Cob and Insulation

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 The Cobber's Companion: How to Build Your Own Earthen Home (Cob Cottage Co., 1998) and co-author of The Art of Natural Building: Design, Construction, Resources (New Society, 2002) and The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage (Chelsea Green, 2002). He teaches practical workshops and provides consultation to owner-builders on a wide variety of natural building techniques, site selection, and design. He lives in an intentional community in Northern California. His informative website is www.strawclaywood.com.

Questions and Answers

Q: I'm living in Tucson, and my yard is definitely made primarily of clay. Last summer, I just about died from the heat coming into the house, which appears to mostly be uninsulated. I am interested in using cob to insulate the walls on the outside, but, I have not been able to find any information specifically about this area of Arizona and the use of cob nor about using cob on pre-existing walls. The walls were painted, last year. Would I have to blast the paint off first or would it be okay to plaster cob directly over the paint? Would I need to use something like chicken wire to help hold the cob on?

A: There is a fair amount of understandable confusion about what different natural building terms mean. The term "cob" is most accurately used to describe a wall system made of a mixture of clay soil, sand, and straw, sculpted in place while wet. Cob walls are generally between 8" and 3' thick and most commonly support the weight of the roof that rests on them. If you are taking a mud mixture (which could have almost the same ingredients as a cob mix) and sticking that onto a pre-existing wall which does all the structural work, I would not call that cob but an earthen plaster.

Another common source of confusion is the difference between insulation and thermal mass. Insulating materials are light-weight and non-conductive, which makes them resist the passage of heat through them. Thermal mass materials are heavy, and can absorb and store a lot of heat. They heat up and cool down very slowly, but heat applied to one side will eventually make its way through to the other side. Earthen materials like cob and earthen plaster are high-mass materials with poor insulation. You can get good thermal results by using them in appropriate ways in a suitable climate, but you need to understand how they work.

If you were to cover the outside of your walls with a thick layer of earthen plaster (or build a cob wall adjacent to your existing wall) it would indeed reduce the amount of heat getting into your wall. It would do that by absorbing heat on the outside surface, holding it until night time, and then re-radiating it out into the night air. To be effective at cooling your home, you would want a very thick layer. The ideal would probably be 12" thick. You would want at least 4" before you would notice much of a difference. Either a 4" layer of earthen plaster or a 12" cob wall attached to the outside of your house is possible, but it would be a big project. It would add an enormous amount of weight to your structure, which may very likely not have been designed for that. You will probably have to widen the foundation to support all that extra weight. You may very well also have to extend the roof overhang to protect the earthen exterior from rain.

Here are some additional strategies to consider for cooling your building, which may have as much or greater effect for less effort than wrapping your house in cob or earthen plaster:

1) Insulate the roof and color it white or silver.
2) Extend your roof overhangs to keep sun off your walls.
3) Add exterior shutters on your windows, especially on the west and east (and south if you have no roof overhang) and keep them closed when you want to keep your home cool.
4) Build porches and/or arbors to shade your west and south walls.
5) Plant a fast-growing shade tree close to your house on the southwest, west, and/or south sides.
6) Irrigate around your house. You can use graywater for this.
7) Insulate the walls, and replace single-pane windows with thermal windows.

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 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: 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 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: 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.

Q: What kind of insulation would you recommend for the roof?

A: For insulation in the roof, there are many options. One of my favorites is the recycled cotton batting called Ultra-Touch, which has been available in California only for the last couple of years. For other options, check out the chapter on roofs in "The Hand-Sculpted House" or the section on natural insulation options in "The Art of Natural Building."

Q: Someone asked you about a double wall and now I am wondering if would it be plausible to fill the gap between the walls with the eco pink spray foam insulation? My husband tells me that it's water-proof and won't rot, it can also be recycled. If this was in between two cob walls that you described, would it let the cob wall breathe enough? If combined with radiant flooring, south facing Trombe walls and a good sized fire place do you think it could realistically heat to -30c? (I live in Southern Ontario.)

A: I'm not familiar with the foam product that you mention. There's a good chance it would work. My main concern would be that the foam would compromise the breathability of the wall. Perhaps you could call the manufacture and ask about vapor permeability of the material. Another question would be how does the foam get installed. Would you simply build the cavity, wait for the cob to dry, and then be able to fill the cavity completely from the top? How would you reach under windowsills and so on?

I have little doubt that something like what you describe could be done, and that it would improve the thermal performance of cob walls a great deal in a cold climate like yours. You would have to work out how much thickness of foam you would need to get the desired insulation for your cold winters. Personally, I have a strong preference for building with natural, low-embodied-energy materials. I believe I could get excellent thermal performance and many of the same sculptural qualities of a cob wall by using straw bales with a thick earthen plaster on both sides. I believe this approach would be simpler, less work, and more environmentally friendly than a cob cavity wall filled with foam.

More experimentation needs to be done with cob cavity walls. Many people have been interested in trying something like this, but so far as I know there have been few actual attempts.

Q: We are planning to build a cob house in south western Missouri. The climate here is very humid year round and it can get very cold in the winter, sometimes below zero but usually between 10 and 30 degrees. Summers can be around 95 for a high but it doesn't cool off much at night. My question regards insulation. Because of the high humidity we cannot use cob/bale construction due to mold issues. I have read about a suggestion of mixing perlite with the cob to add insulation but I'm positive this will not add enough without compromising the strength of the wall. My solution is to have a one and a half to two feet thick wall on the inside and a thinner wall on the outside with a six inch void between to be filled with expanded perlite. This would give us an R value of around 15. My concern is whether the structure will still be able to 'breath' through the loose filled perlite. And if it cannot, will this cause mold problems? I do not want to lose the benefit of breathing walls but insulation is a must.

A: For your climate, insulating the cob walls of your house is an excellent idea. The solution you suggest will probably work, and I don't know of a better one. I don't think the perlite infill will compromise breathability or cause any mold problems. My only concern with the perlite fill is that it could settle over time and leave an uninsulated cavity at the top of the wall. This is a common problem with loose fill insulation.

I have a couple of other suggestions. First, if you're going to take this approach, why not increase the width of your cavity and get yourself a bit better insulation. R-15 is really not very good for your climate. The two feet of cob might give you another R-10 or so. Ideally, I would be looking for something closer to R-30, which you can get just by increasing the cavity width to 8". Depending on your design, you can probably reduce the thickness of the load-bearing cob wall to 16-18", which would allow you a bit more insulation without increasing the total wall thickness.

It would be good to tie together the inside and outside cob walls so they can't separate over time. One solution, if you build the 2 walls up at the same time, would be to lay some wire fencing (something like deer fencing with thick wires and large openings, not chicken wire) across the full thickness of the wall about every 2'-3' as you go up as a bonding layer.

I think your proposal will work. It is a solution I have heard proposed many times, although I don't know if anyone has actually followed through with it. If you do, please let me know so I can pass along whatever you learn to others. The biggest drawback of the approach is likely to be simply that it is a great deal of work to build a wall like this. As an alternative, I do believe that straw bale will work in your climate. I know of several straw bale buildings in Missouri that are doing fine after many years. If I were building a home in your area I would almost certainly use straw bales for some or all of the walls, since they provide better insulation with less work than a cob cavity wall. With a thick earthen plaster applied over the bales, you can get a similar look and feel as you do with a cob wall.

Q: I have just inherited a cottage in Ireland that was built in 1939. The external walls are 500mm thick random rubble sandstone. I would like to insulate them internally using a natural material. I have enough room for up to 100mm thickness of insulation. Would cob be a suitable material to use and should I use plywood shuttering to support it while it is drying or is there a better material you can recommend?

A: I'm afraid that you will find almost no difference in comfort by adding cob, as it is not a good insulator. Actually, the best way to improve the comfort of this home would be to insulate it on the outside, although I realize that this may not be practically or aesthetically possible. Better insulation would be light clay-straw or an industrial panel of insulation. Actually, you could first insulate with the industrial panel, and then cover that with an earthen plaster and have the same look as your cob solution.

I don't want to use PIR insulation boards or panels or any such like. I am not really interested in the aesthetics as much as I am the livability and health of the house. I am also considering hempcrete but it's fairly pricey. Are you saying the house will be as comfortable with stone walls as it would be with cob walls? Does stone have a comparable thermal mass?
Hempcrete people recommend lining the walls either inside or outside to improve thermal mass and heat loss.

(Kelly) Cob and stones are both considered thermal mass materials, with stone being even more dense than cob. In a well designed home that employs passive heating and cooling concepts, the thermal mass is most useful on the inside where it can store and regulate the interior temperature. The insulation on the outside is there to isolate the thermal mass from the fluctuating temperatures outside. So using a lightweight hempcrete on the outside would be the best in your situation.

Q: My husband and I are considering to build a Cob house in North-east Alabama. My main concern is that the building will overheat in the summertime since the nighttime temperatures often stay in the 70's for extended periods of time. In addition to that our area has high ground water levels which makes building underground unfeasible. What would be the best way to make sure the house stays cool in the summertime?

A: There are many things you can do in the design stage to help keep your house cool. An important one is substantial roof overhangs/porches to keep the sun off the walls as much as possible. Another is to limit windows on the east and west walls. But even if the cob is in the shade all day, it is going to tend over time towards the average daily temperature. So if you have a week of nights in the mid-70's, say, followed by days in the mid-90's, at the end of that time the cob will be about 85°, which is warmer than comfort level. Hot humid climates like yours are by far the most challenging for passive cooling. You will probably need to use mechanical cooling to keep your interior temperatures in the normal comfort zone. It will be more efficient than in a mass-less house since you can run your AC at night and use it to flush heat out of the cob at that time. Another thing I'd recommend is a screened sleeping porch where you can take advantage of the cooler nighttime temps and breezes to sleep more comfortably. This could be part of the shading system for the cob walls of the house.

Q: I am living in Southwest Colorado in the four corners area high desert. It is extremely dry and has a ton of sun, but winters can be very cold and bring a lot of snow. I am interested in building a cob house, but from what I am reading, they do not have great insulation. Would cob/strawbale combination be best? Adobe?

A: Adobe blocks have very nearly the same thermal properties as cob. For your climate, I would recommend straw bale exterior walls and cob, adobe or other heavy materials for interior thermal mass. This can be in the form of interior partition walls, built-in furniture, floors, and/or plasters. The most important things if you want a comfortable, thermally efficient home are a very well-considered passive solar design and excellent detailing to prevent excessive air leakage.

Q: I am from India. What insulation do you suggest for my place, Temp from +47 degree Celsius to 2 degree Celsius. Humidity goes to 85% and rainfall abundant, heavy for a fortnight.

A: Insulation is a complex topic. With the heat you experience in the summer, keeping the building cool is more important than heating it in your relatively mild winters. If the building is to be enclosed (no uncontrolled airflow between inside and outside) you will want a lot of lightweight insulation in the roof. I don't know what material to recommend because I don't know what's available in your region. Second, design the building in such a way that there is no direct sun on the cob walls in the summer. This can best be achieved with a wide roof overhang such as a vernada or wraparound porch. Wide roof overhangs also protect cob walls from weather, which is one of the most important factors in building a durable earthen house.

Q: I was recently looking into hempcrete and it looks like an incredibly sustainable building method and I also found that it has excellent insulation properties and I was wondering whether at all hempcrete could be used in combination with cob as some have combined cob and straw bale building for increased home insulation. If so, would you use hempcrete as the ‘inner layer’ in between two layers of cob? As hempcrete is usually used with timber framing as a support would that be unnecessary if cob and hempcrete were used in conjunction?

A: I’ve read quite a bit about hempcrete but have very limited hands-on experience. I’m sure that cob and hempcrete could be used together to make a high-performance hybrid building. There are many ways this could work, and some experimentation would be necessary to find the best techniques. The cavity wall you’re describing is possible, but I think it would be a lot of work - essentially building 2 separate cob walls and insulating between them. I have heard this or similar ideas proposed many times and I’m not aware of anyone who’s actually done it successfully with cob, although I have seen it done with adobe block. Depending on the design of the building, there might be a way to attach a layer of hempcrete to the exterior of a cob wall with minimal additional framing. Or you could use hempcrete for some walls of the house and cob for others. Or you could build the walls out of hempcrete in a fairly standard way and then add a thick sculptural layer of earthen plaster to the inside and/or the outside. It all depends on your design, goals, and available materials.

Q: I am interested in building a cob house in a cold climate. I want to use propane to power my stove and heat my house. I can’t find any information on installing gas heating systems in a cob house. Can you think of any problems that might come from using propane?

A: I don’t see any particular concerns about heating a cob house with propane. However, I do have concerns about building a cob house in a cold climate. Cob walls have a very poor insulation value. Unless you find a good way to insulate them, you will lose a lot of heat through your walls during your cold winters. This will require you to burn a lot more fuel to keep your building warm. I don’t consider this either economically or ecologically wise. Solutions? Either figure out a way to insulate your cob walls, perhaps with the addition of a vapor-permeable material such as hempcrete or straw-clay to the outside of the wall. Or use a better insulating wall system such as straw bale for your exterior walls, and save cob for interior walls and sculptural elements.

Q: I have a 40 x 60 x 22 (height) hay storage shed. The structure is very solid. I would like to use the structure as a frame for a 2 story COB Home. I like the thickness of the COB walls for insulation and ease of keeping that type of structure cool and easy to heat in the winters. Does using a structure like my hay barn provide a good structural foundation for COB built walls inside and out?

A: Cob walls actually have a very poor insulation value and excellent thermal mass. If you live somewhere where the weather is fairly mild year-round, cob can be a good choice for exterior walls. If you live where winters get very cold and/or summers get very hot, you should consider using a better-insulating wall system (such as straw bale or light straw-clay, for example) for your exterior walls. Cob can still be appropriate for interior partitions, built-in furniture, etc.

To be more specific, the R-value of a cob wall varies with its density. A standard cob mix with a lot of sand in it has an R-value between 0.2 and 0.3 per inch of thickness. We have been experimenting with lightweight mixes using lots of straw and little or no sand, and in some cases we can get the R-value up as high as 0.5 per inch. Nobody in this country that I know of is building 3-foot-thick cob walls. The thickest are about 2’ and 16” to 18” is more common. A 2’ thick wall with best case R-value of 0.5 per inch would only give you R-12, which is not sufficient for your climate. On the other hand, you can easily get R-30 or more using straw bales.

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