Q: We are building in a cold wet earthquake zone climate, and are experimenting with a blend of mineral wool (for insulative value and additional strength, pumice and concrete. Straw bale sized lightweight structural concrete. As pumice absorbs water, we are concerned about frost/thaw cycles, and massive rainfall levels in general. Also, how would the cost compare to regular concrete?

A: (Kelly) Sounds like an interesting mix. I wouldn't worry too much about frost-thaw cycles with pumicecrete, since it has been used for many years in extreme climates without deteriorating. Any moisture that is retained in the aggregate would have plenty of room to expand. I would try to keep the walls as dry as possible, though, to enhance their insulating ability. As for cost comparison, I would say that pumicecrete might cost a little more than ordinary concrete, because of the need to buy the pumice, which will likely cost more than the sand/ gravel mix.

Q: I have a house built with 12" scoria, cement, sand walls, do you know what the r value would be for this type of wall?

A: (Kelly) I built my house using scoria in earthbags, and then covered with papercrete; the walls are about 16 inches thick, and are extremely well insulating. I would guess that the R-value is around 40, but this has not been scientifically tested. This guess is based on the performance of the house, which seems similar to a straw bale structure. A 12" pumicecrete wall would certainly be less than this, because it is thinner, and because the cement that binds it is not insulating. The insulating value of such a wall would depend on much cement and sand is used to bind the material together. The more entrained air in the composite, the better the insulation. I would guess that your wall is somewhere between R-10 to R-20.

Q: What are the advantages/disadvantages of using unexpanded perlite? I am referring to perlite that has not been expanded by heat. I am
wondering about the load capacity, general strength etc. i.e. could I reliably use it for building walls or floors.

A: (Bruce) Unexpanded perlite is used in the foundry industry, and for some other very industrial applications, but not normally in any construction applications, or horticultural ones. Unexpanded perlite could be used---although normally it would be much more expensive than regular sand so it isn't used. The only possible problem with using it would be it's tendency to "expand" during a fire---but hopefully by the time the fire reached the expansion temperature, people would no longer be near the building.

Q: Is there a difference between lightweight concrete and lightweight insulating concrete? I am considering the lightweight insulating concrete for a flat roof instead of steel and foam insulation, is this a good idea and how can I get more information on who produces this product?

A: (Bruce) There are many forms of "lightweight" concrete using both lightweight aggregates like expanded shales and clay and/or pumice to what might be called ultra lightweight aggregates like perlite and vermiculite. (See ASTM C-332 for a listing of the different types.) Usually what is called "insulating concrete" would only be concrete made with perlite or vermiculite, and not necessarily with any of the other much heavier lightweight aggregates.

Q: I saw an article in the NY Times about architects commissioned to design green houses to rebuild New Orleans. One of these homes was designed to float in case of a flood and its foundation was said to be made of lightweight concrete. Do you know how this would be done?

A: (Kelly) There are many formulations of lightweight concrete, with the main variation being the type of aggregate that is used. In some cases, such as with styrofoam beads, the concrete could actually float.