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
as styrofoam packing peanuts. The effectiveness of these various approaches will be conjectural until someone undertakes a systematic testing program. Several cob builders in different areas of the country have successfully obtained residential permits for cob houses. The details vary from case to case. In your area, I suspect the most straightforward approach would be to start with existing codes for adobe, and ask your local officials to adapt them as necessary.

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
as rigid foam (or a natural water-resistant insulation like pumice) and then put an earthen floor over that. Do watch out for drainage: you will need to be sure the slab is well drained or moisture can get into the the earthen floor and into your living space. You might even consider installing radiant hydronic heat in the floor to heat your new living space! For more thorough instructions on installing earthen floors, check out the book "The Hand-Sculpted House" by Ianto Evans, Linda Smiley, and myself.

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.

Q: What are your thoughts about using a hybrid cob and straw bale exterior wall in climates like Oklahoma that have both hot and cold temperatures for extended periods of time? Would you recommend a using a cob-bale-cob sandwich? If so, how thick should the cob portion be? Where could I find more information on a hybrid system like this?

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?

A: Ianto Evans at Cob Cottage Company in Oregon has developed a system he calls "bale-cob" which consists of straw bales set into a matrix of cob, sort of like mortar. I don't know what the load-bearing capacity of that system is. You could call or write Cob Cottage for more details. Usually for situations in which good insulation is needed in exterior walls I recommend a straw bale wall. You can certainly make them load-bearing almost as easily as cob. Then you can use higher-mass material like cob for interior walls, floors, built-in furniture, and the like. The system you describe would most likely perform well, but sounds to me like an enormous amount of work. And your foundation would have to be very thick. I think there are simpler solutions.

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.

Please keep in mind, though, that cob walls offer quite poor insulation for such a cold climate. I would not recommend building a home for winter use with all cob walls. Your building will be much more efficient to heat if the exterior walls are made of a better-insulating material like straw bales. Cob can be used for thermal storage and sculptural elements on interior walls, hearths and so on.