Autoclaved Aerated Concrete
(AAC) wall systems
With one product, AAC replaces the wood framing, wood sheathing, wood siding, insulation, synthetic house wrap, and sheet rock used in our conventional fire-prone, rot-prone and termite-prone US building industry. AAC has been manufactured here only since 1994 but it has a 90+ year track record in the rest of the world. Over 200 plants make AAC in 35 countries.
AAC is fireproof, rot-proof, termite-resistant, mold-proof, and substantially soundproof. It can be engineered to withstand hurricanes and earthquakes.
AAC is fireproof. Fires are terrifying catastrophes. We have vicariously experienced an amazing number of them in our neighborhood and among our friends. This is because the US housing supply is wood-based and combustible.
In the last decade, the US sustained 3000-4000 deaths in over 400,000 house fires, plus about 17,000 injuries EVERY YEAR. The financial loss averaged over five billion dollars annually.
Fireproof materials give a feeling of safety to a home. AAC, when combined with our floors and roof systems, receives a class four industrial fire rating – which translates to a less expensive insurance policy.
This AAC house survived the
firebombing of its German city
in World War Two!
AAC is itself rot-proof and termite-proof. When combined with our roofing and floor systems, our houses need no maintenance for decay – and need only an underslab treatment to prevent insect infestation.
AAC is substantially soundproof.
AAC can be engineered to withstand 140 mph hurricane winds.
AAC doesn’t mildew or mold.
AAC is solid, structural, monolithic and self-insulating.
Its exterior is finished in real stucco, which acts as a vapor barrier, and requires no wire lathe for stucco adhesion. Stucco walls never need painting.
Interiors are generally finished in hypoallergenic mineral plaster.We use two coats of stucco and one coat of plaster for a rustic handmade wall finish both inside and out.
AAC has the lowest environmental impact of any wall system (other than hay bale and rammed earth construction. We love these two technologies, but mortgage lenders who also love them can be hard to find. They are also very high maintenance.) AAC is entirely organic.
AAC walls last a LONG time with no maintenance. We expect our houses to be around for hundreds of years.
We are not representatives for any AAC manufacturer. If we find something better than AAC, we will use it.
While we wait for THAT miracle: what is this great stuff?
AAC is made from sand, lime, water and cement; aluminum powder is added to this batter, causing a five-fold expansion of the ingredients. This foamed air-entrained concrete is hardened in a mold and pressurized in an autoclave.
Although AAC panels are available, and are widely used throughout the world, most residential construction in the US employs 8"x8"x24" blocks. Twelve inch block is also available, and we recommend its use in northern climates. Eight inch block is suitable for construction in much of the US.
Building with AAC
It’s natural to think that AAC would handle like ordinary concrete blocks – but the two are little alike. Masons must retool for this material; carpenters seem to take to it enthusiastically because AAC is sculptural like wood.
AAC can be cut with a special handsaw, but most jobs require renting a dedicated AAC bandsaw from the manufacturer, or from a supporting rental company.
AAC is installed on a CMU ("concrete masonry unit," aka cement block) foundation that is set at intervals with J-bolts, and filled solid with concrete.
The first course of AAC is set with type S cement.
Subsequent courses are set with a special mortar and trowel provided by the manufacturers. In each course, cored blocks are placed to line up over the J-bolts; when the walls are erected, lengths of 5/8” threaded steel rod are lowered through the cored block and attached to the j-bolts. They will extend from the foundation through the mudsill, and are part of what makes the house stand up to earthquakes and hurricanes. AAC's great structural strength is also achieved with the construction of the final course: an AAC "U"-shaped block is fitted with two sets of continuous rebar circumvallating the entire structure, and filled with concrete. The treated wood mudsill is attached to this course. Concrete and rebar lentils are built over openings; bucks of treated wood are attached to these openings for the installation of window and door units. Electrical wiring is run in channels made with a router or circular saw. (Plumbing is run in internal walls.) We use standard framing in interior walls and cover them with sheetrock. This interior wall system and the inside of the AAC walls are both plastered with one coat of mineral plaster. The exterior of the AAC walls is coated with real stucco. Both these products are available from Elite Products.
These instructions are intended as a very general introduction to the use of AAC. You must order and read a technical manual from an AAC manufacturer to actually build with the product.
Note: We are not representatives for the product and must refer all technical questions to the technical advisors employed by the various manufacturers. Sorry!
AAC walls have 67% less air infiltration than those of conventional 2"x4" construction. However, this product’s insulation performance varies with different climates. Without taking into account either its reduced air infiltration or its performance in passive solar design, Oak Ridge Laboratories reports that the effective R-values of 8” AAC block range from about R-21 in Phoenix to R-16 in Atlanta, R-14 in Washington, and R-12 in Minneapolis. (For comparison, a finished conventional 2"x4" stud wall has an effective R value of 12.5.) The material gives its best insulation performance in hot climates with wide temperature swings; 8" AAC block is perfect for these climates. More northern climates require the use of 12" AAC block.
Many alternative wall systems have been introduced with bogus claims for their insulation values, and AAC is no exception. The Hebel Company brought AAC to the US with claims of R-36 for 8" block; we wish this splendid figure were correct. But when used as we design with it, AAC’s insulation performance is still great.
As noted in Passive solar design our unheated houses' internal temperatures fall to about 55 degrees F. when outside temperatures are 0 degrees F.; and without cooling systems, they rise to about 78 degrees F. when outside temperatures are 100 degrees F. Unheated AAC houses that are not passive solar can fall below freezing indoors when outdoor temperatures fall into the twenties.
Note: We had the opportunity to closely observe non-passive solar AAC houses built with crawl spaces and wooden floors: they are cold in winter and hot in summer, just like wooden houses. In most of the continental US, it is a mistake to build non-passive solar houses, regardless of their wall systems.
One manufacturer gave us the following 2001 cost comparisons for exterior wall systems: $8/sq. ft. for conventional walls; $11/sq. ft. for AAC; $13 sq. ft. for brick (which is a veneer on conventional framing). We see from these figures that AAC costs more than conventional wood framed walls, but less than brick veneer over framing. The cost of all building materials is too unstable to provide more than relative figures such as these.
Our masons charge approximately the same amount to build with AAC as it costs per block to buy from the manufacturers; shipping costs vary with distance. More exact figures can be obtained by contacting AAC suppliers.
We have been able to offset the higher cost of AAC walls with the use of relatively less-expensive concrete poured-and-scored floors, and by designing uncomplicated roofs that can be built with engineered roof trusses. The combination of these three features allows us to compete in cost with conventional construction while producing a vastly superior house.