Has anyone tried AAC exterior walls without additional insulation in climate zone 4A? What thickness would equate to a 2x6 insulated wall in this region?

I built an AAC house in so central pa, in zone 4 right over the line from zone 3. To answer your question first, a 12 inch block is roughly the same as the best 2x6 stud wall insulation. Dunno if you talk the lingo so bear with me. Stud wall R value is the rating of the insulation itself. A whole-wall number would account for the lesser insulating value of the wood in the wall. A 2x6 wall with studs 24 inches on center is R13.7 ish. A 12-inch AAC block is rated R15. Accounting for vertical concrete cores every four feet, and concrete bond beams at floor heights, it should have a whole-wall value pretty close to the 2x6 stud wall, and better than the typical 2x6 on 16" centers.

There is one more concept to master. Mass effect accounts for the time lag between heat or cold applied to the outside of a high-mass wall and when it finally arrives inside. At times when the ambient temperature ranges above and below "comfortable," the fluxes of hot and cold cancel out before they get inside. I built with 8 inch block and used mass-enhanced value of R17 to get my building permit. Averaged over a year in the nearby Washington DC climate, the mass benefit in spring and fall more than offset the wall's R10 performance in summer and winter. (If ambient temperature are consistently above or below comfortable, heat travels in only direction and there is no mass effect.)  Today's code says walls must be R19, so I'd need 10-inch block now.

I used AAC in conjunction with aggressive passive solar, and count on it to buffer daytime heat rather than for mass effect or heat storage. The fact is that climates in the the northeast are not particularly suited to mass or passive solar. Google UCLA and HEED and you will find the university's Climate Consultant 5. The free software allows you to download historical weather data from the nearest weather station. It has a chart listing the number of hours in a year that high mass will keep the house comfortable (as well as other energy strategies.) It told me ramp up the insulation.

Shudda listened.  The chamfer and chair rail elements below are 12 inch block. The flat surfaces are 8 inch. I could have added an inch of EPS on the flat surfaces for a nominal extra expense and a big bump in performance.

So, why use AAC? It is an architect's wet dream. It has excellent sound qualities. It is fire proof. (As in AAC houses still standing in Dresden the morning after.) Your neighborhood masons can build with it no problems (but many complaints.) It can be less expensive if you leave bare block in garages and closets. For the rest, parge plaster and stucco directly to the block.  

Thanks toddm!

So, you've got all the benefits of AAC aside from a heavy insulation...just how difficult is it to heat/cool your home with 8" walls? Currently, we live in a leaky 2x4 framed house...anything airtight would be a major improvement, and the opportunity to heat with a wood stove through the winter ought to make us feel rich, if not warm.

If you don't mind my asking, I have lots of questions...such as: can a person use locally sourced thinset mortar and stucco, or must you absolutely go with the vendor's specially formulated products? Shipping costs are my greatest concern, along with availability if I miscalculate/spill a bag or two.

Our hvac to date is a 2 ton mini split heat pump conditioning 1600 SF. Our total power bills at about 7 cents/kwh are ~$60/mo. There were two or three episodes this winter when the heat pump struggled and we bundled up. We have yet to power up the radiant heat in our slab on grade, which is fired by a wood stove boiler. Once that's going, our overall energy costs should be $30-$40/mo and we decide how warm we want to be. Walls have relatively little impact on energy costs compared to attic insulation, air tightness and windows. Our attic is R60. As in any concrete stucture, air infiltration comes down to how well you do the doors and windows. Thanks to passive solar, our windows break even on an annual basis in the gray northeast.

Yes, you need AAC formulated products. The thing about AAC is that it is 80 percent air, including the surface to which stucco and plaster must bond. And because it is cured in a autoclave, or steam oven, it will shed moisture for as long as year. The good news is that it isn't a major financial hit. Sider Oxydro shipped plaster from Hawkinsville Ga for ~65 cents/sf and conventional stucco for about $1.30/SF, both of which were super competitive with local prices. Call them directly.

I made two runs to Performance Concrete in Tipp City, O (Dayton area), a distributor for Aercon AAC, for block that wouldn't fit on a two semi flatbeds. (Shipping from Aercon in Haines City Fla. was $1,500/semi.) Aercon sold polymer fortified thinset for $6/bag. When I ran out they told me that Versabond at Home Depot was essentially the same except it cost $15/bag (!?)



The thing about AAC is that it is 80 percent air, including the surface to which stucco and plaster must bond. And because it is cured in a autoclave, or steam oven, it will shed moisture for as long as year. So, yes, you need plaster and stucco formulated for AAC.

I read elsewhere that AAC wicks up moisture quickly and becomes weak and crumbly. Did you have any problems with block absorbing moisture during construction? How long (weeks/months?) can a wall remain exposed before the exterior stucco is applied?

There isn't much hurry. In our case, the house was up nine months before the stucco crew arrived, and 15 months before the plasterer came. Aercon liked the idea of leaving the interior uncovered. The problems happen when builders cover the block too quickly. Aercon wants the blocks kept dry during construction to minimize the normal drying out process. We had two noreasters go through dropping 5 inches of rain each time. I learned that AAC floats, in fact. They dried our once the blocks were vertical and under an overhang. Wet blocks are heavier and harder to cut. And expect some serious humidity in the house for the first year or so, particularly if you parge plaster over the blocks.

Gotta say the claims that wet blocks deteriorate is contrary to my experience. Not even freeze/thaw had any effect on mine. AAC is naturally friable, or breakable, and you have to be careful with edges and corners.

The walls are quite solid once they are together and sans edges and corners. I backed my one-ton truck into the garage wall during construction and left only a dent. Repeating this experiment/accident after the block was under a half inch of stucco, iI got out to find it undamaged. My stucco guy pronounced it superior in impact resistance to stucco over foam, and far superior to dryvit over foam.

A moonlighting mason and I set the block using an ancient box store cement mixer, the genie lift in the picture and this saw: http://www.youtube.com/watch?v=JxpyI3lHGhU

AAC passes moisture readily but isn't much damaged by it, even during freeze/thaw cycling. The finish paints(both interior & exterior) need to be at least somewhat vapor permeable to not end up with paint longevity issues, just as with any unvented masonry wall. As a a moisture reservoir it's some what lower than common brick though, so it's not going to have the same levels of intense moisture drive as solid brick walls- it's probably more comparable to cavity walls or CMU walls for moisture transfer.

A 12" AAC wall with very thin grout/mortar would meet code-min at just shy of 15 whole-wall (as long as it doesn't have a lot of thermal bridging from bond-beams etc,) in most zone 4 locations. But the value of the dynamic thermal mass issues vary widely with the local climate and site conditions. It's WAY better than CMU, but underperforms a minimalist ICF in most applications.

Any air-tight R15-ish wall is a huge improvement over leaky 2x4 construction. Even air tight 2x4 construction (~R10 whole-wall) is a MAJOR improvement over barely insulated leaky 2x4 from both a comfort and thermal performance point of view.

The AAC construction that I am familiar with normally require bond beams that go most of the way through the wall at each floor level.  In some areas, a mid level bond beam may also be required.  Knowing this makes me want to add insulation to the exterior.

I am having trouble reconciling the North American numbers with those presented by Ytong. Basically a 240mm block has a U value of .0158 or R63 if you use Ytongs number of .09W/mk.

What am I missing? These blocks must be different.

As in everything, energy efficiency comes down to compared to what. A comparable home to mine, 1600 sf, at today's code min and heated by a minisplit priced at 7 cents/kwh almost assuredly would put energy bills at $100/mo or less in zone 4. Again, almost assuredly, there is very little you can do to the walls that improves code min enough to recover costs in a normal ownership period. In my approach to green, the emphasis is how to make 1600 sf seem much bigger, and how to DIY wall systems in a way that saves money and increases curb appeal. At the risk of seeming immodest, AAC has been a major winner for me.

If AAC does act as a moisture reserve (comparable to cavity walls, per Dana1), will that cause a challenge to mini-splits for removing humidity?

Todd, how humid is PA? Have you ever experienced condensation on walls if window/doors were opened on a humid day? My in-laws live in an earth-contact, and they have to be very careful about warm moist air flooding in through open doors...I'm hoping not to have an issue with sweaty walls if I go with a thermal mass system.

.I'm hoping not to have an issue with sweaty walls if I go with a thermal mass system.

Cool walls and letting warm moist air in (ie, opening the windows) can certainly create a sweaty walls problem. Unfortunately, opening the windows is what you need to do to maximize the value of interior thermal mass. Consider low mass walls and use the money you save for more insulation.

Posted By AAC-curious on 13 Apr 2013 11:32 AM
If AAC does act as a moisture reserve (comparable to cavity walls, per Dana1), will that cause a challenge to mini-splits for removing humidity?

Todd, how humid is PA? Have you ever experienced condensation on walls if window/doors were opened on a humid day? My in-laws live in an earth-contact, and they have to be very careful about warm moist air flooding in through open doors...I'm hoping not to have an issue with sweaty walls if I go with a thermal mass system.

This should answer your question:

Green Building Advisor - AAC and humidity

Green Building Advisor - AAC

I believe the "Dana" from this forum commented on that forum, as per "Dana Dorsett" from GBA stated:

"Attempts to control vapor diffusion or capillary moisture transfer in AAC with low-perm coatings/paints/sealers are doomed to failure- they blister/peel/flake (whether on the interior or exterior) and only semi-permeable paints can be used. In the cooler drier air of northern Europe it does OK from a moisture performance point of view, but it's so low-R that it doesn't meet current thermal requirements without adding exterior insulation.

AAC is fine as a substitute for old-school CMU, but as a standalone insulation it just barely cuts it for a SE US zone-3A location, even in locations where R13 2x4 (R10 whole-wall) is the tract-house standard. In drier US zones 1B-3B locations with high diurnal temperature swings it may be worth considering for code-min with a bit of a dynamic-mass edge, but not in the muggy-sticky SE."

Sorry. I got busy. Pennsylvania is very humid, I would guess more so than Indiana. According to legend, British quartermasters treated colonial Philadelphia as a tropical post. I thought thermal mass would suffice for the summer and went bare the first season. Well the sensible (air) temperature never got above 83 (on a record 100-degree day), the latent (actual) heat was pretty much unbearable. Bought a two-ton minisplit for the second season and basked in polar comfort. The trick is to buy an undersized AC so it keeps running.

We had horrendous condensation issues the first winter but that was the block/plaster drying out. This winter, the RH in our house was between 50 and 60 percent with no condensation at all; it was much more comfortable than our previous rental houses in Pa, which I would attribute to the tightness of this home.

The GBA posts are nonsense in my opinion. I would NEVER put stucco over wood. Period. If my stucco pops off I can put it back. If a window leaks, it's a problem for that window but not the whole wall. How many CMU buildings have you seen with drainage planes? Coul;d it be because they don't rot?

Aercon told me moisture can't fully penetrate AAC no matter what, and that has been my experience. The AAC planks that make up my garage roof were uncovered for six months, includiing a noreaster that dumped 4 inches of rain. (Two hurricanes, three noreasters and an earthquake so far.) I got penetration through the joints but no penetration of the blocks themselves. Ditto for

Cost? I paid $7/foot for semi DIY, as described above. Stucco and plastering, including skim coats over wall board, brought it to $13/foot for finishes that will knock your socks off. I found sons-of tradesmen who did plaster stucco work with their dads in 1960s and were pure D delighted for a chance to show their chops.

Blistering paint? Well, not after two years, and that first year as I said was a killer. The stucco finish coat is colored, and shouldn't need painting ever. We stained some deck boards inside during the high humidity period and they peeled. The walls? Not at all. Sider Oxydro has been making coatings for AAC since the 1950s.

All that said, I can't say I'd choose AAC for your house because I don't have any notion what you want. My goal was to build a very nice small house that my heirs can sell at a price that makes my effort worthwhile. The challenge: For the moment more people talk about not-so-big houses than actually build or buy them. It's a shame because a code min small house is greener than a McMansion with all the bells and whistles.
Sider-Oxydro

Sorry. I got busy. Pennsylvania is very humid, I would guess more so than Indiana. According to legend, British quartermasters treated colonial Philadelphia as a tropical post. I thought thermal mass would suffice for the summer and went bare the first season. Well the sensible (air) temperature never got above 83 (on a record 100-degree day), the latent (actual) heat was pretty much unbearable. Bought a two-ton minisplit for the second season and basked in polar comfort. The trick is to buy an undersized AC so it keeps running.

We had horrendous condensation issues the first winter but that was the block/plaster drying out. This winter, the RH in our house was between 50 and 60 percent with no condensation at all; it was much more comfortable than our previous rental houses in Pa, which I would attribute to the tightness of this home.

The GBA posts are nonsense in my opinion. I would NEVER put stucco over wood. Period. If my stucco pops off I can put it back. If a window leaks, it's a problem for that window but not the whole wall. How many CMU buildings have you seen with drainage planes? Coul;d it be because they don't rot?

Aercon told me moisture can't fully penetrate AAC no matter what, and that has been my experience. The AAC planks that make up my garage roof were uncovered for six months, includiing a noreaster that dumped 4 inches of rain. (Two hurricanes, three noreasters and an earthquake so far.) I got penetration through the joints but no penetration of the blocks themselves. Ditto for

Cost? I paid $7/foot for semi DIY, as described above. Stucco and plastering, including skim coats over wall board, brought it to $13/foot for finishes that will knock your socks off. I found sons-of tradesmen who did plaster stucco work with their dads in 1960s and were pure D delighted for a chance to show their chops.

Blistering paint? Well, not after two years, and that first year as I said was a killer. The stucco finish coat is colored, and shouldn't need painting ever. We stained some deck boards inside during the high humidity period and they peeled. The walls? Not at all. Sider Oxydro has been making coatings for AAC since the 1950s.

All that said, I can't say I'd choose AAC for your house because I don't have any notion what you want. My goal was to build a very nice small house that my heirs can sell at a price that makes my effort worthwhile. The challenge: For the moment more people talk about not-so-big houses than actually build or buy them. It's a shame because a code min small house is greener than a McMansion with all the bells and whistles.
Sider-Oxydro

Very enlightening, but for the one comment; "The trick is to buy an undersized AC so it keeps running." Any properly designed cooling system will run full time under design conditions and will dehumidify as well, leaving you with an ideal 50% rH at 76°F per ASHRAE standard.

Mass does nothing but buffer outdoor conditions...and noise of course.

You have me curious - how does a typical "compressor located outdoors" air conditioner provide sufficient dehumidification in less than design temperatures? Say 80F and 99% humidity outside. Ie, how does it work?

Posted By toddm on 14 Apr 2013 12:50 PM

The GBA posts are nonsense in my opinion. I would NEVER put stucco over wood. Period. If my stucco pops off I can put it back. If a window leaks, it's a problem for that window but not the whole wall. How many CMU buildings have you seen with drainage planes? Coul;d it be because they don't rot?

toddm,
I live in the land of CMU construction
it is true that the don't rot. However they are not good drainage planes,
they in fact are like a sponge to suck in water and moisture towards the interior of the building, causing mold problems and humidity issues.
wind driven rain will penetrate thu the stucco and porous cores
during hurricane Charlie many masonry homes absorbed and held water several feet high in the core of the wall. that then slowly leached to the interior space, with power out for many days , it created a great mold environment, making many homes uninhabitable

The moldy wallpaper on CMU clad buildings is pretty common. Adding real siding & drain planes (best done with a rainscreen gap) and attention to flashing detail can improve moisture resilience, even in swamps, but in an air conditioned CMU house low permeance insulation on the exterior seems a better way to go.

AAC doesn't wick as strongly or store as much moisture as CMU, but I have to believe that it retains at least SOME of those issues (which is why low-permeance paints blister & fail, even in less-humid more temperate European climates.) More vapor open latex would probably do better, but I like toddm's approach on that better- use colored stucco, and AAC-specific interior finishes, if going that route.

But though it's more resistant to liquid water, AAC is  just as permeable to water vapor as any other cementitious block material. But in answer to AAC-curious' question about the ability of mini-splits to keep up, moisture transport via vapor diffusion is still quite small compared to what's coming in on the ventilation air (or un-intended infiltration in a leaky house.)

Indiana isn't any drier or more humid than Pennsylvania, though you could cherry-pick locations to try to prove either. Most of the summertime air masses in PA came in via IN & OH, but they neither pick up nor lose much moisture along the way. The mean mid-summer dew points in Indianapolis are within a degree of those in Harrisburg according to weatherspark.com datasets.

BTW, Morgan: I thought the ASHRAE standard limit was 65% RH @ 76F, (the threshold of a mold explosion) though most health professionals bound it at 50% over the dust-mite issue.

OK don't believe anything I say about CMU. But I wouldn't worry about paint even if you go with bare blocks, which we did in an uncondiitioned garage. Ivan Burgand at Sider Oxydro, recommended to me by Aercon as the US expert on AAC coatings, told me to use latex paint formulated for CMU. Not a hiccup so far.