To reply to Sas's comment regarding U-value's and what can be achieved using Ytong blocks I have conducted a little research on the Ytong International Website where they list lambda values for their blocks.
For their standard block the lambda value is quoted as being 0.09 W/mK. A block which is 365mm wide achieves a U-Value of 0.23 in only one leaf. On Ecostein's website you can see their current projects, one of which is in Killiney, where they are using a 480mm block in single leaf. This achieves a U-value of 0.16 and has a slightly lower lambda value of 0.08 W/mK. This is extremely close to the passive house benchmark of U-Value 0.15.
Ecostein achieves passive house standard by using a cavity system. On their website you can see another project at Ballyline Stud Co. Kilkenny, where the system they are using comprises of an external leaf of 240mm Ytong block and an internal leaf of 240mm Ytong block with 100mm insulation in between. This achieves a U-Value of 0.12.
I hope this information has cleared up some questions about the Ytong blocks.