As Steven Knapp and his wife plan a new house in Atlanta, indoor air quality (not energy efficiency) is at the top of their priority list. At least that’s how a recent discussion on autoclaved aerated concrete began.
“We both have chemical sensitivities that make ‘usual’ building practices undesirable,” Knapp writes in a Q&A post at GreenBuildingAdvisor. “Before the market bottomed, we were planning to build a healthy house following Baubiologie practices (as much as practical) and using autoclaved aerated concrete for the structure.”
Knapp and his wife put their project on hold as they waited for market conditions to improve, and in the interim, Knapp says, local AAC installers largely disappeared. “As a consequence, we set about designing a house that would use conventional framing and as many no- or low-VOC materials as possible,” he says.
Increased residential construction is pushing up the cost of conventional building materials, and AAC contractors are back in the picture. That’s making Knapp wonder whether the 15% premium he thought he’d have to pay for AAC construction may actually be worthwhile.
“My question is,” he writes, “are you professional builders seeing big price spikes? Are they broad and large enough that I should take another look at AAC?”
While construction costs ostensibly are at the heart of Knapp’s question, the issue also touches on the merits of autoclaved aerated concrete, and that’s the subject of this Q&A Spotlight.
AAC block: higher cost, lower R-value
To GBA senior editor Martin Holladay, an AAC wall will never be able to compete with conventional wood framing on a cost basis.
“Moreover,” Holladay writes, “AAC walls have a very low R-value — most are in the range of R-8 to R-11 — and often require poured concrete bond beams that result in horrendous thermal bridging.”
R-values that low are not what Knapp has been planning on. “The ‘effective’ R-value of an 8-inch block is around 21,” he replies. “I know that’s not stellar, but you can create a very airtight structure with AAC.”
Also, Knapp adds, there is an alternative to a poured concrete bond beam that would have a lower energy penalty.
“Perhaps my best course is to invite the AAC builder to bid on the project and see how it compares,” Knapp says.
Better performance with an alternative
Holladay thinks “effective R-value is an example of deceptive marketing hype.” GBA reader Dana Dorsett doesn’t go quite that far, but he suggests caution when comparing “effective R-values” of this building material to the more reliable R-values of other wall assemblies.
For starters, the “dynamic benefit for massive systems” — one approach to calculating “effective R-value” — most often cited by the people who sell AAC is based on a particular house design tested by the Oak Ridge National Laboratory, Dorsett writes, and unless Knapp is planning building exactly the same house there’s no guarantee he will get the same performance. In fact, the performance of AAC will “vary substantially” with the climate, weather and season, the actual house design and site conditions, he says.
There’s more. “AAC has a very high vapor permeance and moisture absorption — a moisture reservoir that passes both water vapor and liquid water more readily than most building materials, making it a far-less-than-perfect choice for Atlanta’s high latent-load summertime issues,” Dorsett says. “You’d probably be able to build a true/stable R-20 (or even R-30) wall by other methods (and more appropriate moisture control) for the money, and have lower overall cooling/heating loads.”
Low-perm paints and sealers don’t do much to control the problem, Dorsett says, and they simply peel and flake away. Only semi-permeable paints can be used. “In the cooler, drier air of northern Europe it does OK from a moisture performance point of view but it’s so low-R that it doesn’t meet current thermal requirements without adding exterior insulation.”
AAC might be worth considering in some drier climate zones in the U.S. with high daily temperature swings, Dorsett adds. “But not in the muggy-sticky Southeast.”
Comparing AAC to concrete block construction
An alternative that would outperform AAC in the Atlanta area would be a wall made from old-fashioned concrete masonry units (CMUs) wrapped in 1 1/2 in. of polyisocyanurate foam insulation, Dorsett says. That wall would keep all of the thermal mass of the block inside the thermal envelope, and it would meet local codes. “But for the cost of AAC there’s usually more performance to be had elsewhere,” Dorsett says.
If structural integrity and hurricane resistance is a top priority, Knapp might also consider building with insulating concrete forms (ICFs). The reinforced concrete at the core of the ICF wall will be much stronger than either an AAC or CMU wall.
The air quality question
The conversation is enough to get Knapp to take AAC “off the table” and move forward with a stick-built design that will meet the EarthCraft House Platinum standard.
“We are very focused on limiting or eliminating VOCs in the structure,” Knapp writes. “We are also insisting on an accurately sized HVAC (no best guesses) and an energy-recovery unit for air exchanges. The house will be all-electric and have no combustion sources (not even a fireplace).”
Our expert’s opinion
We asked GBA technical director Peter Yost for his take on this question. Here’s his reply:
While I was at the NAHB Research Center (1993–2000), we worked on AAC as part of the DOE Advanced Housing Technology Program, and built one of four townhomes out of AAC, on a Superior Walls basement foundation. The trades grew to like the block because it could be cut and routed and was relatively lightweight. The local fire department (and the Center’s insurance company, no doubt) grew to like it when vandals started a fire in the unfinished AAC unit, with no damage. That said, this was not the first time AAC attempted to find market share in the U.S., and it was not the last, although each time it simply has not taken hold.
For a pretty comprehensive discussion of the pros and cons of AAC, check out the GreenSpec Insights recent blog — including the extensive posted comments discussion — as well as this UC Davis Extension paper.
And it is certainly worth reviewing this ORNL paper, which assesses the energy performance of AAC, by climate.
Individuals with chemical sensitivities should assess materials on a case-by-case basis, focusing their efforts and finances on materials that they know are problematic (and crude as it may be, one of the best indicators is to sleep with your materials, placing small samples of materials on your nightstand and seeing how you feel the next morning).
From a hygrothermal standpoint, the pore structure of AAC can vary considerably from formulation to formulation, significantly affecting its moisture and energy performance (at least one AAC manufacturer has a data file in the latest WUFI Pro materials database, but it is unclear how that applies to other AAC materials).
AAC attempts to be a stand-alone building envelope system for the U.S. market, but it simply has never convinced any industry leaders or bulk market interests to sustain a manufacturing presence in the U.S. There are considerably more reasons not to use this system than there are to use the system.