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An Overlooked Building Material Finds a New Booster

A New York real estate developer thinks there could be a bright future for autoclaved aerated concrete

Posted on Feb 12 2016 by Scott Gibson

Autoclaved aerated concrete is an unusual building material with properties that should make it a hit in residential construction — it's a much better thermal insulator than ordinary concrete, while lightweight, easy to work with and resistant to fire, insects and mold. The only problem is that U.S. builders can't seem to warm up to it.

A product used extensively in Europe just never found much of a toehold here. There is but a single U.S. manufacturing plant, Aercon AAC in Haines City, Florida, and it's not running at full tilt. Some residential builders who specialize in energy-efficient designs have tried using AAC, but most have since left the fold.

Steven Bluestone, a third-generation real estate developer from New York, thinks that the naysayers are wrong and that AAC could still enjoy a bright future in high-performance building. A case in point: his own AAC house in upstate New York that exceeds the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. airtightness requirement and is headed for net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. performance nearly a year after completion.

What's different? Bluestone is using AAC block to build exterior walls, but instead of relying on the block alone he's adding a layer of rigid foam insulation on the outside, and finishing the wall with a vented rain screen and siding.

A long history in Europe, but no translation here

AAC has a lot in common with ordinary concrete, with a few notable exceptions. Instead of fine and coarse aggregate, AAC uses sand or fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info plus aluminum powder to create millions of tiny bubbles in the mix, according to a description published by GreenSpec.

The mixture is placed in molds and cured in an autoclave, which uses steam and pressure to complete the chemical transformation.

AAC blocks, like conventional concrete masonry units, are laid up with mortar. Blocks are placed over lengths of steel rebar that are cast into the foundation walls, and these holes are later filled with grout. Blocks can be cut with the same tools used for wood — bandsaws are commonly used to cut blocks to size.

It takes less energy and fewer raw materials to produce AAC than it does conventional concrete, and the material weighs less, has excellent soundproofing properties, and is both unattractive to insects and fireproof. Its R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. varies by the density of the block, Bluestone says, but typically falls in the range of R-1 to R-1.25 per inch.

More than half of all new construction in Germany used AAC in 2006, GreenSpec reported. But for a variety of reasons — uncertain pricing and weak supply chains, unfamiliarity, and relatively low R-values when used by itself — the market share here hasn't budged. Some builders also were turned off by marketing claims of "effective R-values" that were much higher than testing could support — part of the debate over the value of thermal mass.

"AAC attempts to be a stand-alone building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. 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.," technical director Peter Yost said in a 2013 post at GBA. "There are considerably more reasons not to use this system than there are to use the system."

Starting with a sauna

Steven Bluestone is part of Bluestone Organization in New York City, a family real estate development company with a special interest in energy-efficient construction. He had heard about AAC, and started "playing around with it" on a job in Manhattan where it was used for partition walls in apartment building basements.

Bluestone became interested enough to buy a pickup load of AAC and use it for a wall in a sauna he was building at his Westchester County, New York, home.

Walls begin: AAC walls are reinforced with steel rebar that is cast into foundation walls. Holes for the rebar are later filled with grout.

"I laid up the wall, and I'm anything but a mason," he said. "I thought, 'Wow, this stuff is easy to use, not terribly expensive, flexible.' So I started getting more and more involved and finally I said, 'I want to build my house with it.' "

At the time, Bluestone and his wife owned a piece of land in upstate New York near the Massachusetts border. They planned to build a vacation home that would, within a few years, become their year-round residence. Bluestone approached Bruce Coldham, an architect he'd met through the Northeast Sustainable Energy Association, and asked him to design the house.

Coldham, he said, wanted to use Durisol, a type of insulated concrete form. "He loves it," Bluestone said. "I looked at it and I thought, 'I guess I can do this,' but I wasn't in love with it."

Bluestone wanted to take the lessons learned on his own house and apply them to the kind of project his family's firm was developing — but he didn't think Durisol was especially well suited to the task. "There's no way I could build big buildings with Durisol," he said, calling the expected result "big and funky and unexacting."

So he told Coldham he wanted to go with AAC block, and after many, many rounds of design changes, Bluestone had a project he was ready to build. The finished design had roughly 4,200 square feet of conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. with a main floor and a partially earth-bermed lower level. Bluestone hoped the finished house would deliver Passivhaus-like performance even if it wasn't certified.

Long-distance general contracting

Although Bluestone was working in the city four days a week, he decided he wanted to be the general contractor. He scared up his subs, convinced the local building inspector to approve the construction drawings, and then got to work. The house was completed last summer.

After the 8-inch-thick AAC walls went up, Bluestone attached pressure-treated 2x4s horizontally every two feet with a combination of construction adhesive and screws. Between the 2x4s are 2-foot-wide pieces of 1 1/2-inch thick polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. insulation. After that went two more layers of 1 1/2-inch polyiso, placed vertically with staggered seams. Over the insulation are 1x4 pressure-treated battens attached with screws to the 2x4s, then fiber-cement siding.

Inside, walls are finished with two-coat plaster, about 1/8 inch thick.

Bluestone estimates the R-value of exterior walls at about 40. A blower door test measured airtightness at 0.398 air changes per hour at a pressure difference of 50 pascals, well under the Passivhaus requirement of 0.6 ach50.

The roof is made from 12-inch-thick structural insulated panels (SIPs). Because his wife wanted recessed lighting throughout the house, Bluestone framed down from the SIPs with 2x10s to make room for them without disturbing the SIPs, and filled those cavities with fiberglass insulation. He estimates the total roof R-value at "65-ish."

Other details:

  • Foundation insulation. For the first 4 feet, walls are insulated on the exterior with 4 1/2 inches of extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.). Below that, it's 3 inches of XPS, the same amount placed under the slab.
  • Windows. Passivhaus-certified Zola ThermoPlus Clad, a wood window clad in aluminum with an overall U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.123 (R-8.1).
  • Heating and cooling. A ducted Mitsubishi air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. with a single outdoor compressor and three indoor air handlers. In all, the house has five zones of heating and cooling.
  • Renewables. The all-electric house is powered by a grid-tied 10-kilowatt photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. system, which so far has produced enough power to zero out the utility bills.
  • Water heater: A Stiebel Eltron 80-gallon heat-pump water heaterAn appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed. . A heat-recovery drain pipe picks up waste heat from first floor plumbing.
  • Whole-house ventilation: Zehnder 350 energy-recovery ventilator(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV..

Not a cheap house to build

Bluestone isn't anxious to talk about how much the house cost. "It's more than I want to publish," he said.

Expensive finishes, appliances, cabinets and other detailing — costs that wouldn't necessarily be repeated in another AAC house — are a big part of the reason. There also was the impact of long-distance site management with unfamiliar subcontractors. Although Bluestone said he and his subs worked well together, exchanging plenty of photos as work progressed, he was still working with them on a pay-as-you-go basis, not contract pricing.

Capping the wall: Special U-shaped blocks are placed at the top of the wall, where a steel-reinforced bond beam adds structural reinforcement.

"The meter was spinning really fast, if you want to look at it that way," he said. "If you strip all of that stuff out, the house would have been an expensive house but not as expensive as I paid. I haven't finished doing the math. I put down the books a while ago. I picked them up again because I was curious. I was getting depressed, so I put them down. We did a lot of custom, a lot of fancy stuff."

More important is whether AAC construction would be a financially competitive option for other residential builders if construction techniques can be fine-tuned. Using an exterior insulation and finish system (EIFS) on the outside of the building, for example, would be cheaper than the assembly Bluestone chose.

"I think it could be comparable to a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. with cellulose, without a doubt," Bluestone said. "But the other thing that's more important is the fact that it will be there for a few hundred years. The building is not going anywhere. The life-cycle costing on this AAC concept is not something to dismiss."

Bluestone is enough of a believer in the AAC approach that he's talking with a local Habitat for Humanity architect about it, and is offering to buy the materials for the first two AAC houses the program undertakes.

"I want to see it happen," he said. "I want the buzz. I'm hoping more people buy it... Fireproof, thermal mass, comfortable — what else do you need to know?"

A word from the architect

Bluestone's ally, architect Bruce Coldham, knew his way around high-performance building, and he was slow to get aboard the AAC train.

"I did my level best to talk him out of it," Coldham said, "basically [giving him] all of the technical detail about why it was a good product for a climate where there was a diurnal swing with temperatures above and below [freezing], but it wasn't such a good choice in a climate like this where it went cold and stayed cold."

He initially favored Durisol because it would do a much better job of giving the walls the R-values Bluestone was after. The company had even offered to produce 14-inch block for the job, which would have gotten the wall R-values up in the R-30 neighborhood — much better than AAC could hope to match.

"I was concerned it would be an embarrassment to him sooner rather than later," Coldham said.

But the picture changed when the conversation started to include a continuous layer of insulation on the outside of the AAC walls. In that case, the assembly started to look a lot like the "wrap and strap" treatment a house gets in a deep energy retrofit. In some ways, it didn't matter whether the substrate was AAC block, concrete masonry units, or a wood-framed wall because the exterior insulation was doing most of the work.

Where AAC does start to make more sense is where there are other factors thrown into the mix — for example, fire resistance, durability, aesthetics, or resistance to moisture, rodents, and insects. And unlike Durisol, AAC proved to be a "spectacular air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.," he said.

"My counsel to him initially was very broad-based and conventional common-sensible, and he listened to me and thought about it and came back and said, essentially — my words, not his — 'This is not the whole story, Bruce,' and then proceeded in the following couple of years, to figure it out, to lay out why he was interested, that he wasn't nuts, and then proceeded to act upon his convictions."

Still, Coldham doesn't think AAC is going to be a real competitor to more established wall systems unless there are considerations other than thermal performance and airtightness.

"I think that you need to have something else in play to make you want to choose the AAC over wood frame or concrete masonry or something else," he said. "In Steve's case, it was the aesthetic piece. It was also the simple curiosity of trying a new material. If it were a building that really needed substantial fire separation, that could be a reason for using it."

What about a supply of block?

If AAC houses are going to become more common, it will take a ready supply of AAC block. And for the moment, that doesn't look very likely. In addition to the Florida plant, the are two AAC manufacturers in Mexico, and that's it for North America.

Aercon Sales Manager Mike McCormick says there are six to eight "hot pockets" of residential builders around the country who like AAC, but as much as 95% of his business is on the commercial side. "We're extremely busy with the people we have," he says, without disclosing production numbers for the Florida plant. "It's a very nice business in the commercial market."

AAC is too expensive to compete in residential mass market building, he said, and low margins for small residential jobs leave commercial projects much more attractive to his company. McCormick can spend a lot of time educating builders, inspectors, and homeowners about the benefits of AAC and then land an order for a truckload or two of block for a residential project. Sell a big commercial project, and there will be many truckloads.

Early on, the AAC industry suffered from a lack of effort to grow the market as a whole. When there were more companies vying for AAC orders, they "beat each other up" trying to get customers rather than working together strategically to improve the product and widen the potential pool of buyers, McCormick said. Plus, their pitch to the construction industry seemed a lot like force-feeding. "You don't shove it down someone's throat," McCormick said.

The few manufacturers around formed a trade association in 1998 to share test data and promote the use of AAC. It's still around, but barely.

"It's alive," McCormick said, "but it's on life support, and you can't see a lot of breathing going on."

Still, there are a couple of positives. Aercon enlisted the University of Dayton to work out accurate calculations for R-values based on the wall assembly and climate zone, McCormick said, and he's convinced the industry can offer builders accurate information on how AAC buildings will perform.
Plus, there's another U.S. plant said to be in the works in Bennettsville, South Carolina.

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Image Credits:

  1. Steven Bluestone

Feb 12, 2016 10:21 AM ET

Edited Feb 12, 2016 10:21 AM ET.

Still not convinved
by Ethan T ; Climate Zone 5A ; ~6000HDD

I came here ready to be (re)convinced to look seriously at AAC, but this article didn't do it for me. Yes, it is structure, insulation, fire protection all in one... but then it seems you need to insulate outside and maybe have a service cavity inside... so basically you're building a concrete insulation layer? The only advantage left is the fire resistance... Maybe the way to go would be to lay the bricks lengthwise so you create a 12" wall? But then is cost astronomical? Perhaps we need to see more thorough analysis of the AAC wall's performance in Northeastern climates? The Woodstock Passive House ( is maybe more in line with what a "frugal" AAC home could be? The Woodstock version wraps the AAC in rockwool, not Polyiso.

Feb 12, 2016 12:12 PM ET

It didn't matter whether the substrate was AAC or CMU
by Charlie Sullivan

"In some ways, it didn't matter whether the substrate was AAC block, concrete masonry units, or a wood-framed wall because the exterior insulation was doing most of the work. Where AAC does start to make more sense is where there are other factors thrown into the mix — for example, fire resistance, durability, aesthetics, or resistance to moisture, rodents, and insects. "

That makes sense. And the extra advantages all seem to apply to CMUs as much as to AAC, so it would make sense to focus on a comparison to CMUs. With CMUs, you'd need a little more exterior insulation to the the same total R-value. So we should compare the extra insulation cost to the extra cost of the AAC over the CMUs. Anyone care to offer a cost comparison of AAC blocks vs. CMUs?

Feb 12, 2016 12:27 PM ET

For brick veneer construction there's a place for AAC
by Dana Dorsett

The achilles heel of higher performance brick veneers construction is the thermal bridge at the top of the foundation. A course (or two) of AAC between the top of the poured concrete and the rest can provide substantially higher performance than resting it all on poured concrete.

Feb 12, 2016 2:08 PM ET

reply to Dana
by Jason D

R-values of 1 to 1.25 for AAC hardly seem like it's worth the trouble. If you're looking at a thermal break at the top of the concrete, what about Foamglas? I've been considering the use of Foamglass between the concrete foundation wall and the sill plate of a wood-framed building.

Feb 12, 2016 5:56 PM ET

Foamglas as thermal break
by Ethan T ; Climate Zone 5A ; ~6000HDD

I think Foamglas is susceptible to deterioration if it is in contact with freezing water, but I may be mistaken.

Feb 12, 2016 10:40 PM ET

Edited Feb 12, 2016 10:42 PM ET.

The Woodstock Passive House
by Daniel Levy

I am the builder of the Woodstock Passive House, referenced by E T in comment 1. I’ve been a proponent of AAC since it became readily available in the US in the 1990s, and find it ideal for high-performance, low-maintenance buildings. Steve and I have become friends after meeting as a result of our AAC projects--we often compare notes, and his home is beautiful. Mine is a more basic building—once the siding went up it could blend well in a neighborhood with some nineteenth century homes.

I am not a fan of foam insulation products; instead I’ve used Foamglas™ below grade (glass, despite the word 'foam'), rigid mineral wool for the exterior walls, and cellulose for the ceiling. It is pre-certified by the Passive House Institute US, and though not yet completed the performance and comfort is already astounding. There is an eight-hour lag between outdoor conditions and the inside of an 8-inch AAC wall; I don't know the impact on time lag with exterior insulation. So even when temperatures are low, if the sun comes out the building is warmed. Thermal mass walls with integral insulation outperform fluffy insulation in stud walls at the same R value, reflected in lower R-value requirements in the International Energy Conservation Code for mass walls.

E T is correct that Foamglas can deteriorate if in contact with freezing water. Despite a zero perm rating, if water freezes and expands in surface pores Foamglas can be damaged. This will expose a new surface to deterioration. It needs to be waterproofed.

Feb 13, 2016 2:22 AM ET

Daniel Levy - How would you

Daniel Levy - How would you respond to Charlie's comment, #2, above. I don't see the value of the masonry, despite its insulating properties, if you're going to put insulation and/or siding over it on the outside, and a stud wall inside to run utilities. I can see the value if say the outdide was stuccoed, and an insulated stud wall was inside, but then you lose some of the mass effect from the masonry. I can also see its utility for say a work space that wasn't heated much, or a climate that did not require too much heat, but not for a house in a cold climate.

Feb 13, 2016 9:32 AM ET

Steve Bluestone - responses to comments thus far....
by Steven Bluestone

First, I'd like to thank Scott for his very thorough reporting. Now, I would like to respond to the comments made thus far (not in any particular order).
1] At my house, there was no need for a "service cavity". We purposefully minimized the number of electrical outlets/switches that occurred in the AAC walls. Where possible, floor outlets were used (I know, they are not ADA compliant, but a single family home is not held to that standard). In the few locations where we needed to put them into the AAC, we simply scored two shallow cuts in the wall, chipped out 1/2" to 3/4" of AAC, placed the wire(s) in the gap, patched over them with AAC mortar and/or plaster, and then finished it with the 2 coat system of plaster. Next time, we won't use plaster. AERCON (and Dan Levy) let me know that a thick coat (1/8" +/-) of regular taping compound could be applied directly to the AAC (without any tape or mesh). The compound can be finished smooth or rough, whatever one desires, and then vapor permeable latex paint applied. We avoided running any plumbing lines in the AAC (with the exception of a couple of frost free hose bibs running through it).

2] Some noted why not just use CMU instead of AAC. Here's why. After several experiences of using AAC in lieu of CMU (with multiple masons on different projects), I have come to learn that the installed cost is the same. The AAC costs a little bit more, but is so much faster to put in that the labor savings wash out the difference. The all in cost at my house for installing the AAC was $12.90 /SF (which included all of the AAC precast headers - some of which were massive given some large window openings). From what I've seen, CMU costs that alone (without covering the cost of lintels/headers). CMU offers absolutely nothing in terms of air, vapor, or water sealing. From what I can tell so far, AAC with properly tooled joints provides all of the above without any added coatings, peel and sticks, building wraps, etc., etc. AAC is infinitely easier to cut than CMU. Finally, while it is possible to put in screws (or nails) into CMU, it is not that easy. AAC can easily accept both. One just needs to use coarsely threaded screws, and a gentle finger on the trigger of their screw gun or drill. If you spin the screws too fast, they won't hold. If you drive them into AAC slowly and stop when you feel the resistance of the head at the surface of the AAC or whatever you are attaching, you are good to go. Adding a touch of construction adhesive will guarantee that anything you attach to the AAC will NEVER fall off the wall. Reach out to me directly and I'll tell you a couple of stories that proved this. In other words, in many situations, one would want or need to fur out a CMU wall to accept finishes or create "service cavities". AAC walls allow you to avoid that cost completely (which to me is big).

3] We've used AAC extensively (and quite successfully) on many multifamily buildings as a "starter course" under brick veneer walls. While it's Rvalue of 1 per inch isn't much, having that R8 thermal break with zero thermal bridging at that point of our envelope is a whole lot better than nothing, and, as Dan pointed out, since any material at this location of a building would be susceptible to water, it is much less expensive to install the AAC than FOAMGLAS that would need to be protected.

4] The last thing that should be noted and is hard to comprehend unless you actually spend some time in a building in a cold climate (we're at 4.5 to 5.0) built with AAC on the interior and insulation on the exterior is the comfort level. This winter, I had several relatives who were all in their 80's spend a couple of nights at the house. The interior temperatures (as noted on various Mitsubishi thermostats here and there) were about 64 to 65 degrees. The only complaint that I got from the relatives was that the house was too warm and that they wanted to know if it would be okay to crack open a window in their bedroom a little bit. And the answer is yes, the Zehnder 350 ERV was running at speed number 2 during this time. They were getting plenty of fresh air.


Feb 13, 2016 10:37 AM ET

minor correcction
by Steven Bluestone

I misspoke. I checked and now believe that our house is in a climate zone of 5 or slightly higher. The Columbia County number is 5, but we are at the northern end of the county and are also at a bit of a much higher elevation than the average location. We could possibly be a in a 5.5 zone (for what it's worth).

Feb 13, 2016 10:43 AM ET

Good answer
by Charlie Sullivan

Thanks Steve. Your answer to my question about why AAC in preference to CMU makes a lot of sense. It's relatively easy to compare material costs, but comparing actual construction costs requires experience, so thanks for sharing your experience.

Feb 13, 2016 1:58 PM ET

two more things to consider
by Steven Bluestone

If, after reading all of this, you are interested in trying out AAC, don't do what I did. Do NOT fit the system into your design. Design around the system. The blocks are 24" wide. If you can work out your ROUGH openings to be in 24" increments, you will save a whole lot on materials, cutting of materials, and have less debris to contend with. I can say for sure conservatively that my costs would have been 20% lower (possibly 25%) had I known better (and/or listened to others who may have given me this advice beforehand!).

As mentioned briefly by Scott, one should not underestimate this life cycle analysis/comparison. Buildings made of stone, masonry, and concrete can last a very, very, long time. Buildings made up of wood can be very susceptible to water damage, insect damage, and fire.

Feb 14, 2016 2:18 AM ET

Woodstock Passive House, part 2
by Daniel Levy

Steve has already answered the questions I was asked--thank you, Steve. I'll add a few thoughts. First, although AAC is a masonry product, in many ways it is easier to think of it as wood. It can be cut with hand saws, circular saws, or a masonry cutting band saw (preferable); routed, rasped, carved, nailed, and screwed. I don't care to lift CMUs--AAC is much lighter. Its light weight makes it safer in earthquake zones. You can create sculptural shapes with it, including elaborate moldings. CMUs aren't at all comparable.

Like Steve, I do not use service cavities. Instead I use plaster or drywall compound directly on the interior. First, I try to minimize cutting the AAC for utilities. When needed, I generally cut two slots with a diamond blade in a circular saw and snap out the web in the center. For electrical boxes, I'll drill holes and square the holes with a chisel. In the case of a single-gang box, I use a bit equal to the width of the box and make two overlapping holes, followed by the chisel. It is easy and faster than it may sound, but generates dust. It is definitely quicker to nail a box to a stud, so near intersections of AAC and interior frame walls I'll shift the box to the frame wall if distances allow. There was a time when mortising chisels the size of electrical boxes were sold in the US. I once used a low-speed router at a Hebel training class when Hebel was in the US market. It resembled a worm-drive saw with a gear-like cutter, used in Europe for installing cable or conduit. I don't own one so resort to other methods.

I've cut chases deep enough to leave non-metallic cable in the slot and covered it with fiberglass drywall tape followed by plaster or compound, leaving the cable in an air space. I've also recessed non-metallic conduit.

European manufacturers make anchors specific for AAC, rated for specific loads such as kitchen cabinets and handrails. For many applications I use course screws, as Steve described.

Kevin: I first used AAC in Maryland in 1999, where I built as you suggested with an 8" AAC wall, plastered on the interior and stuccoed directly on the exterior. At that time it provided approximately double the insulation required for mass walls with integral insulation. I can't imagine a simpler, more durable wall system. I've read about but not seen in person an AAC system in Germany of dual-density block--load-bearing AAC on the interior combined with lower-density AAC on the exterior. The system is pre-approved by the Passive House Institute in Germany as meeting the PH energy standard. If AAC reaches greater market penetration in the US, perhaps a wider range of AAC products will be produced here.

In a few weeks I'll be ready to run blower door tests. In a laboratory test in Israel of an AAC wall section extrapolated to a 100 square meter reference building, it was found to have leakage of 0.01 to 0.02 at 150 pascals, compared to 5 to 10 ACH at 150 for CMU. One-hundred fifty pascals is not a typographic error, not our usual reference of fifty pascals. I've also heard of sloppy AAC work with gaps, and the Israeli researcher mentioned that work in the field might not reach the airtightness level they measured.

In conjunction with Prosoco we are testing their R-Guard fluid applied water-resistive and air barrier to investigate its effect on AAC. We are also using their air-sealing products, FastFlash and AirDam for sealing windows, doors, and penetrations.

I'm in Zone 6--it is cold. Until this week the house, with no mechanical equipment including no heat source, remained above 50 degrees F. This week we needed to open doors frequently during work, and the temperature has dropped about one degree F each night, including a night that dropped to 7 F outdoors. I'm writing this after midnight; it is below negative 2 F, and the house is at 44 degrees. When the temperature was slightly warmer, I usually found the temperature in the morning to be higher than the previous night, perhaps because heat stored in the wall from the daytime sun reached the interior. That raises the question of what temperature is comfortable in a Passive House. I find 64 comfortable, as there are no cold surfaces or drafts.

Feb 14, 2016 9:31 AM ET

AAC mortar versus CMU mortar
by Steven Bluestone

AAC "mortar" is more akin to thin set. AAC mortar is latex based, and is applied with notched trowels (similar to how one would lay ceramic/porcelain tiles) as can be seen in the link in the following photo. Regular CMU mortar is applied in a much thicker layer (ie: much heavier, harder to mix, and harder on your back to transport), and is typically done with trowels (ie: much more mess and cleanup). These "little" differences make a really big difference in terms of the ease of use and lowered labor costs.

Feb 15, 2016 1:25 AM ET

Panels, floor systems. and lengthwise/thicker blocks
by Ethan T ; Climate Zone 5A ; ~6000HDD

OK, my curiosity is piqued... Aren't there wall panels available to speed up construction? Is anyone using the 2x20 floor panels in residential construction? Could blocks be laid lengthwise to create s thicker wall? I love the idea of exterior plaster so it seems like avoiding the exterior insulation would be nice. What about a double wythe wall? What about thicker block? Did you use AAC for interior partitions? Why or why not? Is the cost of your building indicative of AAC in general of other design decisions? Since AAC is so limited in the US, can you point us to British, German, out European resources for wall, floor and roof details and assembly methods? Thanks!

Feb 15, 2016 1:28 AM ET

by Ethan T ; Climate Zone 5A ; ~6000HDD

Regarding service cavities... I am reminded that I've begun to appreciate exposed conduit and similar "industrial" aesthetics in residential construction. Perhaps this is another solution to this question of service cavities.

Feb 15, 2016 7:27 AM ET

Panels and thicker blocks
by Steven Bluestone

We are building several homes in Staten Island now using panels. While AAC is light, in panel form it is fairly heavy and requires the use of a small truck crane and two different types of specialized gripping tools (one for floor panels and the other for wall panels) for erection purposes.If one was building a very large project (multifamily and/or commercial building), I'd say go ahead and consider the panels. For a single house, it doesn't make as much sense due to the extra costs of having to rent/hire the specialized equipment. Aercon's website ( has a lot of details and assembly methods within it already. You don't need to go to foreign firms for that information. Aercon sells thicker materials, and, some have claimed that in a very cold climate, the use of thicker blocks (10", 12") could negate the need for additional insulation. I haven't seen any empirical evidence that this is so, and don't personally believe that this would work in colder north American climates. I'd love for someone out there to prove me wrong on this. For my particular house, the higher cost I experienced was definitely related to other design decisions. We put a lot of decorative design oriented bells and whistles in the house that didn't need to be there, and, again, if we were to do it again, I'd consider going with a stucco exterior on top of rigid insulation that was mechanically fastened directly to the blocks. The multiple layered insulation and furring/blocking system that we employed to successfully support the very heavy fiber cement siding/trim system came with at a cost. I think next time I might just go with 1 x 4's outboard of the rigid insulation that were both screwed back into the AAC while also "hung" from the roof structural system and top plate via screws. That would simplify things a bit and lower costs.

Feb 15, 2016 5:25 PM ET

Bond beams
by Ethan T ; Climate Zone 5A ; ~6000HDD

Ok, I found a lot of details for floor and roof panels at Aercon ( and am concerned by the thermal bridging at the bond beams which seem required at wall/roof and wall/floor connections.

Feb 15, 2016 10:52 PM ET

Edited Feb 15, 2016 10:53 PM ET.

Bond Beams
by Daniel Levy

E T is correct that the insulation value of bond beams is lower than the rest of an AAC wall, but I believe the reduction is negligible. Masonry structures are generally reinforced with bond beams at the top of each story, which with AAC is constructed with U-shaped blocks. An 8" wide U-block has two inches of AAC on both sides and the bottom, with more at the lower corners of the U-shape, leaving a 4" wide by 6" high void for rebar and mortar. The area of the bond beam is minor compared to the overall wall area, and the R-value reduction is also small. Since I believe in using external insulation, the bond beam is inboard of the insulation. There may be an argument that the greater thermal mass on the interior is slightly beneficial.

If you are building with AAC in a warm climate and choose to not use external insulation, there is a possible solution. It has been too many years for me to remember for sure which organizations and companies were involved in a study of this issue, but I believe it was the National Association of Homebuilders Research Center and Hebel. The NAHB Research Center built a group of town-homes in the late 1990s in Maryland to test a variety of wood alternatives (sold after construction, so long-term data is not available). One was AAC, and they were concerned about the lower R-value at the bond beams. They suggested adding a piece of rigid insulation vertically against the outer web of AAC, reducing the width of the mortar fill. Hebel personnel agreed that was a viable option. Hebel U-blocks had a rectangular recess, not a U-shape, so adding insulation was easier, but they were more susceptible to damage in shipping than Aercon's U-shape. Since I use exterior insulation, I've not investigated this idea any further. By the way, vagrants set a fire in the NAHB Research Center's Hebel house; it did no damage.

Feb 17, 2016 5:39 AM ET

Hebel technical bulletin
by Ethan T ; Climate Zone 5A ; ~6000HDD

OK, I've finally gotten around to reading the Hebel technical bulletin and I do find the effective R value argument somewhat convincing, especially having spent time in mass homes and felt the difference in sensible comfort.

Mar 21, 2016 11:35 AM ET

Edited Mar 21, 2016 11:35 AM ET.

Durisol vs AAC
by Ethan T ; Climate Zone 5A ; ~6000HDD

I've gone down the Durisol rabbithole, which inadvertently led me back here... I find the following quoted text confusing:

Coldham, he said, wanted to use Durisol, a type of insulated concrete form. "He loves it," Bluestone said. "I looked at it and I thought, 'I guess I can do this,' but I wasn't in love with it."

Bluestone wanted to take the lessons learned on his own house and apply them to the kind of project his family's firm was developing — but he didn't think Durisol was especially well suited to the task. "There's no way I could build big buildings with Durisol," he said, calling the expected result "big and funky and unexacting.

What exactly is the issue with Durisol here? And why would it be more "big and funky and unexacting" to build with Durisol than AAC. It seems that both lend themselves, for obvious reasons, to "blocky" designs.

Mar 21, 2016 12:05 PM ET

Response to Ethan Timm
by Martin Holladay

I'm not going to pretend to speak for Steven Bluestone.

I know that homeowners and builders often have idiosyncratic reasons for choosing certain building materials, so I'm not going to try to talk you out of using Durisol forms if you like them.

As I'm sure you know, Durisol blocks are made from wood shavings and Portland cement. After being dry-stacked, the voids in the blocks are filled with reinforced concrete.

For me, the main problem with Durisol block is the same problem I have with most block walls that include concrete: low R-value. While the manufacturer claims that the R-values of these walls varies (depending on the product you choose) from R-9 to R-20, no independent laboratory has every verified these R-value estimates. It would seem that thermal bridging is likely to undermine the R-value of these walls.

Apr 9, 2016 8:55 PM ET

Edited Apr 9, 2016 8:56 PM ET.

Passive House certification
by Steven Bluestone

I am very pleased to announce that my house passed the final blower door test per the PHIUS protocol. As of Friday, to my knowledge, the house is now the first Passive House structure built with AAC walls in North America. My next project (to be undertaken this year) is to construct a 500 SF shop building on the site out of AAC to PHIUS standards. In lieu of deep foundations and a somewhat expensive and elaborate insulation/cladding system, the goal will be to go with a shallow "Alaskan" slab foundation system, a thick blanket of mineral wool insulation at the exterior of the AAC, and some type of EIFS/stucco finish directly applied to the mineral wool. The goal is to come up with a Passive House structure that has walls built with just three components: AAC blocks, mineral wool, and stucco. Nothing else will be needed for the solid wall areas. No added barrier materials, etc., etc. Hopefully the walls will end up being the same cost or less than most other Passive House walls built for northern, cold (zone 4ish) climates. Stay tuned.....

Apr 21, 2016 2:20 PM ET

energy positive
by Steven Bluestone

.....and, as of yesterday, our electric utility bill confirmed that we are energy positive at the house.

Apr 22, 2016 6:02 AM ET

by Steven Bluestone

Going back to Ethan's comments on Durisol, I believe there's a place for Durisol and also appreciate the fact that it may permit one to avoid foam, but for me it partially comes down to the weight of the units. I have a couple of sample blocks. They are very, very heavy as compared to AAC. Perhaps the fact that I have on and off issues with lower back problems is serving to sway my opinion? The other part relates to the need to apply water, vapor, and air barriers to the very porous Durisol blocks (which is something not required with AAC).

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