[gregmac]

> New buildings are rather efficient in US.

They can be, and there's lots of modern construction techniques that are really good (search term "building science").

One big problem is that a lot of stuff is still built to "minimum code" which just isn't that great. On top of that, sloppy construction work can significantly undermine what is done. In my own house I've fixed several simple things, like missing insulation around vents and holes cut too big (or created by a hammer, rather than cutting).

Here's a good walkthrough of a house under construction showing lots of problems typical to any subdivision (non-custom) build: https://youtu.be/OmU2N_Q732A