The Benefits of AAC

A home designer shares the details of working with autoclaved aerated concrete.

Autoclaved aerated concrete (AAC) is strong, light, insulating, fireproof, mold-resistant, and has excellent design flexibility. AAC will not rot, warp, corrode or otherwise decompose, providing a very durable material that will last for many years. Even when exposed to fire, it releases no toxins or gases. And it's nearly soundproof.

Here's a snapshot of my experiences with using AAC:

Installation and Training

Originally, we were hoping our regular masonry crews would be interested in building with AAC, but their unfamiliarity with the product drove the price up more than we expected. Being hands-on contractors who design and build, we invested in our own AAC crew.

At first we had to work closely with the crew members to get them used to the different processes, but they're not that different. Masonry crews have to adjust to using a special polymer-modified thin-set mortar system, which creates less margin for error. Our masons use a special notched trowel to apply the thin-set mortar and create texture for subsequent pieces of block.

AAC blocks can be installed either directly on a foundation system or on an elevated floor for multilevel construction. As with typical masonry projects, the initial bed joint is critical because the base mortar joint must be level and true to avoid continual adjustments. Initial bed joints are usually from 3/8- to ½-inch thick. Using a waterproof mortar on the floor slab under the first course of block protects the building from water in the soil. A horizontal moisture barrier in the first mortar joint is another way to protect against water intrusion.

Depending on the architectural style, we build the exterior walls with the AAC block and attach the trusses using a top plate, anchor or ledgers consistent with typical construction methods. Ledgers are attached to J-bolts that are installed to the bond-beam block, with 4 inches of concrete around the anchor bolt filled to the face of the wall.

AAC blocks can be cut with a handsaw and sculpted with a rasp, creating endless design possibilities. At first, we bought a big band saw, thinking we would have one table to do all the cutting, but it turned out that the laborer working next to the mason can actually handsaw-cut the pieces right on the spot rather than having to go to the saw cutter and bring the piece back. With handsaws, we were able to cut the blocks right on the scaffolding and put them in place twice as fast as if we had a central saw cutter.

The handsaws are not typical handsaws, though. They are 3-feet long, quarter-inch-carbide-tipped saws. Other masonry tools we use include sanding floats, rubber mallets and levels. The AAC waste created on site can be recycled or disposed of in a regular waste bin.

Exterior surfaces of AAC must be protected from moisture. We use an impermeable synthetic stucco system that helps to avoid moisture issues. For interior wall finishes, we apply a scratch/brown stucco coat topped with regular drywall mud, and usually a smooth Santa Fe finish. Other wall options include painted or veneered plaster left unpainted. Our interior partition walls are wood-framed and receive a 5/8-inch gypsum-board finish to match adjacent walls.

Types

We use three types of AAC block on our jobs. The first is our standard building block, which is 8 inches tall, 24 inches long, and either 8, 10 or 12 inches thick. We typically use 10-inch-thick block on our projects for its combination of insulating value, structural strength and average unit cost.

Then there's a same-size block with a 4-inch bore for corners and door and window openings; it requires vertical rebar and gets filled with concrete.

The third type of block is a bond-beam block (U-block), which is for the top course and has a V-channel cutout for the horizontal and vertical rebar attachments.

AAC is highly adaptable to a variety of architectural designs and can easily be engineered to meet structural load requirements. It can also reduce job-site waste if components are pre-made and tailored to building-specific features.

On the residence pictured, we used 10-inch block to create an exterior wall with a performance R-27 value, and we used cellulose insulation to create a ceiling with an R-40 value. The performance R-value of an AAC block wall depends on the kind of block used and its geographic area. (For the effect of geographic and block-type variables on the performance R-value of an AAC wall, visit TruStone America.)

AAC block provides for walls that are very straight and consistent from an insulating and smoothness perspective. There's less thermal bridging than with wood framing, and you don't have to deal with batt insulation on the walls. When you put your hand on the exterior wall, there's hardly a difference in temperature from the interior wall. According to Oak Ridge National Laboratory, energy demands for a home with AAC walls are about 18 percent less than for wood-frame walls, 36 percent less than two-core CMU and 23 percent less than steel-stud walls.

Learning and Teaching

When we built our first home with AAC, TruStone America representatives were out there showing us how to use it, helping lay the first blocks to get our crew comfortable with the product. Now, TruStone simply provides detailed shop drawings and then our structural engineer provides any custom construction details that may be required.

The electricians are the subcontractors most affected by the use of AAC block. It's a whole new way of pulling wire and installing J-boxes, switches and outlets. They have to use a circular saw to cut out 1 1/2-inch-deep-by-1 1/2-inch-wide channels to accommodate sheathed electrical cable and J-boxes. A circ saw cuts through AAC like butter, but protective eye equipment and respiratory masks are a must because of the dust. That dust is the main reason subs may not like working with AAC, so sometimes we end up doing the cuts ourselves.

Regular screws, clips and metal shields are used to fasten the sheathed electrical cable or conduit in place. We then use channel metal and a special patching material from the AAC manufacturer to cover all the channels made for the electrical chases. We like to keep the plumbing out of the walls, so we fur out the plumbing walls using hat channels to create a cavity for piping and then penetrate the wall rather than the roof to accommodate plumbing vents.

Doug Edwards is head designer and partner for Edwards Design Group, a Phoenix design/build firm. This article is provided by the Partnership for Advancing Technologies in Housing (PATH).

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