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Cob and Insulation |
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Q: My husband and I are very excited about building a cob home for our family. We live in Edmonton, Alberta and the winters get quite cold. Is it even possible to have a heat efficient home in -35 C weather? A: In a very cold region such as yours, I would not recommend building a large living structure entirely out of cob. Cob has great thermal mass but poor insulation value. In very cold weather, especially when the sun doesn't shine for extended periods, you will lose a lot of heat through those cob walls. A more efficient approach would be to make most of the exterior walls out of a more insulating material such as straw bale. Cob can be used for interior walls, for sculptural details on the bale walls, and possibly for some south-facing exterior walls. Q: My wife and I will soon (I hope) be building a house in New Mexico. I've been trying to find how much insulation cob provides. I understand it has great thermal mass but the only information I've seen on its capacity to insulate suggested an R rating of 1 per 1 inch of wall thickness. That sounds great, but I don't know what it was based on. Wouldn't the amount of straw determine the R value as earth has so little? Anyone know what kind of insulation we could have if we built a thick (18 to 24 inch) wall? Any ideas for increasing insulation without resorting to foam? Also, anyone know of successes in getting a permit for a cob house? Particularly in this part of the world, the southwest U.S? A: There has been very little engineering testing done on cob per se. Most of the numbers we use were actually derived from studies of adobe, which is similar in its makeup. Purported R values range from a low of 0.3 per inch to a high of 1 per inch. This variance may depend on the specific components of the material (yes, high straw content should increase the insulation) or on other details of the test. Another way to increase the R value would be to mix pumice into the cob in place of sand, or to add other lightweight materials such Q: I recently purchased a small home in North Eastern AZ. I would like to do an addition and cover the existing wood siding (T 1-11) on the exterior with something insulating and fire resistant. Cob has gotten my attention since my ground appears to have a lot of clay content, there were originally a lot of adobe homes in this area. Is there a way to cover over wooded siding with cob or another natural insulating material? A: The question of how to renovate conventionally-built houses with natural materials to improve efficiency, comfort, and beauty is an extremely important one. There are so many ugly, poorly built, and unhealthy buildings out there. From a materials and energy standpoint, it doesn't make sense to tear down all the old houses and start fresh. But it can be rather tricky to do this sort of remodel in a sensible and elegant way. First of all, cob is not a very good insulator. It is a heavy material that has excellent thermal mass, and can therefore help with efficient passive solar heating and cooling. But unless cooling is your primary need, thermal mass is most useful inside the envelope of the building, rather than outside. To give you better insulation you need a lighter material like straw bales or slipstraw (also called straw light clay). It is certainly possible to build new walls using these techniques (or cob) around the outside of an existing building. However, you need to make sure that you have an adequate foundation to support the additional weight, and a wide roof overhang to keep the walls fairly dry. Another approach would be to replace your existing walls with slipstraw. You could remove the T 1-11, the insulation, and the sheetrock or interior surface, frame the walls out a bit thicker, and pack straw-clay between the studs. This is a major project, of course, and you would wind up with a lot of waste from the demolition. A final option would be to simply apply a natural plaster over your siding. You could use a clay plaster and possibly a finish coat of lime/sand if you live in a place with lots of wind-driven rain or high snow accumulation. This would improve the aesthetic of the house, making it look more natural, and much more fire-resistant, but wouldn't improve the thermal performance very much. Any of these techniques needs to be done carefully, considering the added weight, moisture concerns, and proper connections between the old and new construction. If you don't have much experience with natural building, you should probably find someone more knowledgeable to help you. Q: I live at the corner of New York and Vermont and have found a stone garage with just a slab concrete floor and would like to renovate it into a home. Could I put a cob floor on top of the concrete floor for added warmth/insulation? Also the walls are thin and need insulation, what about cobbing the exterior, cobbing the interior would reduce living space, or maybe a bit on both side of the wall? Is there another form of insulation I should consider? A: You pose an interesting question. However, you should know that despite it's many excellent characteristics, cob does not make very good insulation. An educated guess would be that a cob wall has an R-value of about 0.5 for every inch of thickness. So to get a standard R-19 wall, you would need to build up cob over three feet thick! That would be a lot of work and would probably eat up a lot of the living space in your garage. You could cob the outside of the walls, but remember that cob walls should be protected with a good roof overhang. Unless the existing overhangs are exceptionally wide, this will mean extending the roof. To reduce the amount of work involved, I would probably recommend using another material that is a better insulator than cob. Straw bales are excellent, but of course they are also quite thick (although bales stacked on edge may give you up to R-30 in about 15" of thickness.) Light straw-clay has a similar insulation value per thickness as bales, but can be made to any width you want. Another option would be to build a wooden frame inside or (preferably) outside the stone walls and insulate it with a non-toxic insulation such as cotton batting (check out www.bondedlogic.com for their product ultratouch.) In that case, you would also have to come up with some sort of sheathing to protect the insulation from weather and critters (plywood? cedar boards? lath and lime plaster?) There are many other possibilities; these are just a few. The same goes for the floor: an earthen floor alone will not give you much insulation. However, you could install a better insulator such The same book (and many others) will help you understand the difference between insulation and thermal mass, which is the thermal property that cob really excels at, and is equally as important for the heating and cooling efficiency of a home. Q: I live in Wisconsin and would love to build a round cob house. Reading your comments on the north wall losing heat in cold climates, could the wall be heated with radiant heat tubes inserted in the wall/mud? Using warmed air I guess from solar heat? A: It certainly is possible to heat a cob home in any climate. My comments were merely to the effect that having a cob wall on the north side of the house, especially on the north side, is less efficient than having a more insulating wall like straw bale. So it will take more energy to keep your home a comfortable temperature in the winter. In a place where the winters are very cold, I would tend to make all of the exterior walls (not just the north) out of straw bale, and use cob mainly for floors, plasters, furniture, interior walls and/or trombe walls. Radiant heating is usually done by running hot water or another fluid through the tubes. Running hot air through them is much less efficient since air has such a low heat capacity. Also, no matter what you run through the tubes you are unlikely to be able to heat it entirely from the sun. When the sun is out it will heat your house directly (assuming you design it using passive solar principles). When you will need the auxiliary heat is primarily when the sun isn't out - at night and during cloudy weather. Q: We can at times get to -14 (Australia) in the winter and 40 at times in summer. We have designed a passive solar home and were wondering how insulating cob would be in the winter here. We were thinking of adding more straw in our cob mix. A: Are you familiar with "R-value" as a system to measure insulation value? I don't know whether it is used anywhere else besides the US. As imperfect (and misleading) as that system is, it is the only one I'm familiar with. Under that system, earthen materials like cob have an R-value of about 0.5 per inch of thickness. Code in most cool areas of the country requires a minimum R-value in the walls of R-19, which would require a cob wall more than 3 feet thick. In contrast, a straw bale wall would have an R-value between 40 and 50. Yes, it is possible to increase the R-value of cob by mixing in more lightweight material such as straw, perlite, or lightweight lava rock. But I doubt that would make a huge difference. I would be surprised if you could bring the R-value up to 0.75 per inch, in which case you would still need 2 feet of cob to give you a moderately insulated wall. By all means try it if you are set on using cob. But in very cold areas I would strongly consider switching to a better-insulating material such as straw bales or straw light-clay, especially for (in your hemisphere) south-facing walls which will not receive direct sun and therefore will tend to stay cold in the winter, absorbing heat out of the home. Q: In a climate with highs in the 90's sometimes reaching 100, and lows in the 0 range, with several "rainy" months during the spring, would COB provide good insulation, or will our heating and cooling cost be high? A: Cob is not a good insulator, but it does have excellent thermal mass. This is particularly effective for cooling (especially if temperatures fall at night.) It is also effective for heating in combination with a free heat source like the sun. If you have long periods of cool, cloudy weather, cob may not be the most appropriate choice for exterior walls. A: In a climate with very hot and cold temperatures, you want to maximize the insulation in your exterior walls (and don't forget the roof!). Straw bale is a great way to do that. To help cool the building in the summer and to store heat in the winter, you will want plenty of thermal mass inside the building envelope, especially in places where the winter sun will enter the building and around heat sources like wood stoves etc. Earthen or other masonry floors are a great place to put that thermal mass, as well as cob interior walls and sculptural elements. You will also be plastering the interior surface of your straw bales with a couple of inches of earthen plaster, which provides a lot of additional thermal mass. You can build that plaster out as thickly as you want if you feel you need still more thermal mass, but I don't see the point of making a whole redundant cob wall system, either inside or outside the bales. Q: I am looking at building an environmentally friendly community in Alberta just outside Calgary. I was wondering if a 3 foot cob wall with a 1 foot slipstraw exterior would be sufficient for heating purposes in our climate? A: There's certainly a long of history of using cob in cold climates, such as Ireland and Denmark. And I have no doubt that a well-designed passive cob solar house with dependable backup heating could be very comfortable in any climate. The question for me really is whether it's the best choice for very cold climates, given the availability of much better-insulating options such as straw bale. My guess is that the 4-foot thick wall system you describe would have an R-value of between 30 and 40, which is certainly quite good. However you could get the same insulation value with a 2-foot thick straw bale wall for much less work. Q: I am interested to know if a strawbale house using cob around the bales would do the trick for insulation in cold, wet climates where mold and mildew can be such a problem. I understand that homes hundreds of year old in England often protect the cob with a stucco cover or some such mix for wind damage as well. I'm thinking, a one foot cobbing on outside and inside of each bale, with a couple of inches - or even less if you recommend - on each side, to seal it? So, what would the structural, insulation, moisture issues be in a cobbed/strawbale situation; and would it be easier to pass inspection? A: I also enjoy thinking about how different natural building systems, such as cob and strawbale, complement each other. The ideal situation for efficient heating and cooling is a thermal mass wall such as cob on the inside of the wall, to store heat, with an excellent insulator (like straw bales) around the outside to keep that heat inside the building envelope. The solution you describe reminds me a system called bale-cob which is being developed and promoted by the Cob Cottage Company in Oregon. My personal opinion is that the system you describe sounds like an enormous amount of work. Essentially you are building a laminate of three separate walls, any one of which would probably be sufficient to hold up the roof. I trust straw bales as a structural wall system, so my approach, in order to reduce labor and wall thickness, would be to build straw bale walls and then apply thick earthen plaster. On the exterior, about 2 inches of earthen plaster protected by a fairly thin layer of lime render should be adequate for moisture-proofing. On the interior, you can build out your earthen plasters as thick as you'd like depending on how much thermal mass your climate and design call for. In most situations, about 4 inches of earth on interior surfaces should be plenty of thermal mass, especially if combined with floors of earth or other masonry materials. But if you want more mass, you can build the earthen plaster out thicker, applying it in 2"-thick layers and letting each layer dry before the next is applied. This is still going to be far less work than building a thick cob wall from the ground up. Don't use cement stucco over either cob or straw bale walls - it can lead to significant moisture build-up in the wall. Lime render, in contrast, allows water vapor to escape from the wall before it can cause problems. One thing to understand about building codes is that they are actually quite fluid. Most codes include provisions for "alternative materials and methods" not specifically described in the code. In those circumstances, the responsibility is generally on the builder to demonstrate that the technique in question will be safe and energy efficient. It can be extremely helpful to have the services of a state-licensed structural engineer who understands natural building. I would be surprised if you could not permit a post-and-beam strawbale building virtually anywhere in the country at this point. Q: I would love to build a cob house with good insulation and simultaneously AVOID a post and beam structure, if possible. If I understand correctly, cob is load bearing, so would it make sense to first build the cob house and then place straw bales to the exterior cob walls for the insulation? Or is there a better choice for insulation with cob? Q: Taiwan has a sub-tropic climate and 3 months of the winter are cold. And there are no furnaces in Asia! Would a natural house be warm enough or warmer than a traditional modern house? A: It's hard to predict whether your house would be warm enough without knowing more about the site and climate. Are the days warm when nights are cold? Does the sun come out predictably in the winter? If the answer to either of these questions is 'yes,' and if the building is sited to catch plenty of winter sun, you should be able to design a house that heats itself well using passive solar principles. If the answer to both questions is 'no,' you may want to consider a better-insulating wall system. Are there straw bales available? If so you could make your walls out of straw bales or "bale-cob," a hybrid system developed by the Cob Cottage Company in Oregon. Of course, if the weather is cold and cloudy and you have no supplemental heat source, your house is going to be cold no matter what it's made of. Q: Is it possible to build a cob house in Maine? And if it wasn't finished before the winter months would it get ruined? A:
There is no reason you couldn't build a cob house in Maine. Cob homes are traditional in Denmark and elsewhere in northern Europe with very cold climates. If you didn't finish by winter, you could either cover the walls with a temporary shelter or else erect the permanent roof on temporary posts which you could remove later. The main thing is not to be still cobbing when night-time temperatures plunge below freezing, or the force of expansion of freezing water in the cob can damage the walls. Q: Thanks for a wealth of well presented information. One type of building not covered is wattle and daub. Assuming materials and labor needed are equally available, how would it compare to load-bearing straw bale? A: (Kelly) I consider wattle and daub a sort of offshoot of cob, and it certainly can create a very solid wall, that could be load bearing, if it were thick enough. The big difference between this and strawbale is the insulation factor; strawbales are highly insulating, where wattle and daub would be much less so. So if you intend to build in any area of extreme temperatures, you would be much better off with the strawbales. Q:
Here's a new idea for insulating floors using styrofoam peanuts and cob -- I'm calling it peanut cob. This idea evolved from my zero energy house design. Scoria (or pumice) seems like the best choice, but many people live far from scoria mines and so it makes sense to look for alternatives. Styro peanuts are readily available everywhere in developed countries. But they will crush under a heavy earthen floor (= lowest cost, most sustainable floor option). So I'm thinking of using 8"-12" layer of peanuts mixed with enough cob to create a rigid base. Q: We recently moved to our new 10 acre property in Datil, New Mexico. We are living in a small off grid 400 sq.ft. cabin that was originally a summer cabin, a wood structure that really isn't too efficient. We are researching straw bale & cob building and are wondering what would be the best decision in your opinion. We are at 7500 feet, it can get very cold at night, warm during the day. Some have told us that cob would be better for us & we like the creative outlets that cob building allows but still not sure about the insulating value of cob verses straw bale building. We will have passive solar exposure so that will help There is no code enforcement here so that is not an issue. Is it possible to convert the outer walls of a T111 sided cabin to cob with pleasing results? Which do you recommend? A:
Straw bale definitely has much better insulating properties than cob. Cob works more as a thermal mass material, absorbing heat (especially from the sun), storing it, and releasing it later. The most effective passive solar design incorporates both insulation and thermal mass. In the high desert, either one can work independently as long as you have reliable sun, good solar exposure and correct design. Q: I was cleaning the kitchen about 15 minutes ago, when I spotted a lone popcorn kernel sitting in an infrequently wiped part of the counter. The corn had been popped a few days earlier via stovetop, so there was no butter or cheese on it, as is the norm with microwaveable, bagged popcorn. Before I tossed it in the compost, I took a close look at it, something I'd never done before. It is really amazing how heating a corn kernel can completely change its composition. I scraped it with my fingernail, then split it in two. It was then that it occurred to me how amazingly similar the white, starchy part is to packing peanuts and many types of foam. My question: have you every tried, or heard of anyone trying, to use popcorn as an insulator in cob buildings? I know that styrofoam/packing peanuts have fairly good insulative properties, and I've seen expanded polystyrene-clay slip mixtures used to insulate window cavities. I read of a pending patent for an insulated, biodegradable food container that would be constructed of ground up "heat-expanded grains," specifically popcorn. Would popcorn have value as insulation if given the right environmental conditions? My immediate concern is that, as it is an organic material, it would be susceptible to decomposition, so it would need to stay dry, or otherwise avoid conditions conducive to composting. I live in a Zone 4a climate, so I have great need for insulation in my to-be-built cob cottage. I will be building bale-cob walls, but have yet to identify insulation for the foundation, roof, and floor. I have access to quite a number of concrete blocks, and I am planning to use them for my foundation stem wall. Do you think popcorn could be used to insulate the cavity between blocks in the foundation. How about using it to insulate the roof, assuming that it is sealed from moisture? If I'm crazy, let me know. A: A fascinating question. I agree that popcorn most likely has very good insulating properties. However, it would not keep them long if exposed to moisture, mice, or bugs. Unless you lived on the moon, it would be hard to imagine a part of your building that would not be susceptible to one or more of these forces, at least occasionally. Specifically, you nearly always get condensation on or in insulation because you have different temperature surfaces on either side. So I don't recommend popcorn for insulating any part of your building, or any other material that can't withstand some moisture. Q: I am considering building a cob house here in the interior of BC Canada, but was thinking about the insulating qualities. We hit -30C in the winter so I was considering using a 4" layer of styrofoam sandwiched between 12" layers of cob on either side. The other thing that I was wondering was the use of either vermiculite or perlite in the cob mix to also increase the R-value of the walls. A: In a very cold winter climate like yours, using cob for exterior walls makes your home less efficient to heat than if you used a better-insulating wall system. You can increase the insulation value of cob a little bit by adding lightweight materials to the mix. You mentioned vermiculite or perlite. I haven't tried those but I have tried light lava rock and styrofoam. I don't think it will get you enough additional insulation to be worth the effort. What you really need is a complete thermal barrier to prevent heat from conducting through your walls. Building a cavity wall as you mentioned could be one way to achieve that. I think it will be challenging technically to build the wall that way, leaving the gap in the middle, but I'm sure it could be done with some experimentation. Frankly, if I were in your situation I would not use cob for exterior walls. I would probably use straw bales for most of them, if they are locally available. You can cover the bales with a thick earthen plaster (as thick as you want) to replicate some of the sculptural qualities that cob has, and at the same time protect the bales from moisture and other problems. Even using bales on their edges will give you an R-value of 30 or more. To achieve the same amount of insulation with regular cob, the walls would need to be over 5' thick. Q: I am in the process of obtaining a residential building permit for cob in Klamath County, Oregon. I only need to submit specs for doors and windows and the thermal mass value or R-value for walls 3 feet thick at the base and 2 feet thick at the top. The interior is 25 x 10 feet with a 10 x 8 bedroom off the back. There will be a fireplace on one end of the house. Can you help with the thermal mass or R-values? A: I have never seen any R-value or thermal mass tests done specifically for cob. However, there have been tests done for other kinds of earthen wall materials. Adobe walls are made of a very similar mix and so I assume that the thermal properties are very similar. A review of the literature gave me an average value of about R=0.4 per inch of thickness for an adobe wall. So I would assume that your cob wall would have an R-value of 14.4 at the bottom (36 inches times 0.4) and 9.6 (24 inches times 0.4) at the top. You could average that out for a rough figure of R=12. In addition to answering your question, I will just note that a 3-foot thick cob wall is a VERY thick wall. It's almost surely unnecessarily thick from a structural standpoint, and building such a thick wall will cause you a lot of extra work! Q: Has anybody built a double-wall cob house so that the house could be properly insulated with a cavity in between the two cob walls? I’ve been through your book and numerous Q & A sites where you have discussed insulating for cob (e.g. straw bale, etc.). But I am not keen on straw bale. In the past I have built a double-wall adobe using a 4 inch cavity between the two walls and filling the cavity with perlite or pumice. The finished wall was 24 inches thick (two 10 inch wide adobe bricks and a 4 inch cavity between the two walls). I’m going to soon start an adobe project in northern Wisconsin and was going to use the same technique; double-wall adobe, but my preference is to construct using the cob techniques you described in your book instead of first putting the labor into molding bricks. I don’t really have the space to spread out and mold a few thousand bricks. Wouldn’t (2) ten inch cob walls, tied together every couple of feet with something such as dur-o-wall work the same? It’s not like the cavity facing walls need to be trimmed neatly, and if they did, could I not frame each cob lift with a 1 x 4 section of lumber and treat a 4 inch cob lift (really 3.5 inches) like forming a long, 10 inch wide adobe brick on the wall? As long as the lifts were woven together, is it really problematic to have only a 3.5 inch lift if I needed to frame the lift with the 1 x 4’s? A: Although I don't know that anyone has successfully pulled this off before, I see no reason why what you're proposing wouldn't work. If it were me, I would make the inner wall thicker (say 10-12") and have that one do the load bearing, and the outer wall as thin as possible (say 6"). (Or you could reverse them; wouldn't make too much difference thermally.) That would reduce the amount of cob you have to mix. Some sort of formwork, as you describe, would be very helpful in maintaining the cavity as you build. The height of each lift shouldn't matter structurally, as you say, as long as all the cob is well sewn together. Wait as long as you possibly can to let the cob dry before filling the cavity (several months at a minimum to avoid excessive moisture being trapped in the insulation cavity) and capping over the top. Take lots of pictures, and please let me know how it goes. This could be a major improvement for cob buildings in cold climates.
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Michael
G. Smith
has a background in environmental engineering, ecology, and sustainable
resource management. In 1993, along with Ianto Evans and Linda Smiley,
he started the Cob Cottage Company, a research and teaching group focused
on reviving and improving traditional forms of earthen construction. He
is the author of 
