The biggest challenge probably being the 2x4 walls... if there's one code change to future-proof today's new housing stock, it might be adding 2 inches to the w

The biggest challenge probably being the 2x4 walls... if there's one code change to future-proof today's new housing stock, it might be adding 2 inches to the walls, either with 2x6 studs or 2 inches of rigid foam, for R-20 or so. Much of the rest is possible to retrofit, though not always easy. Actually, while we're on the subject, subslab insulation might be another.