulating exterior walls (not cob exterior walls). I do design natural homes, and yes, you could hire me, though it sounds like your budget is too tight for design services.
      I hope this is helpful.
      Thank you, Sigi

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    2. Thanks so much for your response! Yes, lots of questions, and I've been researching like a madwoman.
      The budget is indeed a shoestring, though I like to fantasize that I'll find time to do much of the labour (despite being a single mother running a company and homeschooling my child... hence the need for some kind of help). But I understand that in the long run I'm likely to save money with a very carefully thought out design. Could I speak with you about designing/planning a workshop?

      What about loadbearing natural buildings? And cob exterior walls? I'm rather hoping to build a whole structure out of cob. Is that not likely to be feasible?

      I'm exploring the idea of starting out by building a very small house (under 120 square feet) without a permit in which we could live while building the 'real' house, with the thought that this would give me practice, allow me to live on my site while building, and reduce the urgency of the building, which I imagine might take me a couple of years. What do you think of that approach?

      Thanks!
      Gryphon

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    3. So definitely you want to be realistic about finances as well as how much time you can dedicate. Otherwise you can get yourself into a jam. Not sure what County you are in, but before you build something without a permit, check with the County office to be sure you are in compliance. Definitely building something small will let you practice & give you a sense of what you are getting in to. And for sure would give you breathing space to build your "real" house over time. I wouldn't build a cob building in this climate (at least not for exterior walls). There is no insulation value and you can get condensation on your walls in winter which can lead to mold issues and a whole can of worms. So I would recommend strawbale or light-clay-straw...something with insulating value...if you are going to live in the building. I'd be happy to discuss design/workshop options. Feel free to call at your convenience M-F 9-5 (though not working tomorrow). Thanks! Sigi

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    4. That makes a lot of sense about not using cob alone. And I will of course make sure everything is very square with my local permit office. Thanks, I'll give you a call on Tuesday. At the DC or the PA number? Happy New Year!
      Gryphon

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    5. Yes, feel free to call. Sorry, not working Monday or Tuesday (forgot the dates!). PA # is best. Thanks!
      Sigi

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  6. Hello Sigi, first I wanted to say that your site provides some amazing information. Thank you.

    Now for the nitty gritty! I was wondering what would your professional opinion be about trying to build a natural home on the Big Island of Hawaii? There is an average of 300 inches of rain a year so I'm concerned about the walls not being able to dry out daily during the rainy season. To keep the cob off of the ground, do I just build a standard foundation?
    Any information you could provide would be amazing!

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    1. Thanks for your kind feedback & for your question. So I first need to give you the disclaimer that I have not designed any buildings for a region with that much rainfall. So what I'm posting here is theoretical and you may want to find someone who designs for your specific climate to verify.

      First line of defense against rain is ALWAYS to lift the floor of the building higher than the ground height and then slope all of the ground away from the building. This is called "positive drainage" and ensures that you don't have pooling water or a river of water heading toward your building when it rains. How high you raise the floor depends on your specific climate and the situation at your specific site. To lift cob off the ground, you still need a solid structure below the cob to support it's incredible weight...that can be stone or concrete or any normal foundation wall system.

      Your second line of defense with that much rain is to create a good roof overhang. This protects your walls from the eroding effects of the water and also keeps the directly rainfall away from the foundation of your building.

      I hope this helps to answer your question. Please feel free to post a follow-up if you have one.
      Sigi

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    2. Good tips, thanks for the super fast response! I'd be happy to supply follow up comments/pictures on how everything goes. It won't be moving forward until after June, so I'm trying to plan ahead as much as I can for the build.
      -Nick

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  7. http://earthbagbuilding.com/projects/hawaii.htm is a project I found that was done using Earth bags with 10% cement due to the extreme moisture.

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    1. So I would just say that there are more natural materials that will also give you high thermal mass that will work in extreme moisture conditions. Whenever you can avoid cement, it's a good thing, as the production of every pound of cement contributes 1.25 pounds of greenhouse gas to our atmosphere. Thanks for sharing your experience.

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  8. What would you recommend using instead of cement in such extreme moisture conditions? I'd like to stay as natural as I can get building homes.
    -Nick

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    1. As a stucco-alternative, I use lime plaster, similar to what was used on historic buildings. It's durable in weather but flexible and breathable.

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    2. Good idea. What about in place of the cement inside of the earth bags? It is for the strength of the walls that they use cement.

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    3. I do not build entire walls from earth bags and never use cement plasters, so I can't really comment on what would be best there.

      I build with super-insulating strawbales. I plaster the inside with clay (usually made with soil from the building site) and plaster the outside with lime stucco, as noted above. Where I need a thermal mass wall, I build with cob, since again I can use clay-soil from the site to build with.

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    4. Thank you for your insight!
      -Nick

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    5. I read recently (unfortunately I can't remember the author or where I read it) that it is a bad idea to use different materials to plaster interior and exterior walls in terms of the passage of moisture through the walls. Any thoughts?

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    6. this is only the case if one of the sides of the wall is non-permeable, for example, using clay plaster inside and cement stucco outside

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  9. Hi Sigi,

    Thank you for sharing your knowledge.
    Please, forgive my English as French is my mother tongue.

    Years in advance, I am planning to build my own straw bales house here, in the province of Québec, Canada. I know the exterior walls must be in straw bales for insulating. But I am not sure I understand what interior cob walls would do.

    My plan is to go the cheapest way, no matter how long. Right now, with the so little knowledge I have, I lack on self-confidence and I consider hiring for the house's base as well as roof and exterior's walls by fear I won't make it ok myself - though it can change over time. So, if I hire for the exterior, but finish the interior myself, can the interior walls all be made with cob or will the insulation be crap in this cold winter country? If it can, how large must they be? Perhaps it's better in straw bale in any way?

    Also, I have neighbours living in a two-stories rented straw bales house and the architecture does not protect the walls from the rain at all. It's been there for about 15 years. We hear about protecting the walls from the rain all the time,yet, here, it does not seem that important. Or perhaps I am missing something out since they rent the house and I do not know the authentic landlord/builder... so, I was wondering what does that mean exactly what we read everywhere about protecting the walls from The rain... how is it possible to do that?

    Finally, I was wondering if you could suggest a fantastic book or vidéo that shows well how to make the natural house's base, roof and plumber & such.

    Thanks for any light you can give!

    Happy day to you,

    Geneviève

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    1. Hi Genevieve, and thanks for your note. I'll start with a book recommendation for you: Serious Strawbale by Paul Lacinski and Michel Bergeron. It deals with strawbale construction in wet & cold climates. I would also direct you to look up Chris Magwood of the Endeavour Centre. He is a Canadian that teaches all types of natural building in Ontario.

      For your specific situation, yes, absolutely, you want something highly insulating as your exterior wall system for your location. Like strawbale. And yes, you want to protect the wall with a good base and a roof overhang. This means, begin your strawbale wall at least 18" (46 cm) from the ground level. There are many details for doing this, so a good book will be helpful here. It also means you want to extend your roof so that water falls off the roof at least 24" (60 cm) away from the exterior face of the wall. This minimizes water running down the face of the wall every time it rains. In your climate, you probably also want ice-and-water shield on the roof to protect snow melt from entering into your house/walls from above. (There may be a different name for that product in Canada...) Your walls don't need additional protection than that unless you plan to use clay plasters on the exterior.

      You may also want to look up Kim Thompson. She is a strawbale designer/builder from Nova Scotia that has over 20 years of experience in a very extreme coastal climate and can give you tons of insight into moisture issues.

      I hope this helps! All the best, Sigi

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  10. My wife and bought a 5 acre parcel in Colorado and I was wondering if at the 7900 ft level if a Cob Home would work for us it is her dream to build one and experience the simple life that comes with it. I want to thank you for all the good information that you shared here,is very useful and helpful as well, any information about the 7900 to 8000 ft level would help a lot as I said it is her dream to build a cob home........

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  11. The elevation is not the issue when deciding on cob vs strawbale...it's the climate. And in Colorado, your energy needs for a well-designed building center around heating. In this scenario, you want an insulator for your exterior walls, so that the heat you use inside in Winter will stay inside, and not creep out continuously (as the heat would for a thermal mass/cob wall). So, what I would recommend for highlands of Colorado is a) design for passive solar (it is possible in sunny Colorado to get all of your heating needs from the sun only), b) use strawbale for your exterior walls, and c) build an interior (or series of interior) wall made from cob to collect and store the heat from low winter sun. Hope that helps!

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  12. Hello. I live in Florida and am looking to build a natural building home. As I am sure you know it is hot and humid with a lot of rain, especially in the summer. So my questions is what would be the best recommended material( ie, cob, straw bale tire wall)? I was planning to build it completely out of Cob in a tree shaded area with a vented roof and strategically placed windows to catch the breeze. The cob will start atleast a foot off the ground and I will have a large overhang on my roof. What do you recommend as a roof design that could catch rain water but not bake us in the summer? From my research a cob house will stay cool if I can keep it out of the sun even if temperatures are pushing 90F plus. Is this accurate? Thank you so much in advance for your website and support!!

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    1. So, traditional hot-climate construction is all about ventilation. So they built double walls and double roofs, and created negative pressure between the cavities to promote air flow. Shaded thermal mass will work as well IF your average temperature in your region is comfort. So you have to look up that weather data. Then you size your thermal mass according to the length of time that you want that thermal cycle to take place. This can make your walls really thick. I remember in Austin, where I lived for several years, the thermal mass thickness was around 21" thick. Shade helps of course, but if heat gain is always your problem, then shade your cob completely, not just one area. I hope this helps. It's a pretty complex topic, so you may want to get a book to read for more detailed information.

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    2. Thank you Sigi! I am on my third book. Is there one you recommend that may guide me best on hot, humid climates?

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    3. The only book I know of is: Climate Responsive Design: A Study of Buildings in Moderate and Hot Humid Climates by Richard Hyde.

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    4. This might help:
      http://www.earthbagbuilding.com/pdf/simpleearthbuildings3.pdf

      The author, Patti Stouter, has published a series of pamphlets on this topic.

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    5. I am not actually a fan of earth bag building unless you really don't have suitable soil for construction. Directly clay-to-clay bond is soooo much stronger and does not use plastic and barbed wire. But thanks for posting your research.

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  13. We live in the Texas Hill Country. I have heard that we might have trouble with fire ants building nests in our strawbale walls. Do you have any tips on preventing that? We are about 3 years out from beginning construction and are in the research and saving phase.

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    1. Hi Jennifer, I have never heard of an issue with fire ants in strawbale buildings. But you might want to check with http://texasnaturalbuilders.info/ as they may have more specifics with your local pest issues and how to resolve them. My very cursory understanding of ants is that they need a source of moisture...the idea when building with straw or any other biodegradable material is that you want to be sure there is no water in the wall...so the ants should naturally look for another location to inhabit. But again, check in with the Texas Natural Builders to see if they can give you more specifics.

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  14. Hi Sigi. Thank you for your wonderful site and open way of helping people build natural homes. I live in Los Angeles and have just started researching building a straw bale or cob house here. Do you know of any groups or builders in the Los Angeles area? Most natural homes seem to be built in more rural areas and I'm wondering if permits and insurance will be problematic in a city? Also from reading these earlier posts it seems that cob would be more appropriate for our area which is generally mild, sunny and 75 degrees, however we can reach the 90-100 in the summer and sometimes have a week of rain in the winter. The day-night temperature does fluctuate a lot, probaly 20degrees cooler most evenings. Also do you deal with rammed earth or pise as well, these are the other options we are considering, however budget will be an issue, so I'm guessing cob will be cheaper? I'd appreciate any insight you are able to give. Thank you for your insight. Dawn

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    1. Hi Dawn, and thanks for posting your question! For your question about permits & insurance in the Los Angeles area, strawbale meets the current building code, plus California has a specific strawbale code in place now. So you are good to go on that front. Cob is also not a problem if it is not used structurally. If you want to use cob or rammed earth as the structure, I would contact Bruce King, a structural engineer in California that specializes in natural building and he could help you navigate that I'm sure. For other California resources, I really only know of CASBA (CA Straw Bale Assoc.). Cob tends to be cheaper than rammed earth for 2 reasons: 1) you don't need formwork and 2) it's a bit more DIY friendly & forgiving, so you can readily do a lot of sweat equity.

      As for which materials to use, you are lucky in that you live in about the easiest climate to design a passively heated & cooled building. I would do a passive solar design that incorporates a high volume of thermal mass inside, placed in such a way that it gets some sunshine in the Winter but absolutely no sun hits it in Summer. The exterior walls recommendation kindof depends on several variables...speed of construction, if insulation is required by your building codes, whether you will install any heating or cooling systems, specific extreme-case temperature data, etc. Certainly a strawbale home with cob or rammed earth interior walls and an earthen floor could be designed to be completely self-regulating in temperature.

      Sorry, a bit of a rambling response to your questions. Hope it all makes sense!

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  15. Hi Sigi,

    I heard from Cob Cottage Company about something called bale cobb? Is that the combination that you were referring to of traditional cobb and straw bale? I was told that bale cobb walls go up much faster (obviously) than just cobb alone? This is what I need because I may be going at this alone or with just my girlfriend. I was also wondering about the expense differences of the three techniques.

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    1. Bale cob is actually a method where you take strawbales and lay them like bricks using cob as mortar. I don't really recommend this method. I would just stack your strawbales without any "mortar" and then plaster them inside and out. What I meant above as a combination of strawbale and cob is to use strawbales for the exterior walls and then strategically placed cob walls inside for temperature performance (moderate temperature swings, absorb excess heat in summer, absorb heat from the sun or a fire source in winter). If you are looking for something speedy, this is what I would recommend.
      Hope this helps to clarify!

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  16. Dear Sigi,

    What a wonderful site and treasure trove of information! I am delighted to have found you because I intend building a natural house in the South West of Ireland which has a very mild climate (being close to the gulf stream) but extremey high rainfall. I am aiming for a passive, carbon-neutral structure, completely earth-covered, but want to address Ireland's perennial problem of internal humidity. If I build the external walls with lime-plastered straw bales and place cob features in strategic places internally, will the cob act to absorb the excess humidity? Also, would you recommend a timber support structure for the earthen roof or will the straw bales be sufficient to take the extra weight?

    Once again, congratulations on your excellent site. Brendan

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    1. Thanks so much for your kind words, and I am pleased to hear you found the information here to be useful. Sounds like you have a pretty inspiring vision for what you are going to build. Certainly your biggest issue sounds like it will be humidity. Is this year-round? I don't have experience designing for a climate with year-round humidity, but in theory, yes, interior cob does act to some degree as a desiccant, absorbing excess humidity from the air. There is a point where it will all come to equilibrium. And if the cob never has the opportunity to dry out...from a less humid season or from being heated (by sun or wood stove etc)...then your air will still feel humid. Does that make sense? In terms of structure, this really depends on the design of the building. It is certainly feasible to support the roof, even a planted roof, on the strawbales in a loadbearing manner. But whether or not that makes sense is too complex a question for this venue. You might want to have a local structural engineer take a look at your design. Hope this helps.
      Sigi

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  17. Hi there,
    I love your site, thanks for sharing! I'm just wondering if you could tell me what is best for hot humid climates? Cob or straw bale?

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    1. Hi and thanks for your positive feedback. For your question...it depends. If you plan to use energy to change the inside temperature (ie, use some form of air conditioning), the you want super insulation, ie, strawbale. If you do not use energy to change your inside temperature anytime during the year, then high thermal mass, such as thick cob, will help you feel cool inside. You will also want excellent natural ventilation and be sure to eliminate points of significant solar heat gain. Hope this helps you.

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    2. Hi Sigi! Thanks so much for the reply. I wont be using AC so I will go with cob. What do you recommend for wall thickness? Cheers:)

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    3. Hmmm...too many variables to be able to advise on that. Depends on your structural loads, your interior volume, your specific climate conditions, your personal comfort level, etc. If you want to stay cool all the time, thicker is better, as a rule, but beyond that I cannot size it for you. Good luck!

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  18. Hi Sigi, which country do you work in?

    I live in Ireland and my boyfriend and I want to build a house in the next few years. I had set my heart on cob, but reading this now I am not so sure.

    Obviously Ireland has quite high rainfall but is fairly temperate. Having said that I think the winters can be awful cold and damp, so I want a building material that will work well in this climate.

    The other thing is, i understand that cob is inflammable, but i can't imagine the same can be said for straw bales? Are there many incidents of straw bale constructions catching fire?

    Thanks very much, your site is really interesting, lots of food for thought.

    Emily :)

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    1. Hi Emily,
      I work in the U.S. (mostly East coast). My suggestion to you would be to reach out to someone with experience designing natural buildings in your region. They will understand your climatic issues best, as well as any building regulations. In terms of which material is best for your exterior walls, I would say that if you heat your home during those cold, damp winters, then an insulating material is a better bet. In terms of flammability, you are correct that cob is inflammable, however, strawbales are also very fire resistant, since they are plastered on both sides with inflammable plaster AND because there is not enough oxygen inside the tight bales to support a fire. (Loose straw is highly flammable, and the surface of a bale will burn, but a plastered wall will not burn.) I hope this helps you! -Sigi

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  19. Hi! Thanks so much. Yours is the best site I've seen about this subject! I have been trying to learn about this, with hopes of building naturally at some point. Two questions: 1) What is the difference, if there is one, between adobe and cob? And 2) Do you know anything (or have any opinions) about building with stone and straw bale--some kind of combination? I live in Virginia, in the Piedmont, and have tons of rocks and stones on my property. I also love the way stone looks, and it's very harmonious with the architecture of this area. But having lived in a stone house previously, I know that stone can be incredibly cold and damp. Thanks so much!!

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    1. Thanks for the kind words. I'm thrilled to hear the you find this all informative.

      Your questions are great. So for #1, cob & adobe are the same ingredients, but used differently. In fact, sometimes cob is referred to as "sculptural adobe". The difference is that cob is mixed dryer and then walls are formed monolithically in place. Whereas adobe is mixed wetter, formed into molds and left to bake in the sun. The adobe bricks are then put together like any masonry wall, using the same clay/sand mixture as the mortar. As for #2, you can absolutely build with stone & straw in the same building. The caveat is that you want to build it in such a way that the straw has a capillary & moisture break between it and the stone (otherwise you will get damp straw over time, and that's not good). Hope this helps clear it up for you.

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  20. Thanks Sigi! Yes, I am so happy to have found your blog. (I recommended it to some people I've been talking to on the permies forum ~~ http://www.permies.com/)

    It sounds like Adobe requires another step and cob allows one to customize a bit more--make it more sculptural. (Though you often see sculpted elements inside adobe houses.) It seems odd that adobe has never gone out of style in the SW, yet cob went out of fashion in most places, SO long ago (notwithstanding its recent comeback in the US and Europe, at least ;-). Do u think this may be due to the dry desert climate that makes it easier to build without worrying about a rainfall destroying the newly formed bricks? Without the need to build temporary shelters? Or are there other reasons you know of that can explain why building with adobe never stopped since ancient times?

    Is adobe generally coated with a limestone finish on outside walls? Or in those dry climates is an adobe finish alone good enough?

    In the end, do you think adobe & cob structures are equally sound, fireproof, heat-retaining, etc.? Do you think cob is so popular right now, rather than adobe, because it is a little less work?

    Would cob work as the capillary & moisture break between the stone & the straw bale or would it have to be something dryer? If so, like what?

    Could you use a homemade cob mix as the mortar when building with stone? (As opposed to the regular cement-type mortar that is generally used these days.)

    Sorry to bombard you with so many questions! Last group! ;-) When you're building with cob, do you do it when the forecast is clear for a few days or do you dry your cob constructions under some kind of temporary shelters? How many days of drying are required before cob (or adobe) could withstand a rainfall? (Or do you just coat everything with limestone on the outside--even ovens--because rain will just wash the cob away? ;-) Then how many days, approximately, would be required for the limestone finish to dry before it could withstand rain? ~~~~Thank you! ;-)

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    1. wow, ok, lots of questions. I'll do my best to address them all. So I wouldn't necessarily say that adobe is more work than cob. They are just different ways of using the same materials. I would choose which is right for you depending on whether you can reasonably sun-bake the bricks and whether you want a masonry wall or a sculpted-in-place wall. For finishes, you can do either a lime plaster or a clay plaster. The difference is maintenance. Clay plasters generally get re-applied annually. If that's not desirable to you, then go with lime. In terms of physical properties (thermal mass, heat retention, sound, etc.), the two are essentially the same. Any differences would have more to do with amounts of straw in your mix rather than if you chose adobe or cob. In terms of mixing, I would mix by foot or by tractor. If you try to use a machine, you will either burn out the motor or you will end up with a mix that is too runny to build vertically with. Drying time for cob is measured in months, not days or weeks. So you need to plan that in to your construction process.

      Hope this helps!
      Sigi

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  21. Quick question, I'd really like to stick to just cob because everything would ultimately be cheaper in building, but it sounds like you're saying it would be more expensive not to do straw bales because of heating cost in winter and the possibility of the house falling apart ("which can lead to mold problems, and even freeze-thaw cycles in your wall (which can cause the wall to break apart").

    So is that something I should worry about in the Georgia area? Temperatures: http://www.intellicast.com/Local/History.aspx?location=USGA0151

    We haven't gotten snow in years, but this year we did drop below freezing. So should i do strawbale to be safe?

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    1. If your drop below freezing is short and an unusual occurrence, then you are probably fine with cob. Unless you plan to put in air conditioning, in which case I would go back to super insulation. If heating is normal for more than a few days in winter, then again, I would do super insulation...ie, straw. Hope this helps you!
      Sigi

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  22. Hi! Great blog, Sigi - thank you so much for all your work!

    I want to build a straw bale/cob house in Massachusetts, but feel lots of resistance. Banks seem wary of financing such a project and I don't have the cash to purchase land outright. Even if I did, I'm not sure I could get a permit. I'd want to hire help (and do as much as I and my fiance can as far as designing and actually building), but again, unless I cash my IRA, I would need a loan. I did talk to one loan officer who said he could "probably" give me a loan on a natural building.

    Do you know of anyone who has done this in MA? I've heard the building code here is very strict and may not allow it. I have also heard there are cob houses in MA, though. There must be a way! We are willing to move to southeastern VT or southwestern NH if necessary... Any advice would be much appreciated! Thanks!

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    1. I'm positive that there are strawbale buildings in Mass. Check http://sbregistry.sustainablesources.com/ to find if any are near you. There is also a link to lenders and insurance companies that have worked with natural buildings. As for codes, you can absolutely design a natural home with strawbale and cob to meet all current building codes. I do it on every project, so I know it is feasible (dare I say, it's a "no brainer".) I have an article explaining this here: http://buildnaturally.blogspot.com/2011/01/newsflash-strawbale-meets-current.html Hope this helps you!

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