Issues with Shotcrete Systems

Dr. Nabil Taha has over 27 years of structural engineering experience. Prior to opening his own engineering firm in Oregon in 1997, he was a Professor of Engineering at Northern Montana State University and at Oregon Institute of Technology. He has structural expertise in a wide range of building systems and can answer questions related to virtually any common building method. His focus is on green design and he is always willing to trying something new. Dr. Taha is dedicated to future sustainability through innovation; he creates solutions for beautiful sustainable and safe structures by melding old and new technologies. He loves a good challenge. He is Licensed in twenty three states and can design buildings and/or consult to assist with structural permitting in these states as well as internationally. As a prior College Professor, Dr. Taha is a teacher at heart. He loves to share his knowledge and offers educational seminars and trainings for the do-it-yourselfers and professionals alike. Dr. Taha's goal is to continue to grow and provide knowledge and services for those trying to make their dream project a reality. No project is too big or too small. For information about Dr. Nabil Taha and his engineering firm see www.structure1.com

Questions and Answers

Q: I really like the greenhomebuilding website and I have learned a lot from it. I have been doing some brainstorming and have come up with a design for a 24 ft. octahedral dome roofed house constructed of shotcrete sprayed, wire mesh/expanded polystyrene panels. The first floor has conventional vertical walls and the second floor has virtually no vertical walls as it enclosed by the dome. I would like to send you a small model and a few photos of the larger model that I have built.

A: (Kelly) Thanks for sending the model and information about your interesting design. I can see that you have put a lot of thought into it. I have always been fascinated with domes and geodesics, and know that they are not really as simple as they appear. I particularly like your expanded, larger design, because it presents more opportunity for passive solar heating.

Your suggested material (shotcrete over wire mesh and EPS) for making such a structure would have the advantage of being fast to erect, durable over time, and a good balance of thermal mass and insulation in the proper locations. On the other hand these materials are all highly industrial and must be transported to the building site, which means more pollution and energy use than simpler, more local and natural materials. Also, these materials would not necessarily be affordable, or even available, in many third world countries. Shotcrete systems require a great deal of specialized equipment and expertise to operate. If your goal is to provide inexpensive housing for the very poor, I'm afraid that this approach might not work.

I encourage you to pursue building such a house for yourself. I see no reason why it wouldn't make a good, strong home, and you would learn a lot about the practicality of it all. Most areas of the country have swimming pool contractors who can do the shotcrete work, and a contractor familiar with erecting the panels should be able to follow your design. Go for it!

Q: You mentioned that a concrete house must have the insulation on the inside to work well. What do you think of the monolithic domes? Are they truly the strongest home you can build and are they well insulated? How do they compare to the geodesic? And I was talking to a man who builds for habitat for humanity here in Florida and he claims his pressure treated termite treated wood houses will do better in a hurricane than concrete because they will give I had always heard a wood house was the worst in a hurricane what do you think? This man also mention aerocrete as being very strong do you like that product? I have decided to look seriously into alternative housebuilding no matter what other people think even if the house looks different if its a well built home someone will want if it comes time to sell.

A: (Kelly) Monolithic domes actually do perform pretty well, because the foam insulation is on the OUTSIDE. They are extremely resistant to any kind of weather or shock, probably more so than any wood frame building. Geodesics are also very strong, but are not particularly easy to build and are prone to leaks. Autoclaved Aerated Concrete (AAC) is an interesting product that is very durable and also fairly insulating. It could make a good house, but it takes extreme precision to use right. Neither Monolithic Domes nor AAC are particularly benign environmentally. They are both highly industrial in manufacture and require considerable transportation of materials, but once the houses are built, if designed right, they can be quite energy efficient.

Q: I think it sounds like a monolithic dome could be a good choice for Florida but there is one last concern: the clean air environment. I wonder if it outgases any chemicals into the inside air or if its clean to breathe? Its hard to balance using natural materials and new strong useful man made inventions how do you know what is best?

A: (Kelly) I wouldn't expect a Monolithic dome to outgas into the interior much, once the concrete is thoroughly cured and plastered or painted or whatever. The urethane foam insulation on the outside could outgas for some time, but it is usually sealed between the cement and the airform membrane. This is a question you might ask the Monolithic Dome people. Choosing the right materials for you is a very personal thing, depending on your sensitivities, commitment to using only natural materials, budget, etc.

Q: I am considering the Monolithic Dome but am confused. The second method said " In this case the polyurethane insulation would also be sprayed on from the outside, with some sort of protective covering for durability" wouldn't you put the foam on the dome from the outside before you put on the rebar and then spray the crete over rebar and foam???

A: (Kelly) I don't think so, because you want the concrete on the inside for thermal mass, and the insulation on the outside to keep that mass from leaking energy.

What is says at the website is: The second of Monolithic's methods of building uses a similar inflated balloon, but the rebar is tied outside of it and the shotcrete is applied from the outside. Once this has set up, the fabric can be deflated and removed for reuse. In this case the polyurethane insulation would also be sprayed on from the outside, with some sort of protective covering for durability.

But then the Monolithic website says: There are many differences between an EcoShell and a Monolithic Dome, but the main difference is that an EcoShell I is constructed without the use of Polyurethane foam. These make good garages, storage sheds, and housing in equatorial climates. The EcoShell II Airform is made out of tougher material than a Monolithic Dome airform, so that it is re-usable, but construction of the EcoShell II is done from the inside as opposed to the outside, so the air pressure that needs to be maintained is not quite as high as is needed during the construction of an EcoShell I.

My comments were based on witnessing a friend building a dome with one of the ecoshells, and I believe he did it like is stated at my website. In any case, the Monolithic folks are the real authorities on this. It might be that in Baja at your site the insulation would not be as necessary...depends on the climate.

Q: I am just starting to plan the design of a new house that will be built on the Oaxacan coast in Mexico which is a big earthquake zone. Currently I am looking at using the foam panel system that is available in Mexico (and a bit in the US www.triditec.com) and while the system creates a much lighter structure that the normal post and beam concrete design used locally I wonder if using lightweight concrete would not be even better. I guess the big issue might be the compressive strength and moisture (salt) absorption problems. There is of course the issue of where to get a lightweight aggregate (normal graded aggregate is not even available). Comments?

A: (Kelly) I would advice you to stick with the standard installation of these panels using concrete stucco, for a couple of reasons: 1) the strength of the wall system has been tested and proven to be adequate structurally (especially important in earthquake country) and 2) having the concrete on the inside would provide much better thermal mass where it is useful in moderating indoor temperatures, whereas the lighter-weight concretes are not as good in this respect.

Q: My wife and I are dreaming of building a "modern style" house with smooth/shiny bare concrete exterior walls looking like those of Tadao Ando's buildings. We suspect that the techniques required to build buildings with bare concrete walls exist only for commercial buildings and may be unfit or unaffordable for residential constructions. Do you know if there is such a thing as a system that would combine ICF blocks (for the inside and for the insulation) and traditional forms for the outside side of the walls and the look?

A: (Kelly) I suggest that you look at the shotcrete sandwich panels. The exterior cement can be finished as smoothly as you want, and you have the advantage of the insulation sandwiched between cement on both sides, which has much better thermal properties than convention ICFs.

Q: I'm trying to find a dome home kit or builder to construct a spider type design dome home as the main center is the living room and kitchen with other rooms branching from the center main room like spider legs; no upper or lower level, has concrete exterior with an R-36 insulation and no wood structure but metal. No wasted space. Reason for this design for disability movement. Any leads about this?

A: (Kelly) What you describe resembles the Monolithic domes described at here; these are somewhat kits, in that you can purchase the air form and many components directly from these folks. I am sure they would advise you about contractors who have experience with their system, which is technologically complex. They use spayed urethane insulation over concrete for insulation. It might be possible to use a more insulating lightweight concrete as well, but to gain R-30, this shell would have to be very thick. You might get Monolithic's opinion about this.

Q: I am building an addition to my home (in the SF bay area). After doing a lot of research I have become very interested in the EPS/wire mesh/shotcrete or stucco panels. However, I was a bit discouraged after speaking to a local shotcrete expert. He said it was much more expensive than conventional wood framing and getting outside corners and inside edges done correctly is difficult. My questions for you: Do you have any advice on reducing the time/cost of applying shotcrete to an EPS panel system? How do we make the inside walls look normal - like stucco or drywall? I think one layer of shotcrete would be less expensive than 3 layers of stucco, but are there more issues with cracking and appearance? What other big issues am I missing?

A: (Kelly) I suggest that you get some second (or third) opinions from local contractors familiar with this technology. It may be more expensive than wood framing, but then you would be getting a much more durable wall that performs better thermally. An experienced finisher of these systems should be able to answer your questions about how to get a good finish. In general, concrete can be finished to as fine and smooth a luster as you might want, although it would probably be easier, and perhaps less expensive, to go with a rougher textured finish. If the wall is finished properly there should be no cracking...again ask the professionals.

C: Have you looked at the MONOLITHIC DOMES? Their website is www.monolithic.com. I have built one, and I would never consider living in anything else, especially after going through a tornado and a forest fire.

Q: I am currently having a house built using foam panels that are about 7.5" thick with 2.5" of shotcrete sprayed on either side. My concern is that there are gaps, not overly large, but still gaps where daylight can be seen, in the foam panels where the layers of shotcrete have not yet been sprayed. The Builder said gaps would be foamed in, but there are still some gaps. He now says any gaps will not effect the integrity or R-Value of the walls. Can you confirm?

A: (Kelly) It is certainly better for there to be no gaps in the insulation, as this provides a way for cold or heat to pass through to the shotcrete layer, and thus to your interior space. That said, the tiniest hairline gaps that might allow sunlight to pass will not much affect your thermal performance. Anything larger than this should be foamed, and this is easy to do with today's foam products. You could do it yourself with a can of spray foam from your local hardware store.

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