The "2x6 construction" mentioned in the initial post does not necessarily indicate a more efficient house. If you run the numbers on a 2x6 wall with only cavity insulation and then compare it to a 2x4 wall with 1" of extruded foam on the exterior, you'll find that the 2x4 wall is more energy efficient (has a higher average R value).
Now, if you go to 2x6* construction but utilize advanced framing techniques to eliminate all but the structurally necessary framing and then wrap in foam then you have a very good wall. Not passive-house to be sure, but better than the average being built nowadays.
* 2x6s are needed for structure using this technique.
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