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AAC, a Novel Stucco, and Innovative Details in Duplex

By Mary James

To say that Dan Levy, a New York Passive House builder and educator, is a fan of autoclaved aerated concrete (AAC) would be a mild understatement. He can easily rattle off a slew of information about its attributes and its prevalence as a building material in diverse countries. AAC—made from a mixture of sand, cement, lime, water, gypsum, and a trace of aluminum—was developed in Sweden in 1923, and is commonly used in most industrialized countries. It apparently holds up well enough in earthquakes to be popular in Japan. Its fire rating is outstanding—a 4-hour fire rating for blocks of 4 inches or more and for the 3-inch panels made by AERCON in Florida. It is airtight, deadens sound transmissions, and delivers an R-value of roughly 1 per inch, depending on the particular AAC block’s density. And, as it’s about 75% air, it’s very light to handle and can be cut with woodworking tools.

Levy is a former wood technology instructor, who still loves wood. Yet, he is well-versed in its potential limitations, and has chosen to eschew wood as a framing material for many years now. In 2016 Levy completed his first Passive House using AAC blocks in Woodstock, New York, which he then sold in 2021. When the opportunity arose to develop a commercial property in the heart of downtown Hillsdale, a small community southeast of Albany, unsurprisingly AAC was Levy’s go-to construction material for his first building there, a duplex. He will live in the larger half of the all-electric duplex and is reserving the smaller two-bedroom unit for short-term rentals, so that travelers can experience the many benefits of a Passive House.

Photos courtesy of Dan Levy
Photos courtesy of Dan Levy

Levy considers AAC to be a major asset when building housing that is resilient in the face of increasingly severe climate events, including wind, fire, and flooding. However, for those California residents starting to face the challenges of rebuilding, it should be noted that the use of AAC there is restricted due to insufficient seismic testing.

“To meet the U. S. where the U.S. is, the major manufacturer of AAC in Florida is now producing three-inch thick panels to use as cladding to wood-frame buildings,” Levy explains, adding, “I think one is far ahead building the entire structure with AAC.” That’s exactly what he’s done in Hillsdale, with eight-inch-thick AAC blocks providing the structure for his 3,000-ft2 duplex. He used basalt rebar in lieu of steel for all block reinforcement after the shallow frost-protected foundation was poured, because he points out, basalt has a higher tensile strength than steel, doesn’t rust, and has little carbon impact.

To improve his walls’ thermal resistance, the AAC blocks are wrapped on the exterior with 6 inches of mineral wool, which is attached with fasteners from EJOT that are designed specifically for AAC. Layered over the mineral wool is a directly applied stucco product, U-Stucco. The approach is unusual enough in the U.S. that his building is a demonstration case for Rockwool to prove that this specialized stucco can be applied in this way here, although it’s not an unusual method in parts of Europe. “It just took finding a crew,” quips Levy, who decided to try tackling it himself just to see if there would be any challenges.

“There was no learning curve to trowel the stucco onto the insulation,” he says, and, “It didn't take more than a few minutes.” He then had the owner of a drywall company look at his sample application, who decided his drywall crew could handle the challenge.  “They just took it and ran with it, and they loved it, because this particular stucco is very lightweight,” notes Levy. According to Levy, U-Stucco is hydrophobic, permeable, has an R-value of 2.2 per inch, and resists fire for 2-hours.  

Levy at the jobsite.
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His roof assembly also features a cutting-edge product: 24 inches of Timber HP loose-fill wood-fiber insulation packed into a scissors truss. He has used cellulose in past projects, but that insulation is becoming more expensive now that fewer newspapers are being printed. In any case, he was pleased with Timber HP’s product, which he held in place with Insulweb breathable fabric above and SIGA’s Majrex vapor barrier below, adhered with their double-sided tape rather than staples. The vapor barrier is reinforced by 1 x 3s installed 16 inches on center to support the weight of the insulation. Above the truss, a Zip system serves as the roof sheathing, which is then topped with a standing seam metal roof. 

As for the windows and doors, Levy has figured out a way to simplify their installation, while achieving optimal air tightness. “I make my openings only a quarter inch bigger than the window,” he explains. Levy double-checked the details and adjusted the openings where needed with an AAC rasp. The units were then carefully sealed directly to the AAC blocks using Prosoco R-Guard products. 

All of this attention to detail paid off when the time came to test the duplex’s airtightness. The results exceeded Passive House certification requirements, averaging out at 0.21 ACH50. “I'm aiming for net zero energy certification with Phius,” he notes.

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The all-electrical mechanical systems were chosen for optimizing the project’s comfort and cost-effectiveness. “It’s all part of my desire to show that we can build a Passive House for, I believe, the same cost as building a traditional code-built house,” says Levy. 

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All mechanical equipment for each unit is independent. For heating and cooling he is planning to use one air-source heat pump for each floor of each residence. One ductless unit will serve each first floor, and a low-static ducted unit will serve the second-story bedrooms. Levy is awaiting equipment with R32 refrigerant. In the meantime, he is proving the adage that a Passive House can be heated with a hairdryer; he’s keeping both buildings warm with three electric space heaters. 

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For ventilation, he is relying on ERVs manufactured by RenewAire. “I meet the Passive House standard by using a much larger unit than needed, and running it at a very low speed, which increases the transfer,” he points out. He’s also using the LumenCache Renata low-voltage DC LED lighting system, which reduces the size of wire required for lighting fixtures. 

With the duplex nearly completed, Levy is starting to elaborate on plans for further developing the highly walkable property that fronts Hillsdale’s main street. His next building will be a workshop that could serve double duty as an ADU. After that, he has some ambitious ideas. “I'm looking at perhaps six more apartments in the front and three stores,” he muses, although those ambitions will require some creative financing to realize. Will those plans get realized using AAC blocks? Stay tuned.

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Author: Mary James
Categories: Article, Detached two-family house, New construction