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.