New Home AAC Zone 6b
We are looking to build a new home this spring in Southern Wisconsin (zone 6b). While the house plan is being finalized we have been debating the value of building with the local standard 2 X 6 frame house vs. AAC concrete (12 inch Aercon AC4). Majority of homes built in the area are of a simple 2 X 6 frame with no AAC homes that I can find in the area. While it does appear good quality insulation can allow for the 2 X 6 frame to meet an R40 rating we wonder if the AAC will actually provide more benefits. In particular from the articles posted AAC appears to provide better sound control, air tightness, storm / fire resistance, and off gassing. Additionally all these added benefits appear to come with no drawbacks. Specifically it appears the cost and time to build will be very similar. My real question is why not use AAC and why is no one else using it?
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I have no experience with AAC, so I cannot comment on that. However, there is no way I know of to achieve R40 in standard 2x6 wall construction. Even closed cell spray foam, would only get you R32 or so. If R40 is your goal, I would suggest taking a hard look at a double stud wall system.
According to the Aercon website the nominal R-value for an 8" Aercon wall is R-11.5, so the 12" wall would be about R17 or so. Don't be misled by the higher 'equivalent' R-values quoted from the Florida Energy Center. Based on thermal mass (thermal capacitance) effects, these are mostly significant only in cooling climates and have no relevance to Wisconsin winter conditions. 12" Aercon is still quite comparable to a conventional 6" framed wall but R40 is nowhere in it.
That said AAC is a good material and well worth considering. Just be sure that your builder is on board with the concept and that your cost calculations include likely upcharges from electricians and plumbers unfamiliar with the construction method.
AAC is a type of concrete. While AAC has more insulation value than regular concrete it still has far less insulation value than standard insulation materials. So how are you going to meet R40 with AAC? Your time is better spend thinking about other important aspects of your new home.
I have a preferred wall assembly (stud wall, use the exterior sheathing as the air barrier by air sealing the seams corners and edges, and add sufficient exterior rigid board insulation to ensure the sheathing is warm) but what is most important is to hire knowledgeable designers and contractors and work within assemblies they can guarantee.
Ask the people you are working with:
What is the air barrier strategy for the entire thermal envelope? How much insulation value should I have in each of my assemblies and why? What is the target for air tightness and how are we properly ventilating? What are the NFRC ratings of the proposed windows and how do your window details assure proper flashing and air sealing?
Build your house right the first time by finding good professionals. Fixing it later is much more expensive.
Further thought - you may want to consider an 8" Aercon veneer on the outside of standard 2 x 4 structural framing with cellulose - you'd end up with R23 or so with the advantages of the Aercon on the outside and conventional service runs on the inside
All thank you for the great feedback and point of focusing on the end result of what is desired; not the product used. The R40 was an estimate rating I was told could be meet with 3 inches of polyisocyanurate on the exterior. My main concern was that it appears builders are not considering AAC in this area and is this for a good reason?
Well, that's a different story. Adding 3" of polyiso to either 12" Aercon or conventionally insulated 6" frame wall could definitely get you in the region of R-35 or above.
I expect builders in your area as in most parts of the US are leery of unfamiliar technologies with an unpredictable learning curve that's likely to eat into their margins. AAC has been popular in Europe for many years because it is a natural progression from traditional masonry construction: the parallel is that builders there have equally tended to regard North American style stick framing with suspicion.
Joe,
You can build a very energy efficient and durable house using common and proven building materials. Most homes built today in the US are energy code minimum with little attention given to airtightness. Develop a wall and roof sytem for your area with R- values in line with superinsulation. R-40 walls and R- 60 ceilings for southern WI and an ACH50 of 1.5 or less will put you in the 1 Btu per sf per hdd range. Like Aaron, I would consider a double wall with blown blanket insulation, exterior foam is not needed if you build the proper wall.
J,
So... you are one of those exterior sheathing-outside-insulationists
the greatest problem/challenge that I see with outside insulation stratagems is with the window details.
J, do you have photos or links to the window details that you use with your "preferred assembly"?
and on another note.... when I look at the European Websites ... I am not really seeing any examples with "AAC"
but I have seen examples of these Ziegel Blocks...which seem to be very interesting
http://www.natural-building.co.uk/PDF/Thermoplan-Ziegel/ThermoPlan%20Manual.PDF
John,
Hope I'm not premature to show a preference towards the air-tight sheathing method.
I haven't sat down and drawn the window details yet but this is an assembly I plan to use on a design in progress. I don't expect the window details to be any more challenging than other super-insulation strategies.
I'll be happy to share the details when complete but it might be awhile. I'm having a hard time finding time to work on the design which is a side project for a family member.
Joe,
I've built a house with the Aercon AAC 12" block. See the attached picture.
AAC has some nice benefits, like you listed. It's also monolithic, durable, involves only three layers (stucco, block, plaster), and has a old-world aesthetic. On the downside, it's extremely heavy, does not provide much insulating value, and it does not provide a drainage plane.
Oh, and your electrician will hate routing cable into the wall ;-)
I think the drainage plane issue is why you don't see it promoted much by the building scientists. It is much like the old three course brick structures---sturdy and durable. But those kinds of structures have their own set of problems.
The house I built was located in Arkansas, with ~ 3,300 HDD. I would definitely not recommend it for your climate, UNLESS you supplement the wall by installing exterior insulation. Just a block wall wouldn't meet code anyway. A good choice would be EPS and soft-coat stucco on the exterior of the AAC block. That would also limit the amount of moisture migrating through the walls.
You could also follow James Morgan's advice for an interior stud wall. You might need to do that in certain locations for utilities. But, since the AAC absorbs a lot of moisture during wet weather, your stud wall cavity would need to provide really good drying potential (no vapor barriers).
For your cold climate, ignore the "dynamic benefit of mass" issue. It just will not work. I can imagine an AAC wall would work really well in the southwest, or anywhere with a significant diurnal temperature fluctuation. In Arkansas, we found it very comfortable during the cooling season, but not so much during the winter. The walls would radiate cold during long stretches of cloudy / cold weather.
Why is nobody using it? They are, down south, where Aercon is based. And it's popular in Europe. Up north, not so much.
For a mason, AAC block is unusual. You can't adjust a course by adding more mortar, or making a thinner joint. The blocks and the courses have to be dead level. When they aren't, the block has to be rasped down to the same plane. A 12" block weighs ~ 70 lb. and it doesn't have a web (it's solid). Before you make a cost comparison, make sure you get a mason to bid on the block installation.
Anyway, those are some thoughts from somebody that has built with AAC. If you have particular questions, you can email me at djncu @ comcastdotnet (replace the dot with a dot). Best of luck.
Note to James - and this is from my memory - standard density AAC has an R-value of 1.0 / inch. The 12" block has a steady state R-value of R-13. The figures you cited might include the DBMS factor.
Joe, sounds like a good assembly to me if built by an experienced crew. I agree with James too as to what is popular. Sticks are the most available and they price out right for most developers.
Edit: " The walls would radiate cold during long stretches of cloudy / cold weather." Per Danial's post below. If you had a woodstove or solar warming that needed to be stored.... Might be the only valid reason for trying your assembly in a heating dominated climate.
Best to find a successful builder in your area building what he knows how to build is my best advice.
Daniel: I got the nominal R-value of 11.5 for an 8" block from the Aercon site: http://www.aerconaac.com/index-2.html, click on the Thermal Efficiency link for a pdf. They claim up to an additional R-9 'equivalent' when allowing for the mass effect, which I entirely agree will be of no significance in a long Wisconsin winter. I see no reference to measured values, only calculated ones which they claim are the result of studies performed by the Florida Solar Energy Center. A quick glance at the FSEC website suggests this is a respectable organization but as their name suggests their focus seems to be on solar thermal and PV systems and I see no obvious reference to any insulation performance studies.
If you're interested in rigid insulated concrete walls, check out TF Concrete Forming Systems in Green Bay.