Issues with Aerated Autoclaved Concrete (AAC)

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 am going to build a small (1000 sf) passive solar home in up-state new york, and I am considering autoclaved aerated concrete. I have been finding conflicting advice about the insulative properties of this material for use in cold climates. Can you offer any advice, or a non-biased resource?

A: I also have heard conflicting opinions about the insulative value of this material. Some even claim that it serves as thermal mass as well. I have known people who built their homes with AAC at over 8000 feet in the Colorado mountains who were quite happy with the comfort of their homes. My recommendation would be to go with the thickest walls that you can afford for maximum R-value.

Q: Would AAC be applicable for earth sheltered construction?

A: (Kelly) I see no reason why not to use AAC for earth-sheltered construction. They claim that it does not wick moisture, but as a precaution, I would suggest using an additional moisture barrier in any bermed locations. With its insulative qualities, I would think that AAC would indeed be a good choice.

Q: I'm looking for a product that: 1. as a skilled owner-builder I can stack myself, 2. will provide greatest insulation bang for the buck, and 3. I can apply stucco and plaster to directly, avoiding additional steps/cost. I'm building a 2200 sf home near the coast of Maine, two stories, about 500 sf of glazing to the south. Of AAC, Durisol, or Faswall, which of these technologies is best for this application?

A: (Kelly) I have seen blocks of AAC being assembled into a building, and I would think that a reasonably skilled person could handle the task. With a large band saw equipped with a carborundum-tipped blade, you can saw these blocks to fit most needs, and with a similar bit in a router you can make channels for some wiring or plumbing. This material is more exacting than ordinary bricks or cement blocks to work with, because it is usually mortared with a thin-set, like tiles, and the joints are more precise. It also has the advantage of being a moderately good insulator, especially with the larger blocks, but even these might only provide R-16 at the most.

I would say that all three of your choices provide moderately good insulation (although not nearly as good as strawbale or some earthbag structures) and would be quite durable and maintenance-free over time. All three can be plastered directly, as far as I know. The ICF's would certainly go up faster and be easier to assemble, however they often require concrete pumpers to fill the voids. I don't know about comparative costs; you'll have to pencil that out based on your local suppliers.

Q: I am going to build a three story house in Florida next year. I am looking at ICF with poured concrete floors and roof. I understand your problems with ICF and the placement of the thermal mass. My question is how would using ACC block on the interior walls instead of sheetrock do to that argument.

A: (Kelly) I consider AAC to be  more of an insulator than thermal mass, so it would add more to the insulation of the walls than to the thermal mass. This would not necessarily be a bad thing, especially if you incorporated some other thermal mass materials in the interior. If I were going to combine ICFs with AAC I would think of putting the AAC on the exterior.

C: Hello from San Miguel de Allende: I agree that the usual brick-and-concrete construction tends to be clammy during the winter and retain heat during the summer. New Contec blocks, made of aerated concrete are become more common. They are made in Mexico and have much higher insulation values.

Q: I am physically building a passive solar Irish cottage using AAC block (http://brigidswood.blogspot.com/). In researching AAC block I discovered conflicting opinions as to whether it was a thermal mass or an insulator. I knew it couldn't be both and I eventually concluded that it was up to me to decide which property to exploit. I decided to proceed as if the AAC block was an insulator. My question is this: "do you see a problem with putting foil/foam/foil (http://www.insulation4less.com/ ) between the AAC block and my interior sheet rock to increase the R value of the wall while taking advantage of the reflective properties of the foil?" It's a relatively cheap "add on" and would increase my wall's R value from approx R-16 to R-30. I wasn't sure about moisture problems, etc.

A: (Kelly) I think that what you propose is a good idea. I wouldn't worry about the foil inducing moisture problems, since it will be on the inside and should not sweat, and the AAC wall is not a breathable wall either, so it should be fine. I think that you are right that AAC is primarily an insulator. Your sheetrock interior will provide some thermal mass, but you might think about adding other mass elements into your interior design.

Q: I have come across a problem of the cracking of plaster and the blocks. What are the reasons for the cracks? Is it the poor quality of blocks or is it due to the improper masonry practices? I have also heard that the blocks have high water absorption and drying shrinkages. Does that play any role in the problem mentioned above?

A: If your block has water high water absorption, then you need to wet the wall before you apply the stucco. Also, cracks in the stucco could be from two sources: 1-Lack or steel mesh in the stucco. 2-Lack of control/contraction and expansion joints in both the masonry and the stucco. If the wall cracks, the stucco will crack. So, make sure that the masonry wall is not the source of the cracks. Please read this publication on stucco: "Plaster/ Stucco Manual by Portland Cement Association" or other available texts.

Comment: responding to the AAC Q&A --- we actually built at 7500 ft in Colorado and the north side of the house is FREEZING. We used thickest blocks. A real lesson. Don't tell people it's ok! We're still trying to figure out what to do!

C: I'd like to add something to think about ....You said "Michael claims that AerBlock is the world's most environmentally green building material, but I wouldn't say that." While I think your contention is first order correct, if you add longevity to the equation AAC really shines. It promises an extraordinarily long life with low maintenance requirements. Many of the (initially) lower energy building systems won't hold up over the long term like AAC.

Q: We are constructing a building in India with AAC block and it has developed a crack on the wall after 2-3 months. What is the reason and cause of this?

A: (Kelly) I am not a specialist in AAC construction, but the most common reason for cracks in masonry walls is settling or shifting of the foundation. The slightest change at this level can cause a crack to appear in the wall. Investigation by a local engineer will probably reveal the truth.

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