[tloup]

Serious question, but what is a prefab home in the US made of? Here in Germany i think Ytong / aerated concrete would be a good choice, since it is a very good isolator already and you can get a pretty good subsidy if you build your house to a certain standard of energy efficiency. Ytong makes that pretty easy to accomplish. Here is a site in German with all of their houses: https://www.ytong-bausatzhaus.de/haeuser/

Here is a developer of prefab houses showing their plus energy house (meaning it produces more energy than it requires to live in): https://www.hanse-haus.de/en/prefabricated-houses/energy-eff... - these are not made out of aerated concrete though, they use timber panels.

[bluGill]

They don't give (at least not that I can find) any specs to how they achieve that efficiency. 20 years ago I looked that up (for a different manufacture) and their vaunted passive house didn't meet the minimum insulation codes for where I lived in the US. The climate in Germany is a lot kinder than US's midwest, and so it probably was passive there.