A pound of concrete does indeed weigh a pound in MA, as does a pound of AAC. But the THERMAL mass of a pound of concrete is only 80% that of a pound of AAC since it has a specific heat of ~0.2 BTU/lb-F as compared to AAC's ~0.25BTU/lb-F. (0.2/0.25= 0.8, or 80%, nicht wahr?)

The average dew point at ANY location in Britain is lower than the average dew point anywhere in the Gulf states. Rainy weather isn't a measure of the absolute humidity of the air, as measured by the dew point. If Louisiana or Texas were as cool as northern Europe it would be raining nearly constantly throughout the summer. Seattle air has much lower humidity than the Gulf coast states too. Dew points above 65F in Seattle are considered newsworthy events, whereas that's lower than the average summertime humidity in most of the southeastern US. In most of the UK and northern Europe the dewpoints are similar to or lower than Seattle's. (Look at Weatherspark.com dew point graphs of weather history & historical averages for a handful of locations, if you have doubts. In foggy-dew Glasgow UK the highest recorded dew point this summer was 58F. The highest dew point for rainy Amsterdam NL was 63F. The AVERAGE dew point for Dallas TX this summer was ~65F, well off it's historical ~ 68F average, but still spiking into the 70s on several occasions.

I wasn't recommmending non-structural slabs as an alternative to AAC, but only as a comparative to the total amount of thermal mass you have in your AAC wall.

And putting the thermal mass outside the insulation is still less effective than putting it inside the thermal envelope.

Seriously, model it in DOE2.

Funny. When BSC wrings its hands about solar driven vapor in brick facade walls, the bogey man is rain rather than dew point. http://www.buildingscience.com/documents/reports/rr-0104-solar-driven-moisture-in-brick-veneer

That would match up with the Brits who divide their island for wall stackup purposes into driven-rain-common and not-so-much. And yet they like AAC. Double funny.

The Brits have their issues with rising-damp in brick, but they're also in a much more moderate climate than the Canadian midwest, with no chance of wintertime moisture accumulation from inteiror drives. I'm not saying that it couldn't be controlled with AAC and made in that climate, but I'd want to know more about it before using it in those there. I saw a lot of AAC construction when I lived in Amsterdam too, but that's also a very modest winter climate compared to Winnipeg.

The capillary wicking of AAC and moisture buffering capacity is probably less than other masonry, but high vapor permeance in a true-cold climate may have some issues. Interior vapor retarders are probably necessary, but it would be interesting to know if that's in fact not the case- I haven't seen any real data on it.

The internal plaster is usually the Airtight-Vapor control layer. AAC is popular in Poland and Northern Germany but it usually has external insulation. Its popular in Scandinavia with the non timber frame brigade, it was invented in Sweden. AAC can suffer from cracking and can be difficult to plaster as the blocks soak in moisture from the plaster making it dry out too quickly. Its medium Hydroscopic so it wicks moisture quicker than concrete blocks.

Thanks for the details on how it's used in those parts of the world! (And on the high capilllary aspects.)

When I saw it used in Holland it was usually (but not always) in a modestly-insulated cavity wall- maybe R5 of XPS/cavity/clay-brick facer, but it's been 20+ years since I lived there, and wouldn't presume that it's done the same way in higher-R buildings.