Tiny Tuesday: Consider Aerated Concrete

recent Boston Globe feature describes a luxury home built with autoclaved aerated concrete (AAC), the first in New England. The material comes in a lightweight masonry block, giving it the flexibility to create unique wall shapes and angles. AAC is a boon for indoor air quality, as it contains no volatile organic compounds (VOCs) and provides excellent air tightness. It’s also fire- and mold-resistant. So why hasn’t AAC caught on in America to match its popularity as a high-performance building material in northern Europe? In short, why have you never heard of it?

AAC3

The first New England home built with autoclaved aerated concrete.

Before we answer that question, let’s look at what AAC actually is. AAC is manufactured by mixing sand with a binding agent (such as cement, fly ash, lime, or some combination), water, and a tiny amount of aluminum powder. Unlike regular concrete, it contains no large aggregate like gravel. Instead, the aluminum powder reacts with the other ingredients to form hydrogen bubbles which greatly increases the volume of the mix. That’s the “aerated” part. The mix is still soft at this point, so it’s cut into blocks and placed in an autoclave, or pressure chamber, until it achieves its full strength.

The result is a block that looks like pumice and weighs about the same – much lighter than regular concrete by volume. And unlike a CMU, this block is solid all the way through. In theory this property makes the block self-insulating.

AAC2

Building a wall of AAC blocks.

AAC’s insulating properties are disputed, though. A Green Building Advisor article states that the R-value of an 8-inch block is only R-8 to R-11. (Compare 8 inches of dense-packed cellulose which is about R-24.) Other issues include moisture and water vapor – both can readily permeate AAC – and structural integrity – there’s no space for reinforcing bars which could resist extreme wind.

Add that to a 15% cost premium over stick-frame construction, and you can understand why American builders show little interest in the material. But consider AAC for yourself and decide whether the health benefits are worth it.

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