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Lighter, more fire-resistant, and a better insulator, autoclaved aerated concrete caught on in the rest of the world ages ago. It's taking a lot longer in the U.S.

To read what manufacturers and distributors say about it, you'd think autoclaved aerated concrete (AAC) was some kind of new, space-age environmental miracle.

Although it certainly has some nifty properties, AAC isn't new and isn't miraculous--but it's certainly popular in Europe, and has been for decades; according to one source, it accounted for 60% of all new construction in Germany in 2006. It has enjoyed pretty flat market share (of near zero) here in the U.S., though, since it was first introduced in the 1990s.

Is there space for AAC in the U.S. market? Should the green building community be working to make space?

How AAC is made

AAC is similar to other concrete types, except that it contains no aggregate; sand or fly ash is included, with aluminum powder added to react with one of these ingredients and "leaven" the concrete, creating tiny bubbles just like baking soda does when it reacts with the buttermilk in your muffin batter. (Your muffins are full of carbon dioxide bubbles, but AAC is full of hydrogen bubbles.)

[Note: Robert Riversong points out in comments that sand is aggregate, which I also thought when I started researching it, but after some more digging, my understanding is that the sand is used as a reactant and is therefore not considered aggregate in AAC. For more, see here.]

The concrete is poured into molds, left to rise, and then "baked" in an autoclave, which uses steam and pressure to complete the chemical reactions and speed up the curing process significantly--completing in hours rather than weeks. The resulting blocks are so full of bubbles that a block of the same size has about one-fifth the material required by regular concrete.

Like conventional concrete masonry units, AAC is sold in a variety of block shapes and sizes, but unlike conventional units, most don't have cores. They are porous and light, like muffins, but not hollow.

Benefits of AAC

The main advantage of AAC when it was first developed in Sweden in the early 20th century was simple: it wasn't wood. It's still not wood, but in North America (unlike in Sweden at the time and in most of Europe now), wood is still plentiful and cheap.

Compared with conventional concrete, AAC still has advantages, though:

• It uses less material--important for concrete, since portland cement is one of the most energy- and carbon-intensive building materials.
• Despite the energy-intensive autoclaving process, manufacturers say it takes about 50% less energy to make, because of the lower portland cement content by volume (we're haven't found anyone to challenge those claims, but are still looking for data).
• It's lighter, which cuts down on transportation costs and fuel use.
• It's a better insulator, with a steady-state R-value just a hair above R-1 as opposed to something more like R-0.2 (neither of these factors in thermal mass, which we'll get to later).
• Air leakage is minimal.
• AAC also has excellent soundproofing properties.
• It can also be used as a firebreak.

Drawbacks of AAC

In a report written for UC–Davis (PDF), Stefan Schnitzler finds few disadvantages to AAC. Here are the two demerits on his list:

• There are few manufacturers in the U.S. (that was in 2006, and now there are almost none, since Xella has moved its Hebel operation to Mexico); this means higher costs, which is a huge barrier for adoption.
• AAC requires a learning curve for builders, because the mortar application is more precise.

We would like to add a few drawbacks that we've found:

• The barriers for builders don't stop with the mortar. According to Derek Taylor, owner of AAC distributor SafeCrete, the only manufacturer in North America right now is a German company whose block dimensions don't work for U.S. builders. These often need to be sawed, adding labor and fuss to a building system that's supposed to be simple. (Taylor's looking forward to two new plants coming online in the States in the next couple years.)
• Since right now your AAC is most likely coming from Mexico, the advantages offered by lighter weight will diminish significantly as the mileage increases.
• Thermal properties are better than those of conventional concrete, but they aren't good enough to make AAC a viable wall material (relative to BuildingGreen-recommended R-values) in most U.S. and Canadian climates without additional insulation. (The European climate, where AAC is popular, is milder.)
• Unless rebar is added--which adds to the weight and amount of material in the blocks--AAC can only be used for low- and mid-rise construction. But it seems to be popular for single-family homes as well as schools.
• Unlike conventional concrete, AAC can't be used as a finish; it is more porous and needs cladding or stucco on the outside so it won't absorb moisture.

Would you use AAC?

That said, AAC does appear to have significant advantages for applications where conventional concrete would normally be the best material--like in the American Southwest and in other climates where thermal mass can increase the "effective" or "mass-enhanced" R-value of the wall. Even then, its performance may still be outmatched by that of insulated concrete forms, depending on the needs of the client.

Unfortunately, much of the information we have on AAC performance in the U.S. comes from manufacturers. We'd like to hear some empirical evidence from the field.

Are you using AAC on any of your projects?

If you've used it, how did it perform? If not, what would it take for you to try it out?

Will environmental product declarations end greenwashing for good? Not so fast.

This is Part 3 in our series on transparency.

Part 1: Why We Care About Product Transparency

Part 2: Why We Need "Nutrition Labels" for Building Products

We all want to know more about where our building products come from and what's in them. Finally, with the emergence of environmental product declarations, we're going to find out!

Aren't we?

The promise of the product transparency movement is huge, and we think this nascent trend is going to play a big role in sustainable manufacturing, design, construction, and operations in the next few years--one reason why we dedicated this month's EBN feature to product transparency.

But things aren't as simple as they might seem in this uncharted realm. There's more to environmental product declarations (EPDs) than meets the eye. And in many cases, there's less than meets the eye too. While many manufacturers are working hard to show leadership on true transparency, there is also a risk of insidious greenwashing like we've never seen before.

Five myths about environmental product declarations

• Products "earn" an EPD. Far from it. Manufacturers pay tens of thousands of dollars to get one, and the fact that a product has one tells you absolutely nothing about its environmental performance. You have to actually read the report to know whether the product is environmentally preferable or not.
• An EPD is like a nutrition label for building products. When's the last time you saw a 20-page nutrition label on your Cheerios? We're guilty of spreading this one ourselves, but it's not really true...at least not yet. Yes, the information in an EPD must follow a standard format, but that's where the similarly ends. For one thing, EPDs are completely voluntary--but also, we're very far from having an at-a-glance on-product label, although there are efforts to develop this. Here's one proposed by forward-thinking designers like Perkins + Will.
• EPDs are about science, not marketing. EPDs are backed by life-cycle assessment, and both have to be done by a third party in accordance with ISO standards. But companies can pick and choose which products they want to have EPDs for, and many will choose for marketing reasons; you won't find out how clean the company overall is by looking at one product's EPD. By contrast, InterfaceFLOR is working to develop EPDs for all its products--but Interface is definitely an exception. You may also find marketing language in some parts of the report.
• EPDs list all the ingredients in a product. Yes, the EPD has to list the materials the manufacturer uses--but you aren't likely to find out what's actually in those materials (like flame retardants or insecticides, for example) by looking at an EPD. We're working with the Healthy Building Network and a variety of manufacturers and other stakeholders to develop a health product declaration (HPD) that could help fill in the gaps. One of the things we're hoping the trend toward EPDs and HPDs will do is to help manufacturers get more information from their suppliers about potentially harmful ingredients.
• EPDs include information on toxicity and ecology. EPDs can sound a lot more comprehensive than they are. Although the life-cycle assessment behind the EPD will look at "human health" or "ecological" effects, those categories sound broader than their names justify. (See our recent coverage of a life-cycle assessment of hand-drying methods, where "ecosystem quality" was narrowly defined as "potentially disappeared fraction of plant species per square meter per year.") As far as human health goes, the HPD could help here too.

Where EPDs come from: the infographic

The success of product transparency depends on the design community knowing what an EPD is--and what it isn't.

In the process of writing the feature article, we put together an at-a-glance chart to explain where EPDs actually come from and to show a few key points you should know about them. Click above to view a larger version. Click here to download a PDF for printing.

Why we need EPDs, even though they're flawed

While you're printing it out, take five minutes to watch our fun video exploring what we think is so great about product transparency--including what you can do to make sure all the building product information you need is at your fingertips.

Our pellet stove has DC fans and a kit that allows us to hook it up to a battery to power those fans in the event of a power outage. Photo: Alex Wilson. Click on image to enlarge.

House location and design are the starting points in achieving resilience--where the house located, how well it can weather storms and flooding, and how effectively it retains heat and utilizes passive solar for heating and daylighting. Beyond that, we should look to more active renewable energy systems for back-up heat, water heating, and electricity. This week we'll review these options.

Wood stoves



In rural areas, clean-burning wood stoves provide an easy option for back-up heat. With a compact, highly energy-efficient (resilient) home, a single, small wood stove can effectively heat the entire house when there is a power outage or interruption in heating fuel. Even in our current home, which is far from what I would call a "resilient" (relative to energy performance), we use a wood stove as our primary heat source--albeit accepting significantly cooler temperatures in parts of the house that are distant from the wood stove.
Wood stoves are dirty, though--even EPA-compliant models (as all new wood stoves sold new today must be). In a rural area, such as where I live, reliance on wood heat may be acceptable, but in more densely populated areas extensive use of wood heat would cause significant pollution problems. Even in our area, when there is a power outage and more residents fire up their wood stoves, the air quality deteriorates. Thus, wood heating makes the most sense when the house to be heated is highly energy efficient so that little wood needs to be burned to maintain comfortable, safe conditions. And then, the wood stove should be operated for maximum combustion efficiency (minimum smoke production).

Pellet stoves



Like wood stoves, pellet stoves can do a good job of heating an energy-efficient house. Because of the fan-supplied combustion air, pellet stoves tend to be much cleaner-burning than wood stoves. The need for electricity to operate, though, makes pellet stoves inherently less resilient.

Our pellet stove--the sole heat for the apartment above our garage--works like most pellet stoves when AC electricity is available: electric coils ignite the pellets during start-up, a fan brings combustion air to the burn-pot in the stove, and another fan blows the heated air into the room. In the event of a power outage, however, our pellet stove--unlike most--can still be operated. The fans in our Quadra-Fire Mt. Vernon AE have DC motors, and we have jumper cables that allow us to operate the stove during a power outage by clipping them to an automotive or other deep-cycle 12-volt battery. This back-up power isn't enough to start the pellet stove (we have to do that manually with pellet starter gel or some kindling), but the battery can power the two fans.

This photovoltaic system in southern Vermont provides back-up power during power outages. Photo: Alex Wilson. Click on image to enlarge.

Solar electricity



The ultimate in resilience can be achieved with a solar-electric (photovoltaic) power system that can be used when the grid is down. Photovoltaic (PV) systems directly convert sunlight into electricity. PV modules can be installed on a roof or on ground-mounted racks. Most use silicon wafers that are specially made so that photons of light excite electrons and generate direct current (DC) electricity. An inverter in most PV systems then converts that DC electricity into alternating current (AC) that can be used by standard household appliances and also fed into the utility grid through a net-metering system.

The problem with most grid-connected PV systems is that when the grid goes down (during a power outage), you can't use the electricity. This is a safety feature with grid-connected PV systems to prevent them from feeding electricity into the power grid when linemen may be repairing down wires. To serve as a power source during a power outage (key to resilience), it is generally necessary to install some battery back-up and a "hybrid" PV system. These systems are more complex (and costly), because they include not only a battery bank, but also controls that send power either to the battery bank or power grid, depending on the charge state of the batteries and status of the grid. There are apparently some specialized inverters that allow electricity to be used in the home (during the daytime when the PV system is producing electricity) even when the system is disconnected from the grid during an outage, but these inverters are uncommon.

This solar water heater on a Guilford, Vermont home is augmented by a heat exchanger in the wood stove, which offers back-up water heating in the event of a power outage--relying on passive thermosiphoning. Photo: Alex Wilson. Click on image to enlarge.

Solar water heating



To heat water when the electric grid is down the best option is a solar water heating system that can operate without AC electricity. Some "active" solar water heaters have DC pumps with integral PV modules that operate the pump when the sun is shining--thus the PV module serves both as the controller and the pumping power. There are also two types of passive solar water heaters that require no electricity. Thermosiphoning systems have the solar collector mounted below the storage tank, and solar-heated water rises through natural convection into the storage tank when the sun is shining. With batch or integral-collector-storage (ICS) solar water heaters, the water is stored right where it is heated (with water pressure delivering that water to a collector on the collector on the roof).

A solar water heating system can be augmented with water heating coils in a wood stove to ensure adequate hot water during the winter months when there is less solar energy.

About this series



Throughout this resilient design series, I'm covering how our homes and communities can continue to function in the event of extended power outages, interruptions in heating fuel, or shortages of water. Resilient design is a life-safety issue that is critical for the security and wellbeing of families in a future of climate uncertainty.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

This exterior window shade in Florida blocks most of the solar gain, yet allows some view out. Photo: Alex Wilson. Click on image to enlarge.

Over the past month-and-a-half, I've been focusing on resilient design--which will become all the more important in this age of climate change. Achieving resilience in homes not only involves keeping them comfortable in the winter months through lots of insulation and some passive solar gain (which I've covered in the previous two blogs), it also involves keeping them from getting too hot in the summer months if we lose power and our air conditioning systems stop working. This week, despite the freezing weather, we'll look at cooling-load-avoidance strategies and natural ventilation.

Orientation and building geometry



With new houses, we can relatively easily control orientation and geometrical form to minimize unwanted solar gain. The optimal orientation for a house is with the long axis running east-west, so that the longer walls face south and north. This allows the house to benefit from the sun when we want that heat, but keep it out when we don't want it. The sun always rises in the east and sets in the west, but in the summer it rises much higher in the sky. By having more windows facing south, most of the sunlight will glance off that glass during the summer when the sun high overhead, while in the winter, with the lower-angle sunlight, most of that sunlight shines through those windows--providing passive solar heating (see last week's blog).
At the same time, having fewer windows on the east and west make sense relative to summertime overheating. Significantly more sunlight shines through a square foot of east- or west-facing window during the course of a day in the summer than through a square foot of south- or north-facing window, so limiting east and west windows helps to prevent overheating.

Window selection

The type of glazing in our windows has a major impact on how much sunlight is transmitted through them. This is why it almost always makes sense in well-insulated buildings to "tune" the windows by orientation. By this, I mean using glass (glazing) on the south that transmits a high percentage of the sunlight striking it and glass on the east and west that transmits less sunlight. We refer to this property as the solar heat gain coefficient (SHGC); it is the fraction of total solar energy transmitted through the glass (assuming the sunlight strikes the glass at a normal (perpendicular) angle.

A good rule-of-thumb is to select south-facing windows that have SHGC values of 0.6 or higher (0.5 or higher with triple-glazed windows), and east- and west-facing windows with SHGC values of 0.3 or lower. Windows with SHGC values of 0.6 will transmit twice as much solar energy as windows with SHGC values of 0.3. The beauty of recent advances in glazings it that we can now have fairly large window areas (to provide views and natural lighting) without nearly the energy penalty (both from heat loss and unwanted solar gain) we had two or three decades ago.

Shading windows from direct sun



On the south, we can also use simple overhangs or awnings to block virtually all of the direct sun. On the east and west, different shading strategies are better, because the sun is lower in the sky. For these windows, exterior shade screens or roller blinds can be very effective. So can plantings of tall annuals like hollyhocks or vines like clematis, morning glory, and grape.

Designers and builders in the south learned the principles of shading windows long ago. Traditional architecture in hot climates often included wrap-around porches that kept direct sun out of the house, while providing pleasant outdoor living space. (Part of resilient design is looking at how our grandparents or great grandparents built--and returning to some of this vernacular architecture that is so well-adapted to the local climate.)

Reflective roofs and walls

Light-colored roofs and walls reflect, rather than absorb, most of the sunlight striking them. By not heating up as much, less heat is transmitted through to the interior. With high insulation levels in roofs and walls (see below), the need for reflective exterior surfaces is less important, but this strategy can still make a difference.

High insulation levels and tight construction



Just as an energy-efficient building envelope reduces heat loss in the winter, it also reduces unwanted heat gain during the summer--thus helping to control cooling loads and maintain comfort. If we follow the sort of recommendations for insulation levels for resilient homes that were outlined a couple weeks ago, unwanted heat gain will be very effectively controlled in the summer--as long as windows are closed during the hottest days.

Natural ventilation

Finally, we can achieve resilient homes that won't get too hot if power is lost and air conditioning doesn't work through natural ventilation. This strategy is particularly effective at night, when it's cooler outside than in. Simple operable windows with screens offer the primary strategy here, but we can go further. In hot, sunny climates, such as the Southwest, one can build solar chimneys that use the natural buoyancy of warm, rising air to pull in cooler outside air--sometimes through inlet tubes buried in the ground (earth tubes). Operable windows high on a wall or skylights can also serve as solar chimneys.

All of these natural cooling strategies can keep a house safe and reasonably comfortable in the summer during power outages. During normal times, such measures will significantly reduce the amount of time an air conditioner has to operate, while keeping the house more comfortable.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.